ALBERT

Notes: Abstract Microprobe analyses of feldspars in granite mylonites containing flame perthite give compositions that invariably plot as three distinct clusters on a ternary feldspar diagram: orthoclase (Or92–97), albite and oligoclase-andesine. The albite occurs as grains in the matrix, as flame-shaped lamellae in orthoclase, and in patches within plagioclase grains.We present a metamorphic model for albite flame growth in the K-feldspar in these rocks that is related to reactions in plagioclase, rather than alkali feldspar exsolution. Flame growth is attributed to replacement and results from a combination of two retrograde reactions and one exchange reaction under greenschist facies conditions. Reaction 1 is a continuous or discontinuous (across the peristerite solvus) reaction in plagioclase, in which the An component forms epidote or zoisite. Most of the albite component liberated by Reaction 1 stays to form albite in the host plagioclase, but some Na migrates to form the flames within the K-feldspar. Reaction 2 is the exchange of K for Na in K-feldspar. Reaction 3 is the retrograde formation of muscovite (as ‘sericite’) and has all of the chemical components of a hydration reaction of K-feldspar. The Si and Al made available in the plagioclase from Reaction 1 are combined with the K liberated from the K-feldspar, to produce muscovite in Reaction 3. The muscovite forms in the plagioclase, rather than the K-feldspar, as a result of the greater mobility of K relative to Al. The composition of the albite flames is controlled by both the peristerite and the alkali feldspar miscibility gaps and depends on the position of these solvi at the pressure and temperature that existed during the reaction. Using an initial plagioclase composition of An20, the total reaction can be summarized as:20 oligoclase + 1 K-feldspar + 2 H2O = 2 zoisite + muscovite + 2 quartz + 15 albiteplagioclase+ 1 albiteflame.This model does not require that any additional feldspar framework be accreted at replacement sites: Na and K are the only components that must migrate a significant distance (e.g. from one grain to the next), allowing Al to remain within the altering plagioclase grain. The resulting saussuritization is isovolumetric.The temperature and extent of replacement depends on when, and how much, water infiltrates the rock. The fugacity of the water, and therefore the pressure of the fluid, may have been significantly lower than lithostatic during flame growth.

Notes: Abstract In metapelitic schists of the north-eastern Weekeroo Inliers, Olary Block, Willyama Supergroup, South Australia, syn-S1 and syn-S2 assemblages involving staurolite, garnet, biotite and another mineral, most probably cordierite, were overgrown by large syn-S3 andalusite porphyroblasts, owing to isobaric heating from metamorphic conditions that existed during the development of S2. Conditions during the development of S3 probably just reached the andalusite—sillimanite transition. During the development of S4, at somewhat lower temperatures than those that accompanied the development of S3, the following reaction occurred:staurolite + chlorite + muscovite ± biotite + andalusite + quartz + H2O.The amount of retrogression is controlled primarily by the amount of H2O added by infiltration. As the syn-S3 matrix assemblage was stable during the development of S4, but the andalusite porphyroblasts were no longer stable with the matrix when H2O was added, the retrogression is focused in and around the porphyroblasts. With enough H2O available, and if quartz was consumed before biotite in a porphyroblast, then the following reaction occurred:staurolite + chlorite + muscovite + corundum ± biotite + andalusite + H2O.This reaction allowed corundum inclusions in the andalusite to grow, regardless of the presence of quartz in the matrix assemblage.

Notes: Abstract Effects of post-entrapment fluid-inclusion modification are examined with reference to retrogression-related quartz veins from the Caledonian, Øse Thrust, northern Norway. The inclusions occur in secondary trails, and contain high-density hypersaline aqueous fluids. On morphological characteristics, they are subdivided into, Type A: elongate, ellipsoidal and/or irregular inclusions, and Type B: more equant, regular, and/or negative crystal form. With reference to previous research on post-entrapment modification of inclusions in quartz it is proposed that Type A inclusions experienced little or no post-entrapment modification, whereas Type B inclusions show features characteristic of post-entrapment permanent inelastic stretching and/or leakage. This produces increased homogenization temperatures (Th), associated with increased inclusion volume and lowering of density, whilst maintaining constant salinity. The similarity of data for degree of fill and salinity between Type A and Type B inclusions indicates that Type B inclusions have primarily modified by stretch rather than leakage. However, the spread towards slightly larger volume of vapour in Type B inclusions suggests that some leakage has also occurred. Because stretched and/or partially leaked inclusions have increased Th, isochore projections significantly underestimate trapping pressure (Pt) relative to unmodified inclusions. Therefore, recognition of post-entrapment inclusion modification due to overpressure is crucial to avoid misinterpretation of data, but has considerable potential for constraining the detail of P-T trajectories of individual rocks. On this basis, rocks from the Øse Thrust zone, north Norway, are shown to have experienced rapid uplift on a ‘clockwise’P-T-t path during the final stages of Caledonian (Scandian) orogenesis.

Notes: Abstract The Erzgebirge Crystalline Complex (ECC) is a rare example where both‘crustal’eclogites and mantle-derived garnet-bearing ultramafic rocks (GBUs) occur in the same tectonic unit. Thus, the ECC represents a key complex for studying tectonic processes such as crustal thickening or incorporation of mantle-derived material into the continental crust. This study provides the first evidence that high-pressure metamorphism in the ECC is of Variscan age. Sm-Nd isochrons define ages of 333 ± 6 (Grt-WR), 337± 5 (Grt-WR), 360± 7 (Grt-Cpx-WR) (eclogites) and 353 ± 7 Ma (Grt-WR) (garnet-pyroxenite). 40Ar/39Ar spectra of phengite from two eclogite samples give plateau ages of 348 ± 2 and 355 ± 2 Ma. The overlap of ages from isotopic systems with blocking temperatures that differ by about 300 ° C indicates extremely fast tectonic uplift rates. Minimum cooling rates were about 50° C Myr-1. As a consequence, the closure temperature of the specific isotopic system is of minor importance, and the ages correspond to the time of high-pressure metamorphism. Despite textural equilibrium and metamorphic temperatures in excess of 800° C, clinopyroxene, garnet and whole rock do not define a three-point isochron in three of four samples. The metamorphic clinopyroxenes seem to have inherited their isotopic signature from magmatic precursors. Rapid tectonic burial and uplift within only a few million years might be the reason for the observed Sm-Nd disequilibrium. The εNd values of the eclogites (+4.4 to +6.9) suggest the protoliths were derived from a long-term depleted mantle, probably a MORB source, whereas the isotopically enriched garnet-pyroxenite (εNd–2.9) might represent subcontinental mantle material, emplaced into the crust prior to or during collision. The similarity of ages of the two different rock types suggests a shared metamorphic history.

Notes: Abstract The crystalline core of the Himalayan orogen in the Langtang area of Nepal, located between the Annapurna-Manaslu region and the Everest region, contains middle to upper amphibolite grade pelitic gneisses and schists. These rocks are intimately associated with the Main Central Thrust (MCT), one of the major compressional structures in the northern Indian plate, which forms a 3.7-km-wide zone containing rocks of both footwall and hangingwall affinity. An inverted metamorphic gradient is noticeable from upper footwall through hangingwall rocks, where metamorphic conditions increase from garnet grade near the MCT zone to sillimanite + K-feldspar grade in the upper hangingwall. Petrographic data distinguish two metamorphic episodes that have affected the area: a high-pressure, moderate-temperature episode (M1) and a moderate-pressure, high-temperature episode (M2). Comparison with appropriate reaction boundaries suggests that conditions for M1 in the hangingwall were approximately 900–1200 MPa and 425–525°C. Thermobarometric results for 24 samples from the footwall, MCT zone and hangingwall reflect P-T conditions during the M2 phase of 400–1200 MPa and 490–660° C. The decrease in estimated palaeopressures from footwall to hangingwall approximate a lithostatic gradient of 27 MPa km-1, with slight fluctuations in the MCT zone reflecting structural discontinuities. In contrast to the palaeopressures, palaeotemperatures are indistinguishable across the entire area sampled. Although field evidence suggests the presence of the inverted palaeothermal gradient well known in the Himalaya, quantitative thermobarometry indicates that temperatures of final equilibration were all within error of each other across 17 km of section. At Langtang, change in pressure is responsible for the presence of the sequence of index minerals through the section. I interpret these data to reflect diachronous attainment of equilibrium temperature conditions in a lithostatic palaeopressure profile after ductile faulting of the sequence.

Notes: Abstract High-pressure-temperature metapelites that occur in close proximity to eclogitized mafic rocks in the southern part of the Gagnon terrane (Parautochthonous Belt, eastern Grenville Province) were investigated in order to constrain depths of burial and P-T paths. Mineral assemblages and partial melting relationships in these metapelites are consistent with peak temperatures in the range between 700 and 800° C. However, growth zoning is apparently well preserved in garnets and only narrow rims (width = 100–500 μm) are obviously affected by diffusional retrograde resetting. Despite uncertainties regarding mineral assemblages and compositions of matrix minerals at early stages of garnet growth, it can be shown that the observed growth zoning profiles of garnets imply increase of both pressure and temperature up to a common maximum at pressures between 1300 and 1600 MPa, and that thermal relaxation did not occur during the initial stages of unloading. On the other hand, calculated retrograde P-T conditions are consistent with steep decompression paths. The inferred ‘hair-pin’-shaped P-T path is consistent with independent evidence of rapid, tectonically driven exhumation, resulting in the preservation of growth zoning in garnets from such a high-temperature regime.

Notes: Abstract A petrogenetic grid and related diagrams derived from KFMASH-system experiments demonstrate that osumilite is stable in relatively magnesian bulk rock compositions (XMg 〉 0.6) at temperatures in excess of 875° C and pressures less than 11 kbar. The experiments, involving the dehydration melting of biotite in synthetic metapelites, were conducted in the range 850–1000° C. Both the mineral assemblages and phase compositions reported from well-documented natural examples of osumilite-bearing rocks are reproduced by the experiments at P-T conditions similar to those previously estimated for these occurrences. Peak metamorphic P-T conditions can be reliably inferred from distinctive osumilite-bearing assemblages identified in the phase diagrams, thereby avoiding the problems of diffusional re-equilibration that often prohibits conventional geothermobarometry from recovering peak conditions. Integration of the experimental data with recent independent experiments, after correcting the latter for an underestimated friction correction, allows extension of the petrogenetic grid to higher temperatures. The extended grid is applied to assess and refine the metamorphic history of the Napier Complex, East Antarctica: the high-P stability limit for osumilite in the Napier Complex is 9–10 kbar, the prograde P-T-t path is not necessarily anticlockwise and isobaric cooling in the Scott and Tula mountains occurred, respectively, at pressures greater and less than reactions in the range 8–9 kbar. The stability range for osumilite predicted by the KFMASH-system petrogenetic grid overlaps many more metamorphic terranes than osumilite is found in. Whilst osumilite is not distinctive in thin section and is prone to retrogression, it is possible that carbon dioxide present in the natural system stabilizes cordierite at the expense of osumilite.

Notes: Abstract Paragonite in textural equilibrium with garnet, omphacite and kyanite is found in two eclogites in the ultrahigh-pressure metamorphic terrane in Dabie Shan, China. Equilibrium reactions between paragonite, omphacite and kyanite indicate a pressure of about 19 kbar at c. 700° C. However, one of the paragonite eclogites also contains clear quartz pseudomorphs after coesite as inclusions in garnet, suggesting minimum pressures of 27 kbar at the same temperature. The disparate pressure estimates from the same rock suggest that the matrix minerals in the ultrahigh-pressure eclogites have recrystallized at lower pressures and do not represent the peak ultrahigh-pressure assemblages. This hypothesis is tested by calibrating a garnet + zoisite/clinozoisite + kyanite + quartz/coesite geobarometer and applying it to the appropriate eclogite facies rocks from ultrahigh- and high-pressure terranes. These four minerals coexist from 10 to 60 kbar and in this wide pressure range the grossular content of garnet reflects the equilibrium pressure on the basis of the reaction zoisite/clinozoisite = grossular + kyanite + quartz/coesite + H2O. The results of the geobarometer agree well with independent pressure estimates from eclogites from other orogenic belts. For the paragonite eclogites in Dabie Shan the geobarometer indicates pressures in the quartz stability field, confirming that the former coesite-bearing paragonite-eclogite has re-equilibrated at lower pressures. On the other hand, garnets from other coesite-bearing but paragonite-free kyanite-zoisite eclogites show a very wide variation in grossular content, corresponding to a pressure variation from coesite into the quartz field. This wide variation, partly due to a rimward decrease in grossular component in garnet, is caused by partial equilibration of the mineral assemblage during the exhumation.

Notes: Abstract The hydrothermal metamorphism of a sequence of Pliocene-aged seamount extrusive and volcanoclastic rocks on La Palma includes a relatively complete low-P-T facies series encompassing the zeolite, prehnite-pumpellyite, and greenschist facies. The observed mineral zonations imply metamorphic gradients of 200–300° C km-1.The transition from smectite to chlorite in the La Palma seamount series is characterized by discontinuous steps between discrete smectite, corrensite and chlorite, which occur ubiquitously as vesicles and, to a much lesser extent, vein in-fillings. Trioctahedral smectites [(Mg/(Fe + Mg) = 0.4–0.75] occur with palagonite and Na-Ca zeolites such as analcime and a thompsonite/natrolite solid solution. Corrensite [(Mg/(Fe + Mg) = 0.5–0.65] first appears at stratigraphic depths closely corresponding to the disappearance of analcime and first appearance of pumpellyite. Discrete chlorite [(Mg/(Fe + Mg) = 0.4–0.6] becomes the dominant layer silicate mineral coincident with the appearance of epidote and andraditic garnet.Within the stratigraphic section there is some overlap in the distribution of the three discrete layer silicate phases, although random interstratifications of these phases have not been observed. Although smectite occurs as both low- and high-charge forms, the La Palma corrensite is a compositionally restricted, 1:1 mixture of low-charge, trioctahedral smectite and chlorite. Electron microprobe analyses of coarse-grained corrensite yield structural formulae close to ideal values based on 50 negative charge recalculations. Calcium (average 0.20 cations/formula unit) is the dominant interlayer cation, with lesser Mg, K and Na.The absence of randomly interlayered chlorite/smectite in the La Palma seamount series may reflect high, time-integrated fluid fluxes through the seamount sequence. This is consistent with the ubiquity of high-variance metamorphic mineral assemblages and the general absence of relict igneous minerals in these samples.

Notes: Abstract Quartz-hosted, synthetic CO2-H2O fluid inclusions behave as open systems with respect to diffusional transfer of hydrogen during laboratory-simulated metamorphic re-equilibration at 650, 750 and 825°C and 1.5 kbar total pressure with fO2 defined by the C-CH4 buffer. Microthermometry and Raman spectroscopy show that the initial CO2-H2O inclusions become CO2-CH4-H2-H2Oinclusions after diffusive influx of hydrogen from the reducing confining medium. Measurable changes are observed in inclusion compositions after only 15 days of re-equilibration, implying significant hydrogen mobility at still lower temperatures over geological time spans. Results of synthetic inclusion re-equilibrium experiments have profound implications for the interpretation of natural fluid-inclusion data; failure to account for potential hydrogen migration in inclusions from high-temperature geological environments may lead to erroneous estimates of P-T, and/or the compositions of metamorphic fluids.

Notes: Abstract Fe-Mg carpholite occurs in metasediments of tectonically disrupted basement, shelf and foreland basin units that structurally underlie the Semail ophiolite in NE Oman. In the lower grade, structurally higher units, Fe-rich carpholite coexists with paragonite, quartz, illite, kaolinite and chlorite, whereas in deeper units, Fe-Mg carpholite occurs with pyrophyllite, sudoite, phengite and/or chloritoid. Mineral compositions in these units indicate that chlorite is more magnesian than coexisting Fe-Mg carpholite at low temperatures and pressures but, at higher metamorphic grades, XMg decreases in the order sudoite 〉 carpholite 〉 chlorite 〉 chloritoid. This suggests a reversal in Fe-Mg partitioning between Fe-Mg carpholite and chlorite at temperatures below or close to those of the breakdown of kaolinite + quartz to pyrophyllite and at XMg= 0.35.Phase relations and mineral equilibria indicate that the P-T conditions of formation of the Fe-Mg-carpholite-bearing rocks of NE Oman range from 280–315° C, 3–6 kbar for the structurally highest units to 325–440° C, 6–9.5 kbar for the deepest units, indicating a systematic down-section increase in metamorphic grade. Textural relations in these rocks, interpreted in the context of pertinent equilibria, are consistent with the clockwise P-T paths previously constrained for these units from petrological studies of interlayered isofacial mafic rocks.

Notes: Metamorphic Petrology. By Akiho Miyashiro. UCL Press Ltd, London, 1994. ISBN 1-85728-037-7 (HB), 1-85728-038-7 (PB)

Notes: Abstract The Santiago Schists are located in the Basal Unit of the Ordenes Complex, one of the allochthonous complexes outcropping in the inner part of the Hercynian Belt in the north-west of the Iberian Peninsula. Their tectonothermal evolution is characterized by the development of an eo-Hercynian metamorphic episode (c. 374 Ma) of high-P, low- to intermediate-T. The mineral assemblage of the high-P episode is preserved as a very thin Si= S1 foliation included in albite porphyroblasts, being composed of: albite + garnet-I + white mica-1 + chlorite-1 + epidote + quartz + rutile ± ilmenite. The equilibrium conditions for this mineral assemblage have been estimated by means of different thermobarometers at 495 ± 10 °C and 14.7 ± 0.7 kbar (probably minimum pressure). The later evolution (syn-D2) of the schists defines a decompressive and slightly prograde P-T path which reached its thermal peak at c. 525 ± 10 °C and 7 kbar. Decompression of the unit occurred contemporaneously with an inversion of the metamorphic gradient, so that the zones of garnet-II, biotite (with an upper subzone with chloritoid) and staurolite developed from bottom to top of the formation.The estimated P-T path for the Santiago Schists suggests that the Basal Unit, probably a fragment of the Gondwana continental margin, was uplifted immediately after its subduction at the beginning of the Hercynian Orogeny. It also suggests that the greater part of the unroofing history of the unit took place in a context of ductile extension, probably related to the continued subduction of the Gondwana continental margin and the contemporaneous development of compensatory extension above it. The inverted metamorphic gradient seems related to conductive heat transferred from a zone of the mantle wedge above the subducted continental margin, when it came into contact with the upper parts of the schists along a detachment, probably of extensional character.The general metamorphic evolution of the Santiago Schists, with the development of high-P assemblages with garnet prior to decompressive and prograde parageneses with biotite, is unusual in the context of the European Hercynian Belt, and shows a close similarity to the tectonothermal evolution of several high-P, low- to intermediate-T circum-Pacific belts.

Notes: Abstract Low-pressure/high-temperature (low-P/high-T) metamorphic rocks of the Cooma Complex, southeastern Australia, show evidence of an anticlockwise pressure-temperature-time-deformation (P-T-t-D) path, similar to those of some other low-P/high-T metamorphic areas of Australia. Prograde paths are reasonably well constrained in cordierite-andalusite schists, cordierite-K-feldspar gneisses and andalusite-K-feldspar gneisses. These paths are inferred to be convex to the temperature axis, involving increase in pressure with increase in temperature. Evidence of the retrograde path is inconclusive, but is consistent with approximately isobaric cooling, as are available isotopic data on the Cooma Granodiorite, which indicate initially rapid cooling following attainment of peak temperatures. The retrograde path is inconsistent with either a clockwise P-T-t-D path involving rapid or even moderate decompression immediately post-dating the peak of metamorphism, or a path in which the retrograde component simply reverses the prograde component, because both these paths should cross reactions forming cordierite from aluminosilicate, for which no evidence has been observed.Determination of the deformational-metamorphic history of the complex is not straightfoward and depends on careful examination of critical samples. Evidence necessary for successful elucidation of the prograde, and part of the retrograde, deformational-metamorphic history in the Cooma Complex includes: (1) sequentially grown porphyroblasts that can be timed relative to surrounding foliations; (2) partial replacement microstructures providing relative timing of metamorphic reactions that cannot be timed relative to foliation development; (3) a tectonic marker foliation (S4 at Cooma) that allows correlation of foliations from one location to another; and (4) single samples containing all of the foliations and all generations of porphyroblast growth within a single metamorphic zone. The latest two or three foliations involve low strain accumulation, allowing relative timing relationships between foliations and porphyroblasts to be more clearly determined.Sequential porphyroblast growth and foliation development in the cordierite-andalusite schists is examined for situations involving rotation and non-rotation of porphyroblasts relative to geographically fixed coordinates. Although the number of foliations developed varies in the rotational situation, depending on the deformation history proposed, the sequential order of porphyroblast growths does not differ from the non-rotational situation. Thus, whether or not porphyroblasts rotated in the Cooma rocks, the sequence of reactions, and therefore P-T-t paths inferred from the relative timing of porphyroblast growths, remain the same, for the deformational histories evaluated.

Notes: Abstract Observations and microthermometric data on fluid inclusions from a terrane that underwent deformation following peak metamorphic conditions show that grain-boundary migration recrystallization favours the entrapment of carbonic inclusions whereas microfracturing during brittle deformation favours the infiltration and eventual entrapment of aqueous fluids. Our results imply that pure CO2 fluid inclusions in metamorphic rocks are likely to be the residue of deformation-recrystallization process rather than representing a primary metamorphic fluid.Where the temperature of deformation can be deduced by other means, the densities of fluid inclusions trapped during recrystallization, which we call recrystallization-primary fluid inclusions, can be used to constrain the ambient pressure during deformation. Using these constraints, the data imply that the post-metamorphic Hercynian exhumation in Sardinia brought rocks at 300° C to within 3km of the surface. This conclusion is similar to that described for the rapidly uplifted Southern Alps in New Zealand.

Notes: Abstract Metapelitic and charnockitic granulites exposed around Chilka Lake in the northern sector of the Eastern Ghats, India, preserve a multi-stage P—T record. A high-T decompression from above 10 kbar to 8 kbar around 1100°C has been determined from Mg-rich metapelites (XMg〉0.60) with quartz-cordierite-orthopyroxene-sillimanite and cordierite—orthopyroxene—sapphirine—spinel assemblages. Between this and a second decompression to 6.0 kbar, isobaric cooling from 830 to 670°C at 8 kbar is evident. These changes are registered by the rim compositions of orthopyroxene and garnet in charnockites and metapelites with an orthopyroxene—quartz—garnet—plagioclase—cordierite assemblage, and are further supported by the garnet + quartz ± orthopyroxene + cordierite and biotite-producing reactions in sapphirine-bearing metapelites. Another indication of isobaric cooling from 800 to 650°C at 6.0 kbar is evident from rim compositions of orthopyroxene and garnet in patchy charnockites. Two sets of P—T values are obtained from metapelites with a quartz—plagioclase—garnet—sillimanite—cordierite assemblage: garnet and plagioclase cores yield 6.2 kbar, 700°C and the rims 5 kbar, 650°C, suggesting a third decompression.The earliest deformation (F1) structures are preserved in the larger charnockite bodies and the metapelites which retain the high P—T record. The effects of post-crystalline F2 deformation are observed in garnet megacrysts formed during or prior to F1 in some metapelites. Fold styles indicate a compressional regime during F1 and an extensional regime during F2. These lines of evidence and two phases of cooling at different pressures point to a discontinuity after the first cooling, and imply reworking.Two segments of the present P—T path replicate parts of the P—T paths suggested for four other granulite terranes in the Eastern Ghats, and the sense of all the paths is the same. This, plus the signature of three phases of deformation identified in the Eastern Ghats, suggests that the Chilka Lake granulites could epitomize the metamorphic evolution of the Eastern Ghats.

Notes: Abstract The Chuncheon amphibolite, part of the Gubongsan Group which overlies the Yongduri gneiss complex, is interlayered with calc-silicate rock, marble, quartzite, biotite schist and quartzofeldspathic gneiss in the central Gyeonggi massif, South Korea. Metamorphic pressures and temperatures estimated from the amphibolite are 5.5–10.6 kbar and 615–714°C. These P—T conditions are close to those defined by the reaction curve between kyanite and sillimanite, and suggest medium-pressure-type metamorphism of the Chuncheon amphibolite. For two metapelites intercalated with the amphibolite, temperatures are estimated to be 607–699° C, consistent with those obtained from the amphibolite. On the other hand, pressures estimated from these metapelites are significantly different, 4–6 kbar and 9–13 kbar, when rim and core compositions of garnet are, respectively, used. These P—T estimates obtained from the amphibolite and metapelite suggest a nearly isothermal decompression of 3–7 kbar during denudation. Rapid decompression is likely on the basis of the results of mineral chemistry, phase equilibria and geothermobarometer. Moreover, in conjunction with the occurrence of kyanite in the adjacent Gyeonggi gneiss complex, P—T estimates of the Chuncheon amphibolite and metapelite suggest a clockwise P—T—t path. This evolutionary path may be related to the amalgamation of continents during the late Proterozoic event which corresponds to the Jinningian orogeny in the Qinling belt of China.

Notes: Abstract The Ruby terrane is an elongate fragment of continental crustal rocks that is structurally overlain by thrust slices of oceanic crust. Our results from the Kokrines Hills, in the south-central part of the Ruby terrane, demonstrate that the low-angle schistose fabric formed under high-P/low-T conditions, at peak conditions of 10.8-13.2 kbar and 425-550° C, consistent with the rare occurrence of glaucophane. White mica 40Ar/39Ar cooling ages from these blueschists indicate that the metamorphism occurred prior to 144 ± 1 Ma. The blueschist facies assemblages are partially replaced by greenschist facies assemblages in the eastern Kokrines Hills. In contrast, in the central and western Kokrines Hills, upper amphibolite to lower granulite facies metamorphism associated with extensive late Early Cretaceous plutonism has completely overprinted any evidence of an earlier high-P/T metamorphic history. Deformation accompanying the plutonism produced recumbent isoclinal folds in the plutonic rocks and pelitic gneisses of the wallrock; decompression reactions in the pelitic gneisses suggest that the deformation occurred during exhumation. Thermochronological data bracket the time of intrusion and cooling below 500° C between 118 ± 3 and 109 ± 1 Ma.Our data from the schists of the Ruby terrane support the general assumption of many authors that the Ruby terrane was subducted beneath an oceanic island arc. This tectonic history is similar to that described for other large continental crustal blocks in northern and central Alaska, in the Brooks Range, Seward Peninsula and Yukon-Tanana Upland. The current orientation of the Ruby terrane at an oblique angle to these other crustal blocks and to the Cordilleran trend is due to post-collisional tectonic processes that have greatly modified the original continental margin.

Notes: Abstract Metagreywackes in the Eastern Belt of the Franciscan Complex contain the assemblage: Qtz + Ab + Lws + Chl + Ph + Pmp + Fgl + Hem ° Cal/Arg or compatible subassemblages. Blue amphibole first appears in the westernmost part of the belt and pumpellyite is absent in the eastern part. The compositions of the coexisting minerals and the nature of the continuous reactions in these low-grade blueschists suggest that the distribution of blue amphibole and pumpellyite in the Eastern Belt of the Franciscan Complex reflects differences of effective bulk composition rather than differences in physical conditions of metamorphism. In rocks lacking pumpellyite, white mica may be essential to the growth of blue amphibole, but carbonate plays only a limited role. The continuous reaction that limits the appearance of blue amphibole and the disappearance of coexisting pumpellyite has the general form: Pmp + Chl + Ab + Qtz + Hem + H2O + FeMg-1= Fgl + Lws. This reaction requires significant hydration as pressure increases in order to produce blue amphibole. Most of the Eastern Belt of the Franciscan Complex formed in limited ranges of temperature and pressure, which are estimated to be 240—280° C, 6.5-7.5 kbar. Pressures in the westernmost part of the area were about 1 kbar lower than in the east. Pressures of about 8.5-10 kbar are estimated for tectonic blocks that contain sodic clinopyroxene.

Notes: Abstract Petrological study of highly strained carbonate and pelitic rocks within the contact aureole surrounding the western part of the Papoose Flat pluton yields thermal profiles (plots of metamorphic temperature versus distance) across the aureole that show temperature gradients which are relatively flat and narrow (〈100m). The gradients occur close to the contact and indicate a slight decrease in temperature from 500–550°C at the pluton/wall rock contact to 450–500°C at the outer margin of the aureole. One thermal profile across low-strain metasedimentary rocks located in the southern part of the aureole shows that thermal effects from emplacement extend no further than 600 m from the contact. Coexistence of andalusite and cordierite in pelitic rocks of the aureole constrain pressures to 〈4 kbar. Thermal modelling using an analytical solution of the conductive heat flow equation for a rectangular-shaped pluton reproduces the observed thermal maxima and profile shape. Conductive rather than convective cooling also is supported by isotopic and field evidence for limited fluid flow along the strongly deformed margin of the pluton. Simple thermal models coupled with observed high-temperature deformation features and a measured 90% attenuation of stratigraphic units in the plastically deformed western part of the pluton's aureole indicate that strain rates may have been of the order of 10-12s-1. Evidence for episodic heating, such as two distinct generations of andalusite growth in pelites from the aureole, alternatively may indicate a longer heating event and, therefore, slower strain rates. Thermal models also indicate that parts of the pluton still may have been above the solidus during deformation of the pluton margin and aureole.

Notes: Abstract The Protogine Zone comprises a system of anastomosing deformation zones which approximately parallel the eastern boundary of the Sveconorwegian (1200–900 Ma) province in south-west Sweden. Ages of granulite facies metamorphism in the Sveconorwegian province require exhumation from c. 30 to 35 km crustal depths after 920–880 Ma. 40Ar/39 Ar cooling ages are presented for muscovite from high-alumina rocks formed by hydrothermal leaching associated with the Protogine Zone. Growth of fabric-defining minerals was associated with a ductile deformational event; muscovite from these rocks cooled below argon retention temperatures (c. 375 ± 25° C) at c. 965–955 Ma. Muscovite from granofels in zones of intense alteration indicates that temperatures 〉 375 ± 25° C were maintained until c. 940 Ma. Textural relations of Al2SiO5 polymorphs and chloritoid suggest that dated fabrics formed during exhumation. The process of exhumation, brittle overprint on ductile structures and hydrothermal activity along faults within the Protogine Zone tentatively are interpreted as the peripheral effects of initial Neoproterozoic exhumation of the granulite region of south-western Sweden.Muscovite in phyllonites associated with the ‘Sveconorwegian thrust system’cooled below argon retention temperatures at c. 927 Ma. Exhumation associated with this cooling could have been related to extension and onset of brittle-ductile deformation superimposed on Sveconorwegian contraction.

Notes: Abstract Orientated symplectites have been observed in deformed granulite facies metabasic rocks from the Ivrea-Verbano zone in northern Italy. The area underwent lower crustal extension, accommodated by movement on localized high-T shear zones. In areas of relatively low strain, such as at the margins of shear zones, symplectites of orthopyroxene, plagioclase and spinel have formed. The symplectites are vermiform and orientated parallel to the main foliation and in the regional stretching direction. The reaction was synkinematic with the deformation, and only developed in potentially dilatant grain boundaries in the rock. It was presumably inhibited in grain boundaries subjected to higher normal stress due to the relatively large volume increase involved in the reaction.The observations support the interpretation that the deformation was related to regional extension under high-T granulite facies conditions, the symplectites forming as a result of decreasing pressure.

Notes: Abstract The E-W-trending Kohistan terrane in the NW Himalaya is a sandwich of a magmatic arc between the collided Karakoram (Asian) and Indian plates. The southern part of the Kohistan arc is principally made up of amphibolites derived from volcanic and plutonic rocks of Early Cretaceous age. Gabbroic relics in the amphibolites display calc-alkaline character, and their mineralogy is similar to low-P plutonic rocks reported from modern and ancient island arcs. The largest of these relics, occurring along the southern margin of the amphibolite belt near Khwaza Khela, is subcircular in outline and is about 1 km across. It consists of cumulate gabbros and related rocks displaying a record of cooling and crustal thickening. Primary olivine and anorthite reacted to produce coronas consisting of two pyroxenes +Mg-Fe2+-Al spinel ± tschermakitic hornblende at about 800° C, 5.5–7.5 kbar. This thermotectonic event is of regional extent and may be related to the overthrusting of the Karakoram plate onto the Kohistan arc some 85 Ma ago, or even earlier. Later the gabbros were locally traversed by veins containing high-P assemblages: garnet, kyanite, zoisite, paragonite, oligoclase, calcite, scapolite and quartz ° Chlorite ° Corundum ± diopside. Formed in the range 510–600° C, and 10–12 kbar, these suggest further thickening and cooling of the crust before its uplift during the Tertiary. This paper presents microprobe data on the minerals, and discusses the tectonic implications of the coronitic and vein assemblages in the gabbros.

Notes: Abstract Granulite facies marbles from the Upper Calcsilicate Unit of the Reynolds Range, central Australia, contain metre-scale wollastonite-bearing layers formed by infiltration of water-rich (XCO2= 0.1–0.3) fluids close to the peak of regional metamorphism at c. 700° C. Within the wollastonite marbles, zones that contain 〈10% wollastonite alternate on a millimetre scale with zones containing up to 66% wollastonite. Adjacent wollastonite-free marbles contain up to 11% quartz that is uniformly distributed. This suggests that, although some wollastonite formed by the reaction calcite + quartz = wollastonite + CO2, the wollastonite-rich zones also underwent silica metasomatism. Time-integrated fluid fluxes required to cause silica metasomatism are one to two orders of magnitude higher than those required to hydrate the rocks, implying that time-integrated fluid fluxes varied markedly on a millimetre scale. Interlayered millimetre -to centimetre-thick marls within the wollastonite marbles contain calcite + quartz without wollastonite. These marls were probably not infiltrated by significant volumes of water-rich fluids, providing further evidence of local fluid channelling. Zones dominated by grandite garnet at the margins of the marl layers and marbles in the wollastonite-bearing rocks probably formed by Fe metasomatism, and may record even higher fluid fluxes. The fluid flow also reset stable isotope ratios. The wollastonite marbles have average calcite (Cc) δ18O values of 15.4 ± 1.6% that are lower than the average δ18O(Cc) value of wollastonite-free marbles (c. 17.2 ± 1.2%). δ13C(Cc) values for the wollastonite marbles vary from 0.4% to as low as -5.3%, and correlations between δ18O(Cc) and δ13C(Cc) values probably result from the combination of fluid infiltration and devolatilization. Fluids were probably derived from aluminous pegmatites, and the pattern of mineralogical and stable isotope resetting implies that fluid flow was largely parallel to strike.

Notes: In north-central Wopmay Orogen, syntectonic low-P(Buchan-type) suites of mineral isograds outline regional metamorphic temperature culminations that are associated, at the higher structural levels, with emplacement of early Proterozoic plutons in the west part of a deformed and eastward transported continental margin prism. The mapped isograds mark the first occurrence of biotite, staurolite, andalusite, sillimanite, sillimanite-K feldspar and K feldspar-plagioclase-quartz ± muscovite (granitic) pods in metapelites, with increasing proximity to the plutons.Microprobe analyses and field observations have resulted in the formulation of reactions for the ‘ideal’pelitic system K2O-Na2O-FeO-MgO-Al2O3-SiO2-H2O-Al2O3-SiO2-H2O, to account for the various mineral assemblages of each metamorphic zone. A P-T petrogenetic grid showing erosion surface P-T curves for the northern Wopmay Orogen pelites, compiled on the basis of the mapped isograds and the inferred reaction(s) for each metamorphic zone, documents a variation in exposed metamorphic pressure ranging between 2 and 4 kbar.The configuration of a new bathograd, based on the invariant model reaction sillimanite + K feldspar + plagioclase + biotite + quartz + vapor ± muscovite + liquid and interpolated across three metamorphic suites, is consistent with a major regional structure culmination and with independently determined pressures obtained from anorthite-grossular-quartz-Al2SiO5 geobarometry. The positive correlation between the configuration of the bathograd and the structural and pressure culmination points to the pressure-dependence of anatectic-granitic-pod mineral associations.

Notes: Variations in assemblage and composition of the constituent minerals in basic and intermediate metavolcanics encountered in the Zarouchla Group of the Phyllite-Quartzite Series are consistent with a progressive sequence, corresponding to temperature conditions estimated at 290-380°C (minimum values) under a total pressure greater than 3°5kbar and possibly as high as 5 kbar. In the absence of more critical evidence, the parageneses recorded in the metavolcanic rocks are interpreted as belonging to a prograde facies series from the lawsonite-albitechlorite facies through the pumpellyite-actinolite facies to the greenschist facies. The present distribution of mineral assemblages does not show a simple increase of metamorphic grade in a given direction but is apparently related to the tectonic evolution of the metamorphic sequence.

Notes: Abstract Mylonites from shear zones cutting Hercynian gneisses in the central Pyrenees have been studied in thin section and using the electron microprobe. The shear zones contain retrogressive greenschist facies assemblages implying introduction of an aqueous fluid during deformation in the zones. Textural evidence suggests that fluid-rock interaction occurred throughout the active life of the shear zones.Whole-rock chemical changes during deformation are documented in a variety of mylonitic lithologies and retrogressed country rocks. The overall effect was to reduce chemical differences between lithologies. Activity diagrams show that this would be expected if a hydrous fluid was circulating between different lithologies during deformation. In most cases fluid/rock ratios were relatively small resulting in gradual chemical changes and repeated recrystallization. ‘Open-system’behaviour with reduction in the number of phases is seen in some granite mylonites, suggesting focusing of fluid movement in parts of the shear zones. Continual fluid-rock interaction may have led to reaction-enhanced ductility in the shear zones over a long period of time. The source of fluid is uncertain, but may be related to underthrusting of material beneath the area investigated.

Notes: Abstract The hornblende-bearing basic gneisses in the Uvete area, central Kenya, were metamorphosed under a narrow range of P and T (6.5 ± 0.5kbar and 530 ± 40°C) of the staurolitekyanite zone in the Mozambique metamorphic belt. They show a wide variety of divariant and trivariant mineral assemblages consisting of hornblende, cumminatonite, gedrite, anthophyllite, chlorite, garnet, epidote, clinopyroxene, plagio-clase and quartz. The bulk and mineral chemistries and the graphical representation of phase relations show that each mineral assemblage approaches chemical equilibrium and defines a unique composition volume in the A′(Al + Fe3+− (13/7)Na)-F(Fe2+)-M′(Mg)-C′(Ca-(3/7)Na) tetrahedron. The composition volumes are distributed quite regularly and do not overlap each other.The phase relations in the Uvete area are in contrast with those in the staurolite-kyanite zone amphibolites in the Mt. Cube quadrangle, Vermont. The amphibolites there contain low-variance mineral assemblages formed under different values of μH2O and μCO2. These assemblages define overlapping composition volumes in the A′-F′-M′-C’tetrahedron.The mineral assemblages in the Uvete area are interpreted as having formed in equilibrium with fluid at a high and nearly constant μH2O value. Such a fluid composition was externally controlled by the supply of H2O-rich fluid expelled from the surrounding pelitic and psammitic rocks. The body size of the basic gneisses in the Uvete area (less than 400m in thickness) was small enough for the fluid to migrate completely.

Notes: Abstract Blueschist-facies rocks on the Seward Peninsula constitute a structurally coherent terrane measuring at least 100 × 150 km. Radiometric age data indicate that high-pressure metamorphism probably occurred in Jurassic rather than in Palaeozoic or Precambrian time, as previously suggested. Protolith sediments (Nome Group) are of intracontinental basin or continental margin type, and of lower Palaeozoic and possibly late Precambrian age, thus predating the high pressure metamorphism by more than 200 m.y.Blueschist-facies mineral assemblages were developed in almost all lithologies of the Nome Group, and are best preserved in FeTi-rich metabasites (glaucophane + almandine + epidote) and pelites (glaucophane + chloritoid + phengite). A lawsonite–crossite subfacies was developed in possible Nome Group rocks on the east flank of the Darby Mountains. Albite–epidote–amphibolite facies assemblages characterize Nome Group rocks in the southwestern part of the Peninsula. Metamorphism in the central zone of the terrane passed from early lawsonitic to subsequent epidote–almandine–glaucophane schist subfacies with the local development (east of the Nome River) of eclogitic assemblages.The high pressure metamorphic minerals were synkinematic with the development of mesoscopic-scale intrafolial isoclinal folds and a flattening foliation of consistent orientation. Initiation of uplift probably corresponded to the growth of barroisite rims on earlier sodic and actinolitic amphiboles, and partial post-kinematic greenschist facies replacements record later stages of decompression. Ophiolites and melange are not associated with the Seward Peninsula blueschists. The high-pressure metamorphism was caused by tectonic loading of a continental plate by an allochthon of indeterminate origin. The PT conditions of high pressure metamorphism were approximately 9–11 kbar, 400–450°C, thus falling between the PT paths of the Shuksan and Franciscan terranes.

Notes: Abstract Two periods of garnet growth (Gt1 and Gt2) have been found in the Finnmarkian nappes of north Norway. In the Kolvik Nappe (the lowest nappe) Gt1 has preserved an S2 syntectonic spiral inclusion fabric; in the Olderfjord Nappe an earlier S1 fabric and an interkinematic inter-D1–D2 fabric have been preserved in Gt1 whilst only the S1 fabric has been found in Gt1 in the Brennsvik Nappe (the highest nappe). In each nappe Gt2 overgrew a penetrative fabric (S2) wrapped around Gt1. In the Kolvik Nappe inclusion fabrics may be continuous from Gt1 into Gt2 but in the higher nappes there is a distinct break. Gt2 may have been partially syntectonic with D3 in the Brennsvik Nappe.Chemically Gt1 in the Kolvik Nappe and in parts of the Olderfjord and Brennsvik Nappes has antithetic Fe-Mn zoning. In all nappes XCa and XMg are weakly zoned in Gt1; XMg increases outwards and is greater in the higher nappes in Gt1 suggesting higher nucleation temperatures. In the Olderfjord and Brennsvik Nappes Gt2 is marked by increasing XCa, probably due to changing garnet-plagioclase equilibria, although the Fe/Mg ratio remains constant. XMg is higher in Gt2 than Gt1.Basement rocks within the nappe pile have an early pre-Finnmarkian growth (Gt1) and a later Finnmarkian growth (GtH) correlated with Gt2 on the basis of chemical zoning patterns.The diachroneity of Gt1 is ascribed to progressively earlier (compared to the structural development) cessation of overstepping of garnet-forming reactions before peak metamorphism in the higher nappes, resulting in earlier structural events being preserved.

Notes: Abstract Fluid inclusion studies of rocks from the late Archaean amphibolite-facies to granulite-facies transition zone of southern India provide support for the hypothesis that CO2,-rich H2O-poor fluids were a major factor in the origin of the high-grade terrain. Charnockites, closely associated leucogranites and quartzo-feldspathic veins contain vast numbers of large CO2-rich inclusions in planar arrays in quartz and feldspar, whereas amphibole-bearing gray gneisses of essentially the same compositions as adjacent charnockites in mixed-facies quarries contain no large fluid inclusions. Inclusions in the northernmost incipient charnockites, as at Kabbal, Karnataka, occasionally contain about 25 mol. % of immiscible H2O lining cavity walls, whereas inclusions from the charnockite massif terrane farther south do not have visibile H2OMicrothermometry of CO2 inclusions shows that miscible CH4 and N2 must be small, probably less than 10mol.%combined. Densities of CO2 increase steadily from north to south across the transitional terrane. Entrapment pressures calculated from the CO2 equation of state range from 5 kbar in the north to 7.5 kbar in the south at the mineralogically inferred average metamorphic temperature of 750°C, in quantitative agreement with mineralogic geobarometry. This agreement leads to the inference that the fluid inclusions were trapped at or near peak metamorphic conditions.Calculations on the stability of the charnockite assemblage biotite-orthopyroxene-K-feldspar-quartz show that an associated fluid phase must have less than 0.35 H2O activity at the inferred P and T conditions, which agrees with the petrographic observations. High TiO2 content of biotite stabilizes it to lower H2O activities, and the steady increase of biotite TiO2 southward in the area suggests progressive decrease of aH2O with increasing grade. Oxygen fugacities calculated from orthopyroxene-magnetite-quartz are considerably higher than the graphite CO2-O2 buffer, which explains the absence of graphite in the charnockites.The present study quantifies the nature of the vapours in the southern India granulite metamorphism. It remains to be determined whether CO2-flushing of the crust can, by itself, create large terranes of largeion lithophile-depleted granulites, or whether removal of H2O-bearing anatectic melts is essential.

Notes: Abstract There are discrete masses of un-deformed metabasite within the blueschist series of the island of Syros. Greece. Around the margins of these masses are zonal sequences through rocks showing intracrystalline deformation but without a geometric fabric, to rocks with discrete and anastomosing shear zones, and finally to penetratively foliated rocks with isolated relics of the original undeformed texture. Textural relics suggest that this spatial sequence is at least qualitatively also a temporal sequence.This progressive shear zone deformation took place concurrently with a glaucophane-epidote to eclogite reaction. The reaction pathways in the rocks that underwent the shear zone deformation can be compared with those in rocks of a similar composition that suffered a longer deformation history and show no relics of an undeformed parent. Although the final assemblages are in both cases the same, the pathways are different. These differences are in part related to reactions promoted by the change from local to bulk equilibrium on the onset of deformation in the rocks. They are also related to the crystallization and later breakdown during the sequence of progressive equilibration of a metastable phase, in this case an impure glaucophane.

Notes: Abstract The Rockley Volcanics from near Oberon, New South Wales occur within the aureole of the Carboniferous Bathurst Batholith and have been contact metamorphosed at P ∼ 100 ± 50MPa (10.5kbar) and a maximum T ∼ 565°C in the presence of a C–O–H fluid. Prior to contact metamorphism the volcanics were regionally metamorphosed and altered with the extensive development of actinolite, chlorite, plagioclase, quartz and calcite. The contact metamorphosed equivalents of these rocks have been subdivided into: Ca-poor (cordierite + gedrite), Mg-rich (amphibole + olivine + spinel), mafic (amphibole + plagioclase) and Ca-rich (amphibole + garnet + diopside; diopside + plagioclase; garnet + diopside + wollastonite) rocks.The chemistry of the minerals in the hornfelses was controlled by the bulk rock chemistry and fluid composition. Pargasites and hastingsites as well as an unusual phlogopite with blue green pleochroism, are found in Ca-rich hornfelses. A comparison of the assemblages with experimentally derived equilibria suggests that the fluid phase associated with the Ca-rich hornfelses was water-rich (Xco2= 0.1 to 0.3) while that associated with the Mg-rich hornfelses was enriched in CO2 (Xco2 〉 0.7). The different hornfels types have reacted to contact metamorphism independently in both their solid and fluid chemistries.

Notes: Abstract A garnet–hornblende Fe–Mg exchange geothermometer has been calibrated against the garnet–clinopyroxene geothermometer of Ellis & Green (1979) using data on coexisting garnet + hornblende + clinopyroxene in amphibolite and granulite facies metamorphic assemblages. Data for the Fe–Mg exchange reaction between garnet and hornblende have been fitted to the equation. In KD=Δ (XCa,g) where KD is the Fe–Mg distribution coefficient, using a robust regression approach, giving a thermometer of the form: with very satisfactory agreement between garnet–hornblende and garnet–clinopyroxene temperatures. The thermometer is applicable below about 850°C to rocks with Mn-poor garnet and common hornblende of widely varying chemistry metamorphosed at low aO2.Application of the garnet–hornblende geothermometer to Dalradian garnet amphibolites gives temperatures in good agreement with those predicted by pelite petrogenetic grids, ranging from 520°C for the lower garnet zone to 565–610°C for the staurolite to kyanite zones. These results suggest that systematic errors introduced by closure temperature problems in the application of the garnet–clinopyroxene geothermometer to the ‘calibration’data set are not serious. Application to ‘eclogitic’garnet amphibolites suggests that garnet and hornblende seldom attain Fe–Mg exchange equilibrium in these rocks.Quartzo-feldspathic and mafic schists of the Pelona Schist on Sierra Pelona, Southern California, were metamorphosed under high pressure greenschist, epidote–amphibolite and (oligoclase) amphibolite facies beneath the Vincent Thrust at pressures deduced to be 10±1 kbar using the phengite geobarometer, and 8–9kbar using the jadeite content of clinopyroxene in equilibrium with oligoclase and quartz. Application of the garnet–hornblende thermometer gives temperatures ranging from about 480°C at the garnet isograd through 570°C at the oligoclase isograd to a maximum of 620–650°C near the thrust. Inverted thermal gradients beneath the Vincent Thrust were in the range 170 to 250°C per km close to the thrust.

Notes: Abstract Rock fracture enhances permeability and provides pathways through which fluids migrate. During contact metamorphism, fluids contained in isolated pores and fractures expand in response to temperature increases caused by the dissipation of heat from magmas. Heat transport calculations and thermomechanical properties of water-rich fluids demonstrate (1) that thermal energy is a viable mechanism to produce and maintain pore fluid pressure (Pf) in a contact metamorphic aureole; (2) that the magnitude of Pf generated is sufficient to propagate fractures during the prograde thermal history (cause hydrofracture) and enhance permeability; and (3) that Pf-driven fracture propagation is episodic with time-scales ranging from years to thousands of years. Because Pf dissipation is orders of magnitude faster than P, f buildup, Pf oscillations and cyclical behaviour are generated as thermal heating continues. The Pf cycle amplitude depends on the initial fracture length, geometry and the rock's resistance to failure whereas the frequency of fracture depends on the rate of heating. Consequently, oscillation frequency also varies spatially with distance from the heat source.Time series of fluid pressures caused by this process suggest that cyclical fracture events are restricted to an early time period of the prograde thermal event near the intrusive contact. In the far field, however, individual fracture events have a lower frequency but continue to occur over a longer time interval. Numerous fracture cycles are possible within a single thermal event. This provides a provisional explanation for multiple generations of veins observed in outcrop. P f cycling and oscillations may explain several petrological features. If pore fluids are trapped at various positions along a pressure cycle, the large amplitude of Pf variations for small fractures may account for different pressures recorded by fluid inclusions analysed from a single sample. Pf oscillations, during a single thermal episode, also drive chemical reactions which can produce complex mineral textures and assemblages for discontinuous reactions and/or zoning patterns for continuous reactions. These can mimic polymetamorphic or disequilibrium features.Temporal aspects of fracture propagation and permeability enhancement also constrain the likely timing of fluid flow and fluid-mineral interactions. These data suggest that fluid flow and fluid-mineral reactions are likely to be restricted to an early period in the prograde thermal history, characterized by high Pf coincident with relatively high temperatures, fracture propagation and consequent increases in permeability. This early prograde hydration event is followed by diffusional peak metamorphic reactions. This relationship is evident in the complex mineralogical textures common in some metamorphosed rocks.

Notes: Abstract Andalusite-bearing veins formed during contact metamorphism in the aureole of the Vedrette di Ries tonalite. In the veins, quartz crystals that are completely armoured by andalusite or that occur in strain shadow areas contain three generations of fluid inclusions: low-salinity H2O-CO2-CH4 mixtures with CH4/(CO2+ CH4) ± 0.35 (type A); low-salinity aqueous fluids (type B); H2O-free, CO2-CH4 fluids with the same carbonic speciation as A (type C). Carbonic types A and C typically have a dark appearance, which is attributed to graphite coatings on inclusion walls. Microstructural analysis of the host quartz and calculated densities indicate that type A inclusions were likely trapped during vein formation. These inclusions underwent strain-assisted re-equilibration during cooling that resulted in density increases without change of composition. After the rocks had cooled below about 350 ° C, type C inclusions appear to have formed from one of the immiscible fractions after unmixing of the H2O-CO2-CH4 fluid mixtures. Aqueous type B inclusions, apparently trapped between 225 and 350 ° C, could represent an independent fluid, or could be the H2O-rich fraction of unmixed type A fluids. Taking account of the uncertainties, the composition and density of the complex type A inclusion fluids are in good agreement with the properties of primary fluids calculated from the petrological data. The fluid inclusion data support the model of vein formation by hydrofracturing as a result of dehydration of graphitic metapelites. These new results also demonstrate the importance of considering strain in the interpretation of metamorphic fluid inclusions.

Notes: Abstract Aqueous fluid released in metamorphism is transported upwards from depth to the Earth's surface. I propose a hydrofracturing model for the fluid transport. In the model, fluid is transported by the upward propagation of a two-dimensional vertical fluid-filled crack from a fluid reservoir (e.g. overpressured compartment under a seal) at depth to the Earth's surface; fluid is injected consecutively from the reservoir into the crack at a given (but not necessarily constant) injection rate; some of the injected fluid is lost by infiltration from the crack walls into the surrounding permeable rock. An approximate solution of the crack propagation is obtained using fluid dynamics for turbulent film flow and linear elastic fracture mechanics. The solution shows the transition from a regime in which the excess pressure of the fluid in the reservoir drives the propagation to a regime in which the buoyancy of the fluid in the crack drives the propagation. For example, if the net injection rate of H2O is 1 m2/s, the regime transition occurs when the vertical crack length becomes 280 m; after the transition, the propagation velocity and average aperture are constant: 21 m/s and 4.8 cm. If the injection rate is lower than a critical value, hydrofracturing cannot be an effective mode for the fluid transport because of the significant fluid loss by infiltration from the crack walls into the surrounding permeable rock. Assuming a fluid-saturated crust with hydrostatic pore fluid pressure, a lower limit can be estimated for the injection rate required to transport H2O by hydrofracturing without significant fluid loss. For example, the lower limit for transport from a depth of 15 km to the Earth's surface is estimated at 0.2 m2/s if the crustal permeability is 10-17 m2. The lower limit decreases with decreasing crustal permeability.

Notes: Abstract Microstructural and chemical analysis of plagioclase in 20 superficially similar amphibolite facies ductile shear zones in metagabbors and amphibolites of the Ivrea Zone in Italy reveals significant differences in An and Ba contents. Plagioclase, which was deformed at P-T conditions lower than those of the wall rocks, occurs in the following four different microstructural situations with different chemical compositions: (i) relatively undeformed porphyroclasts, (ii) dynamically recrystallized grains and subgrains rimming the porphyroclasts, (iii) infill of microcracks cross-cutting the porphyroclasts and (iv) fine-grained recrystallized grains in the matrix of the shear zones. The differences in the An and Ba contents are caused by partial chemical equilibration of plagioclase in the shear zones during and partly after deformation. Changes in An and Ba contents were caused by fluid-assisted grain-boundary migration recrystallization, as well as by solid-state diffusion, while fluid activity was high. The relation between the composition and microstructures of the plagioclase in the shear zones indicates that in the different shear zones, fluids ceased to be active during different stages in the late shear zone deformation history.The interpretation of the variations in composition and microstructures reveals that only grains that developed by grain-boundary migration recrystallization and that are not adjacent to porphyroclasts reflect P-T conditions during the dominant shear-zone deformation.

Notes: Abstract TEM and XRD techniques were used to study crystal growth characteristics of the fabric-forming phyllosilicates which developed in response to low-grade metamorphism and tectonic imbrication in part of the Southern Uplands thrust terrane. Prograde regional metamorphism, ranging from late diagenesis through the anchizone to the epizone, was accompanied by the development of a slaty cleavage which is commonly bedding-parallel. TEM-measured mean thicknesses of white mica and chlorite crystallite populations increase with advancing grade and correlate with XRD-measured crystallinity indices. Analytical TEM data show that prograde changes in composition lead to a net loss of Si, Ca and minor Fe from the fabric-forming phyllosilicates. White micas are paragonite-poor phengites with a mean b lattice parameter of 9.037 Å, and indicate an intermediate pressure series of metamorphism with a field gradient of 〈25° C km-1. Chlorite compositions evolved from diabantite (with intergrown corrensite) to ripidolite over an estimated temperature range of 150–320° C. Field gradient and temperature estimates suggest that crystal growth and fabric development occurred at burial depths ranging from 6 km to at least 13 km in the thrust terrane. During late diagenesis, crystal growth of white mica and chlorite was predominantly a consequence of polytypic and phase transitions, and resulted in similar size distributions which resemble typical Ostwald ripening curves. Under anchizonal and epizonal conditions, white mica grew more rapidly than chlorite because of its greater ability to store strain energy and recover from subgrain development; as a result crystal thickness distributions are not typical of Ostwald ripening. In contrast, chlorite crystals which grew under these conditions developed subgrain boundaries at high strain rates which were only partially recovered at low strain rates; these retained dislocations reduce the crystallite thicknesses detected by TEM and XRD, compared with those of white mica. These differences in strain-induced crystal growth indicate that white mica (illite) and chlorite crystallinity indices are likely to show significant differences where low-grade metamorphism is closely associated with tectonic fabric development.

Notes: Abstract The metamorphic history of the Archaean Superior Province crystalline basement in the Palaeoproterozoic Ungava Orogen attests to the importance of structural and geohydrological controls on a retrograde amphibolite-granulite transition. Two distinct metamorphic suites, separated in age by nearly one billion years, are recognized in extensively exposed tonalitic to dioritic metaplutonic gneisses. The older suite comprises c. 2.7-Ga granulite facies assemblages (orthopyroxene-clinopyroxene-hornblende-plagioclase-ilmenite ± biotite ± quartz) that record moderate pressures (±5 kbar) and high temperatures (±800° C). A younger, c. 1.8-Ga suite resulted from amphibolitization of the granulites and is characterized by regionally extensive amphibolite facies mineral zones that broadly parallel the basal décollement of the overlying Proterozoic Cape Smith Thrust Belt. Deformation/mineral growth relationships in the amphibolitized basement indicate that extensive hydration and re-equilibration of the Archaean granulites occurred during thrust belt deformation. The transition from granulite facies to amphibolite facies assemblages is characterized by the growth of garnet-hornblende-quartz ° Cummingtonite coronas between plagioclase and orthopyroxene-clinopyroxene, as well as titanite coronas on ilmenite. Multi-equilibrium thermobarometry on the coronitic assemblages documents re-equilibration of the granulitic gneiss to 7.7 kbar at 644° C in the south and 9.8 kbar at 700° C in the north. The variably deformed, amphibolite facies domain sandwiched between the coronitic garnet zone and the basal décollement is marked by significant metasomatic changes in major element concentrations within tonalite. These changes are compatible with equilibrium flow of an aqueous-chloride fluid down a temperature gradient. The source of fluids for basement hydration/metasomatism is interpreted to be dehydrating clastic rocks in the overlying thrust belt, with fluid flow probably focused along the basal décollement.

Notes: Coexisting Ca-poor and Ca-rich pyroxenes in granulites at Cape Riche, in the Precambrian Albany-Fraser Province, Western Australia, are dominantly chemically homogeneous within individual samples, suggesting a major episode of equilibration. However, occasional grains in a few samples contain exsolved domains interpreted as relics of an earlier, higher-T assemblage. Pyroxene pairs in ten, presumably isothermal, samples from a restricted area are used to (i) assess the suitability of several versions of the two-pyroxene thermometer for application to metamorphic rocks, and (ii) determine the thermal history of the Cape Riche pyroxenes.The various versions of the two-pyroxene thermometer applied to the well-equilibrated homogeneous pyroxene grains show poor to good precision and yield mean temperatures varying widely from 683° to 893°C, in the following order of increasing T: Lindsley (1983; opx version), 683°± 11°C; Kretz (1982; KD version), 705°± 19°C; Ross & Huebner (1975), 709°± 30°C; Kretz (1982; solvus version), 735°± 24°C; Fonarev & Graphchikov (1982; opx version), 〈750°C; Lindsley (1983; cpx version), 784°± 40°C; Fonarev & Graphchikov (1982; cpx version), ~820°± 30°C; Wood & Banno (1973), 849°± 16°C; Powell (1978), 854°± 23°C; Wells (1977), 893°± 10°C. Independent T estimates, based on mafic assemblages and garnet-biotite thermometry, suggest that the major episode of metamorphism occurred at 700-800°C (P ~ 5 kbar). Therefore the Wells, Powell, Wood & Banno and Fonarev & Graphchikov (cpx) temperatures are almost certainly too high. In the absence of a more precise independent T estimate it is difficult to assess the relative merits of the results obtained from the remaining versions of the two-pyroxene thermometer, none of which can be unequivocally demonstrated to be seriously in error, though the Lindsley (opx) T is probably too low. Other significant shortcomings evident in the results include the relatively poor precision obtained from the three methods based on purely graphical representation of the augite limb of the solvus (i.e., the Ross & Huebner, Fonarev & Graphchikov (cpx) and Lindsley (cpx) versions), and the apparent dependence of derived T on Mg/Fe2+ ratio for the Powell, Wood & Banno and Lindsley (cpx) methods.For the bulk compositions of exsolved domains, the different versions of the two-pyroxene thermometer yield mean temperatures 23° to 82°C (overall mean, 65°C) higher than for homogeneous grains in the same samples. These exsolved domains are interpreted as relics of a higher-T (peak?) metamorphic assemblage, rather than an igneous precursor.

Notes: Abstract We have deduced the steady-state lithospheric geotherm at c. 1 Ga in the south Indian shield area using the available data on the concentration of radioactive elements, and the P-T conditions of Proterozoic mantle xenoliths in the south Indian kimberlites as constraints. The geotherm was adjusted back to 2.5 Ga by keeping the surface temperature constant and calculating the temperature change at the top of convecting upper mantle. The reduced or mantle heat flux, which was treated as an adjustable parameter, was 20.9–21.3 mW/m2 at 1–2.5 Ga. Comparison of the calculated steady-state geotherm with the available P-T data of the Archaean (c. 2.5 Ga) charnockites and granulites from southern India suggests that the granulite facies metamorphism in this region had resulted from a major thermal perturbation, which was c. 400° C at 25 km.Seismic tomographic and gravity data essentially preclude any significant magma underplating of the granulitic crust in southern India. Previous workers have suggested that the formation of charnockites in this region was associated with copious CO2 influx from a deep-seated source, possibly the mantle. In this work, we have evaluated both the transient and steady-state thermal effects of the heat convected by CO2 outgassing from upper mantle. It is shown that the thermobarometric array of charnockites and granulites can be produced by the convective perturbation of the steady-state geotherm, and that a flux of CO2 of ±90 mol/m2 yr (corresponding to Darcy velocity of ±0.30 cm/yr) for a period of ±30 Ma was needed to produce the required perturbation. This is c. 150 times the average CO2 flux through the tectonically active area of the Earth's crust at the present time. There is, however, an uncertainty of a factor of 3 in this value.Seismic tomographic and gravity data independently suggest thickening of the crust beneath the granulite terrane compared with the adjacent Dharwar craton. This suggests thermal perturbation due to overthrusting as a major potential cause for the granulite facies metamorphism in south India. Overthrusting of a 30–35-km-thick thrust block was needed to produce the required thermal effect. The estimated thickness of the original crust from geobarometric and seismic tomographic data south of the orthopyroxene isograd or ‘transition zone’is compatible with the emplacement of a thrust block of this magnitude. However, the latter fails to match the estimated pre-uplift crustal thickness at the transition zone, if it is assumed that the crust has not thinned by non-erosional processes since the Archaean. Thus, we propose a combination of overthrusting and CO2 fluxing from a deep-seated source as the cause for the formation of charnockites in this zone. The required focusing of CO2 in this case is c. 40% of that estimated in the model where CO2 fluxing was considered to be the sole reason for thermal perturbation. This combined thrusting—CO2 fluxing model also helps explain the development of patchy charnockites in the transition zone from amphibolite facies rocks.

Notes: Abstract End-member, continuous and degenerate reactions are derived for the multisystem with the six components Na2O, CaO, (Mg/Fe)O, Al2O3, SiO2, H2O among the phases plagioclasess, garnetss, amphiboless, cpx, opx, olivine, spinel, quartz and an aqueous fluid. The chemography of this system is degenerate due to the co-linearity 2Opx = Ol + Qtz. This co-linearity has its implications both on reaction space and phase equilibria.From a total of 28 reaction systems, reaction space is derived for nine subsystems (phases in parentheses are absent): Case A1: (Cpx,Ol) (Cpx,Opx) and (Cpx,Qtz), Case A2: (Spl,Ol) (Spl,Opx) and (Spl,Qtz), Case B: (Ol,Opx) (Ol,Qtz) and (Opx,Qtz).In the absence of either cpx or spl (case A), three reactions form an invariant point, either [Cpx] or [Spl], where the co-linear phases olivine, opx and quartz coexist on the transformation line 2Opx = Ol + Qtz. Changing mineral compositions force invariant points to move along the line with the different reaction curves changing their relative position according to Schreinemakers’rules. Zero contours, i.e. the location where (a) phase(s) disappear(s) in reaction space correspond to singular points in phase diagrams. Two types are distinguished; singular points of indispensable and of substitutable phases. In the first case the phase disappears from the entire bundle while in the second it disappears from a single reaction. In the specific case where the substitutable phases are also the co-linear ones, two of the three co-linear phases disappear simultaneously. Two of the three reaction curves coincide.In the system including Cpx and Spl (Case B) three reactions, (Ol,Opx) (Ol,Qtz) and (Opx,Qtz), oppose three invariant points, [Ol], [Opx] and [Qtz]. Invariant points no longer move along the line 2Opx = Ol + Qtz. The coincidence of the zero contours of all three co-linear phases in reaction space-the result of the chemographic degeneracy-causes the respective singular points to coincide in the phase diagrams. This is the location where curves must be rearranged in a bundle to conform Schreinemakers’rules.The reaction Grs1Prp2= 2 Ol + An is fourth order degenerate and part of all nine subsystems (cases A and B). It can be used to relate the different phase diagrams to one another.

Notes: Abstract We report structural and metamorphic data from a c. 25-km transect across the eastern Grenville Front Tectonic Zone (GFTZ) to the Britt domain at the northern end of Georgian Bay near Key Harbour, Ontario. Constrasting Grenvillian structural and fabric elements characterize the eastern GFTZ, northern Britt domain and a narrow Transition Zone between them.Moderately to steeply dipping foliations with strong down-dip lineations in all three divisions appear to be associated with NW-directed thrusting. In the Transition Zone and northern Britt domain, early S = L fabrics with steep lineations are overprinted by younger structures (S 〉 L) with shallow, SE-SSE-plunging lineations in which sparse, dominantly (but not exclusively) normal-sense kinematic indicators are recognized. Pressure and temperature estimates from Grenvillian metamorphic assemblages in metadi-abase indicate that conditions of P ± 12 kbar and T c 800° C were achieved before or during the thrust-related deformation, with P-T-t paths that indicate near-isothermal decompression to P c. 4 kbar and T c. 700° C. Correlation of fabric elements with points on the P-T-t paths suggests that exhumation occurred during two stages, the first associated with thrusting (≥1035 Ma) and the second with extension and thrusting (pre-1003 Ma).The GFTZ contains steeply to moderately dipping, thrust-related fabrics and lacks shallow, extensional structures; the latest episode of thrusting in the GFTZ is inferred to have taken place at 990-980 Ma. The data are interpreted in terms of a tectonic model involving two stages of propagation of the Grenville orogen towards its foreland (≥1035 Ma and ≥980 Ma), with an intervening period of extension, although the tectonic regime probably remained compressional on the scale of the orogen.

Notes: Abstract 40Ar/39Ar data (on hornblende, muscovite and K-feldspar) are presented for samples from the western Grenville Province taken along a 140-km traverse from the Grenville Front into the Britt domain. Our interpretation is based on 28 new analyses, synthesized with 20 previously reported from the traverse area.In regions where comparisons are possible, muscovite and (large domain) K-feldspar apparent ages appear similar (at c. 920–930 Ma), but throughout the traverse, these are c. 60–70 Myr younger than the hornblende ages. The inferred cooling rate over the c. 350–500°C temperature range, c.2°C Myr-1, is appropriate for exhumation controlled by post-orogenic erosional unroofing. At the Grenville Front Tectonic Zone (GFTZ) — Britt domain boundary there is a c. 25-Myr offset in both hornblende and muscovite/K-feldspar ages. We interpret the lower ages in the Britt domain to reflect variations in crustal thickness and geothermal gradient between the flank and interior of a thick orogen.The argon data from the GFTZ are interpreted in the context of an asymmetric crustal-scale antiformal structure developed during a late episode of convergence. Hornblende from rocks on either side of the core of the antiform has an apparent age of c. 990 Ma, our estimate of the age of the compressional event. In the west, we infer that these date the short-lived thermal event associated with the development of the crustal-scale antiform previously postulated. In the east, the ages reflect the cooling of material brought toward the surface in the flank of the antiform. Hornblendes from the antiform core appear to contain excess radiogenic argon. We suggest that this was the ambient argon in rocks transported from depth that was subsequently trapped when the rocks cooled rapidly.

Notes: Abstract Three generations of mylonites discovered in the northern Prince Charles Mountains (nPCM) are associated with episodes of crustal thickening and thinning. First-generation mylonites (MY1) are shallow thrusts which pre-date both folding and peak metamorphic conditions, and formed during early crustal thickening. Second-generation mylonites (MY2) are significant subvertical normal faults that formed at conditions of c. 5 kbar and 700° C, and throughout the nPCM consistently display NW-block uplift. It is argued that MY2 uplift was rapid in the north-west, produced exhumation of approximately 6–7 km, and caused re-equilibration of most nPCM assemblages at lower pressures. It is suggested that features of this terrane may be reconciled with a tectonic model involving simultaneous crustal thickening and lithospheric thinning; MY2 uplift may reflect isostatically induced uplift. In contrast, the adjacent east Lambert Glacier Region (eLGR) was unaffected by MY2 uplift and remained at lower crustal levels. P-T trajectories across this oblique terrane thus reflect a gradual transition in uplift rates: nPCM paths preserve mostly cooling after partial MY2 exhumation, while those in the eLGR are dominated by slower uplift which facilitated the retrograde growth of coronas and symplectites at amphibolite facies conditions. Amphibolite facies third-generation mylonites, MY3, post-date the preserved P-T segments and are low-angle normal faults which indicate consistent easterly transport across the entire terrane. It is proposed that they are related to tectonic collapse.

Notes: Abstract Five whole-rock 40Ar/39Ar plateau ages from low-grade sectors of the Sambagawa belt (Besshi nappe complex) range between 87 and 97 Ma. Two whole-rock phyllite samples from the Mikabu greenstone belt record well-defined 40Ar/39Ar plateau ages of 96 and 98 Ma. Together these ages suggest that a high-pressure metamorphism occurred in both the Sambagawa and Mikabu belts at c. 90–100 Ma. The northern Chichibu sub-belt may consist of several distinct geochronological units because metamorphic ages increase systematically from north (c. 110 Ma) to south (c. 215 Ma). The northern Chichibu sub-belt is correlated with the Kuma nappe complex (Sambagawa belt). Two whole-rock phyllite samples from the Kurosegawa terrane display markedly older metamorphic ages than either the Sambagawa or the Chichibu belts.Accretion of Sambagawa-Chichibu protoliths began prior to the middle Jurrasic. Depositional ages decrease from middle Jurassic (Kuma-Chichibu nappe complex) to c. 100 Ma (Oboke nappe complex) toward lower tectonostratigraphic units. The ages of metamorphic culmination also decrease from upper to lower tectonostratigraphic units. The Kurosegawa belt and the geological units to the south belong to distinctly different terrances than the Sambagawa-Chichibu belts. These have been juxtaposed as a result of transcurrent faulting during the Cretaceous.

Notes: Abstract 40Ar/39Ar data collected from hornblende, muscovite, biotite and K-feldspar constrain the P-T-t history of the Cordillera Darwin metamorphic complex, Tierra del Fuego, Chile. These data show two periods of rapid cooling, the first between c. 500 and c. 325° C at rates ≥25° C Ma-1, and the second between c. 250 and c. 200°C. For high-T cooling, 40Ar/39Ar ages are spatially disparate and depend on metamorphic grade: rocks that record deeper and hotter peak metamorphic conditions have younger 40Ar/39Ar ages. Sillimanite- and kyanite-grade rocks in the south-central part of the complex cooled latest: 40Ar/39Ar Hbl = 73–77 Ma, Ms = 67–70 Ma, Bt = 68 Ma, and oldest Kfs = 65 Ma. Thermobarometry and P-T path studies of these rocks indicate that maximum burial of 26–30 km at 575–625° C may have been followed by as much as 10 km of exhumation with heating of 25–50° C. Staurolite-grade rocks have intermediate 40Ar/39Ar ages: Hbl = 84–86 Ma, Ms = 71 Ma, Bt = 72–75 Ma, and oldest Kfs = 80 Ma. Thermobarometry on these rocks indicates maximum burial of 19–26 km at temperatures of 550–580° C. Garnet-grade rocks have the oldest ages: Ms = 72 Ma and oldest Kfs = 91 Ma; peak P-T conditions were 525–550° C and 5–7 kbar. Regional metamorphic temperatures for greenschist facies rocks south of the Beagle Channel did not exceed c. 300–325° C from 110 Ma to the present, although the rocks are only 2 km from kyanite-bearing rocks to the north.One-dimensional thermal models allow limits to be placed on exhumation rates. Assuming a stable geothermal gradient of 20–25° C km-1, the maximum exhumation rate for the St-grade rocks is c. 2.5 mm yr-1, whereas the minimum exhumation rate for the Ky + Sil-grade rocks is c. 1.0 mm yr-1. Uniform exhumation rates cannot explain the disparity in cooling histories for rocks at different grades, and so early differential exhumation is inferred to have occurred. Petrological and geochronological comparisons with other metamorphic complexes suggest that single exhumation events typically remove less than c. 20 km of overburden. This behaviour can be explained in terms of a continental deformation model in which brittle extensional faults in the upper crust are rooted to shallowly dipping ductile shear zones or regions of homogeneous thinning at mid- to deep-crustal levels. The P-T-t data from Cordillera Darwin (1) are best explained by a ‘wedge extrusion’model, in which extensional exhumation in the southern rear of the complex was coeval with thrusting in the north along the margin of the complex and into the Magallanes sedimentary basin, (2) suggest that differential exhumation occurred initially, with St-grade rocks exhuming faster than Ky + Sil-grade rocks, and (3) show variations in cooling rate through time that correlate both with local deformation events and with changes in plate motions and interactions.

Notes: Abstract The Rand and Pelona Schists consist of eugeoclinal rock types overlain by continental basement along the Vincent-Chocolate Mountains (VCM) faults. Both schists display inverted metamorphic zonation, defined in part by a systematic variation in composition of calcic to sodic-calcic amphibole in mafic schist structurally upward. The compositional progressions include increase of total A1, A1IV and Ti, but decrease in the ratios of Na/(Na + Ca) to A1/(A1 + Si), and NaM4 to (A1VI+ Fe3++ Ti). These variations imply that structurally high rocks belong to a lower-pressure metamorphic fades series than those at depth. This result is consistent with previous views that the inverted metamorphic zonations represent intact structural sequences.Amphibole composition is dependent not only on structural position (i.e. P-T), but also upon bulk-rock composition. The important controls are whole-rock Mg/(Mg + Fe2++ Mn) and Fe3+/Fe2+. The greatest impact of these factors, however, is on the absolute values of Na and Al, rather than their ratio. Thus, interpretation of facies series is not seriously hindered by compositional variability.Sodic amphibole in epidote blueschists from the Rand Schist is extensively replaced by sodic-calcic amphibole. Sodic-calcic amphibole in the Rand Schist and Pelona Schist is, itself, rimmed by actinolitic amphibole. Similar blueschist to greenschist transitions in other metamorphic terranes are typically attributed to exhumation. In the Rand and Pelona Schists, the sequence probably formed during burial.

Notes: Abstract 40Ar/39Ar step-heating and single-grain laser fusion ages from phengites from the polydeformed and polymetamorphosed blueschist-greenschist facies Nome Group fall into two groups. Samples from the upper part of the structural section that have experienced a relatively weak metamorphic and deformational post-blueschist facies overprint and one sample from the Cape Nome orthogneiss yield plateau ages of 116-125 Ma. More intensely overprinted samples yield hump-shaped spectra with minimum ages of 123 Ma and maximum ages of 334 Ma. Samples with hump-shaped spectra are derived from a greater structural depth than most samples with plateau ages. Unreasonably old maximum ages from some of the disturbed spectra suggest that the hump-shaped spectra result from the incorporation of excess 40Ar. This interpretation conflicts with previous interpretations of similarly disturbed spectra from the Brooks Range, which have been argued to provide minimum ages for blueschist facies metamorphism. Since the maximum temperatures achieved by all samples were probably above the blocking temperature of Ar in phengite, the 116-125 Ma plateau ages are a minimum age for blueschist facies metamorphism on the Seward Peninsula, Alaska.

Notes: Abstract Granitic orthogneiss is widespread throughout the metamorphic core of the Brooks Range in both the ductilely deformed blueschist/greenschist facies Schist Belt and the lower grade Central Belt (= Skajit allochthon) to the north. Orthogneiss occurs as large metaplutonic massifs and in small bodies enclosed within metasedimentary rocks. Crystallization ages for the granitic protoliths range from Proterozoic through Devonian (U-Pb zircon); the K-Ar system was reset during Cretaceous metamorphism. Mineral assemblages of the orthogneisses reflect nearly complete re-equilibration during Jurassic-Cretaceous collisional orogenesis in northern Alaska. The most common metamorphic paragenesis in orthogneiss is: Qtz + Kfs + Ab + Phe + Bt ± Ep, Ttn, Rt, Ap, Chl, Cal.Constituent minerals from 16 Brooks Range orthogneiss samples were analysed with the electron microprobe. Phengite from the Schist Belt samples is highly enriched in Al-celadonite, with Si values up to 3.50 per formula unit (on an 11-oxygen basis). Central Belt samples contain phengite with lower Si content (±3.38 p.f.u.). In nearly all samples, Si content of phengite varies considerably, reflecting partial re-equilibration to lower pressure and/or higher temperature conditions. Metamorphic conditions were estimated using the Phe-Bt-Kfs-Qtz barometer and the two-feldspar solvus thermometer. The results indicate that the Schist belt underwent high-pressure/low-temperature metamorphism (generally 9-12 kbar at 375-430° C), consistent with the widespread development of glaucophane + epidote/clinozoisite and lawsonite pseudomorphs in other rock types. The Central Belt also experienced a relatively high P-T metamorphism, with most samples yielding pressure estimates in the range 5-8 kbar (at 325-415° C). These results confirm the existence of two metamorphic belts in the core of the Brooks Range that differ in metamorphic conditions by up to 5 kbar. The range in Si content in phengite from Schist Belt samples is consistent with isothermal decompression of up to 5 kbar.

Notes: This work presents the results of a fluid inclusion study of an amphibolite-granulite facies transition in West Uusimaa, S.W. Finland. Early fluid-inclusions in the granulite facies area are characteristically carbonic (CO2), in contrast to predominantly aqueous early inclusions in the amphibolite facies area. These early inclusions can be related to peak metamorphic conditions (750-820°C and 3-5 kbar for peak granulite facies metamorphism). Relatively young CO2 inclusions with low densities (〈0.8g/cm3) indicate that the first part of the cooling history of the rocks was characterized by a near isothermal uplift.N2-CH4 inclusions, with compositions ranging between pure CH4 and pure N2 (Raman spectral analysis), were found in the whole area. They are probably syn- or even pre-early inclusions. Only nearly critical homogenizing inclusions have been found (low density). Pressure estimates, based on densities of early fluid inclusions, show that the rapid transition of amphibolite towards granulite facies metamorphism is virtually isobaric. Granulite facies metamorphism in West Uusimaa is a thermal event, probably induced by the influx of hot, CO2-bearing fluids.

Notes: Multisystems of n+k (k 〉 3) phases are very complicated and knowledge of them has suffered as a result. The successful solution of the topological relationships in n+ 3 phase multisystems by Zen (1966, 1967) and Zen & Roseboom (1972) has aroused much interest regarding what will happen in a multisystem of more than n+ 3 phases. Since 1979, some important research results on this topic have been published. These results have expounded the substantial rules governing the appearance of phase relations in phase diagrams of n - k (k 〉 3) phase multisystems. The most significant conclusions include: (1) It is impossible to incorporate all the possible phase relations in an n+k (k 〉 3) phase multisystem in a single closed net. Therefore, it is no longer enough to use only a single closed net to depict the topological relations involved in these types of multisystems. Instead, one or more groups of closed nets, namely the complete system(s) of closed nets are necessary for this purpose. (2) A principle called the Combination Principle has been proposed and proved. It states: Any closed net of one n+k (k 〉 3) phase multisystem must be a combination of two or more distinct n+ 3 order submultisystem closed nets belonging to the given n+k phase multisystem, if it is not one of the n+ 3 order submultisystem closed nets itself. The combination principle provides both a theoretical basis and a practical method for the construction of closed nets and, hence, for the derivation of the real phase diagrams for any n+k (k 〉 3) phase multisystem. (3) A theorem on divariant-assemblage-characteristic-stability-polygons is also important to our understanding of the n+k (k± 3) phase multisystem closed nets. This theorem can be stated as follows: A divariant assemblage of an n+k (k± 3) phase multisystem will be stable in an l-polygon lacking diagonals in an appropriate set of closed-net-diagrams, and this l-polygon may be at least a triangle, and at most a k-polygon. In addition, the closed-net-diagrams of unary and binary n+ 4 phase multisystems derived respectively by Guo (1980b, 1980c, 1981a) and by Roseboom & Zen (1982) have also been summarized. The combination principle is applied to a practical petrological problem in this paper, dealing with 7 phases in the system FeO-Fe2O3-SiO2.

Notes: Abstract The stability of quartz-chloritoid-staurolite-almandine-cordierite and aluminium silicates is used to constrain both metamorphic conditions and pressure-temperature trajectories for two localities within the 2700 Ma Archaean Yilgarn Block in Western Australia. Available experimental data are used to calculate thermodynamic data for a self-consistent set of equilibria between these minerals. A lower amphibolite facies locality from the margin of a lower strain area contains assemblages including quartz-chloritoid-staurolite-garnet-biotite with altered cordierite replacing chloritoid, quartz-staurolite-andalusite, and quartz-cordierite-andalusite-biotite. This locality was heated to 530–560°C in the andalusite field, at 4.2 kbar. A sample from a mid- to upper-amphibolite facies, highly strained locality contains relict staurolite enclosed by andalusite, in turn replaced by cordierite and muscovite with biotite and sillimanite in the matrix. The assemblage was heated isobarically from conditions near the maximum experienced by the lower grade locality of 560°C at 4.2 kbar to temperatures in excess of the andalusite-sillimanite transition but within the quartz plus muscovite stability field (600–650°C). The higher grade locality is close to a granitoid dome and sections based on gravity profiles reveal that this locality is underlain by granitoid at shallow depths. The higher grade metamorphism apparently reflects superposition of the thermal aureole on regional metamorphic conditions similar to those in the lower grade areas.

Notes: Abstract A major system of steep Caledonian shear zones, of regional extent, has been identified in NE Scotland. The shear zones affect a wide range of lithologies, including Argyll and Southern Highland Group Dalradian, ‘Younger Basic’intrusives and their hornfelses, and also the earlier of the more acid intrusions. The observed fabrics and parageneses are consistent with low-pressure amphibolite facies metamorphism. These shear zones represent a phase of movement which occurred in the 490-465 Ma interval when ambient temperatures were still high, and it is concluded that this is the principal control on the metamorphic grade achieved within the shear zones, although local anomalies may exist.

Notes: Abstract An experimental study of the system CaCO3–MgCO3–FeCO3 was undertaken in order to calibrate the iron correction to the calcite–dolomite geothermometer, which is based on the solubility of magnesium in calcite in the assemblage calcite + dolomite. The experiments, at 450°C and lower temperatures, resulted in products with a very small grain size and incomplete equilibration. However, application of a carefully-devised automatic data processing algorithm to analyses of the phases in experimental charges, combined with a thermodynamic analysis, results in geothermometer diagrams which should be preferred to previous theoretical predictions.

Notes: Abstract Biotite, garnet, staurolite and kyanite isograds in pelitic metasedimentary rocks are developed as a result of thermal metamorphism around syntectonic granitoids in Eastern Rouergue (France). Temperature estimates range between 400°C and 650°C at about 6.5 kbar. Geothermobarometry shows a steep isobaric T gradient which is consistent with the interpretation that the metamorphic highs are thermal aureoles. High grade rocks show evidence of two staurolite forming reactions in the presence of plagioclase and the absence of chlorite that have not been described previously in the literature. The reaction that occurs in the middle staurolite zone, alm-rich ga + Ca-rich pla + Na-rich mu gro-rich ga + Na-rich pla + st + Na-poor mu, is considered to be prograde, whereas the reaction that occurs in the kyanite zone, alm-rich ga + Ca-rich pla + w st + Ca-rich ga + Na-rich pla + qz, is retrograde. The topology of these reactions is illustrated in terms of end member compositions for the systems KNaFASH and KCaFASH, respectively.

Notes: The occurrence of lawsonite is described from pelitic schists of the lower-grade part of the pumpellyite-bearing subzone of the chlorite zone in the Asemi River area of central Shikoku. The lawsonite-bearing parageneses are consistent with the generally accepted view that the Sanbagawa facies series represents higher pressures than the lawsonite-bearing facies series in New Zealand.

Notes: Abstract Late Archaean orthogneisses and aluminous and iron-rich metasedimentary rocks intruded by anorthosite and a ferrodiorite-granite suite were completely recrystallized during Proterozoic granulite facies metamorphism. Geobarometry and geothermometry indicate P-T conditions of around 7.5kbar. 700°C, with a CO2-rich fluid phase and logfO2 at or below -16. A two-stage high-grade history of near isochemical corona growth is preserved in metasediments with the reaction cycle opx + plag + H2O → hbl+gar+SiO2→ opx+plag+H2O. End product compositions resemble those of the initial phases, and the only mobile components were SiO2 and/or H2O. The coronas reflect shortlived fluctuations in chemical activity at essentially constant P and T, contrary to simple progressive change in equilibrium parameters recorded by most corona-bearing textures.

Notes: Abstract Phase relations and mineral chemistry for garnet (Grt), orthopyroxene (Opx), sapphirine (Spr), water-undersaturated cordierite (Crd), osumilite (Osu), sillimanite (Sil), K-feldspar (Kfs), quartz (Qtz) and a water-undersaturated liquid (Liq) have been determined experimentally in the system KFMASH (K2O-FeO-MgO-Al2O3-SiO2-H2O) under low PH2O and fO2 conditions. Four compositions have been studied with 100 [Mg/(Mg + Fe)] ranging from 65.6 to 89.7. Based on our experimental data, a P-T grid is derived for the KFMASH system in the presence of quartz, orthopyroxene and liquid. Osumilite has been found in various mineral assemblages from 950 to 1100°C and 7.5 to 11 kbar. In the temperature range 1000-1100°C, the pair Os-Grt is stable over a pressure range of about 3kbar. The divariant reaction Os + Opx = Grt + Kfs + Qtz runs to the right with increasing pressure. Because osumilite is the most magnesian phase it is restricted to Mg-rich compositions at high pressure. The reaction defining the upper pressure stability limit of Os-Grt is located around 11 kbar with a nearly flat dP/dT slope over the temperature range 950–100°C. Over the entire temperature range investigated osumilite is not stable beyond 12 kbar. The data imply a restricted pressure range between 11 and 12 kbar for the stability of the assemblage Os-Opx-Sil-Kfs-Qtz. At 1050°C and above, osumilite occurs in various mineral assemblages together with the high-T pair Spr-Qtz.When coexisting with garnet, orthopyroxene or sapphirine, osumilite is always the most magnesian phase. At 1050 and 1100°C, liquid is invariably the most Fe-rich phase in the run product.Our data support a theoretical P-T grid for the KFMAS system in which osumilite is stable outside the field of the high-T assemblage Spr-Qtz. Moreover, our grid indicates that Os-Opx-Sil-Kfs-Qtz has a more restricted pressure and compositional stability domain than Os-Grt, in agreement with natural occurrences. Osumilite is stable over a large pressure range, such that in Mg-rich rocks, and at high temperature, it can occur at any depth in normal thickness continental crust.

Notes: Abstract Mg-gabbros from East Ligurian ophiolites (Northern Apennines, Italy) display a high-temperature/low-pressure recrystallization localized along ductile shear zones. In deformed gabbros, the igneous diopside is recrystallized into granoblastic aggregates of neoblastic diopside and minor red-brown amphibole. The latter displays a pargasitic composition, with high amounts of AlIV Na(A) and Ti (± 1.8, 0.7 and 0.4 atoms per formula unit, respectively). Major element composition of neoblastic minerals highlight equilibration temperature conditions in the range 800–950° C. Red-brown Ti-pargasite also occurs as a minor interstitial constituent, presumably growing from a residual trapped liquid, in the differentiated lithologies (Fe-Ti-diorites) of the plutonic ophiolitic complex.By means of ion microprobe (SIMS technique), rare earth (La, Ce, Nd, Sm, Eu, Gd, Dy, Er, Yb) and selected trace elements (Sr, Y, Cr, V, Sc, Zr, Ti) have been analysed in igneous and neoblastic diopside, as well as in Ti-pargasites. Ti-pargasites have also been analysed for F and Cl, and compared with the halogen composition of the amphiboles, mainly hornblendes to actinolites, which are related to the subsequent low-temperature brittle evolution.Neoblastic Ti-pargasite from deformed Mg-gabbros bears close compositional similarities with igneous Ti-pargasite from undeformed Fe-Ti-diorites, whereas it is geochemically distinct from the amphiboles post-dating the ductile event. In particular, Ti-pargasites have relatively high contents of F, REE, Y, Zr and Sr, which are not consistent with crystallization in the presence of seawater-derived hydrothermal fluids.High-grade recrystallization probably developed in the presence of volatile-rich igneous fluids, either trapped between the cumulus minerals or injected into the ductile shear zones from outside the local system. An alternative hypothesis comprises the absence of fluid phase and the development of ductile shear zones in Ti-pargasite-rich layers. The petrological features of deformed gabbros recovered from present-day slow-spreading ridges and fracture zones bear close similarities with the investigated ophiolitic metagabbros. In East Ligurian ophiolites, high-grade ductile shear zones have been related to the initial stages of the uplift of the gabbro-peridotite complex to the sea-floor.

Notes: Abstract A petrographic and petrological analysis of exceptionally well-preserved hydrothermal veins from the Merrimack synclinorium, north-eastern Connecticut, has been carried out in order to place new field-based constraints on fracture aperture dimensions and porosity in the lower continental crust. The veins preserve substantial open space today in outcrop, and contain mineral assemblages including subhedral to euhedral crystals of quartz, kyanite and almandine-rich garnet. Textural evidence indicates unequivocally that the vein minerals grew into macroscopic (mm- to cm-scale) open space between the vein walls. The veins are interpreted to have been large-aperture fractures along which significant advective fluid infiltration and chemical reaction occurred. The porosity of the rock mass due to open space between fracture walls today is c. 0.3%, but it could have been as large as several percent when the flow system was active. Quantitative thermobarometry results from vein mineral assemblages indicate that the fractures formed at pressures and corresponding crustal depths of c. 0.8 GPa and c. 30km, and temperatures of 550–600° C. The depth of fracture formation corresponds to published estimates of the maximum burial depth of the Merrimack synclinorium during the Acadian orogeny. The formation of large-aperture fractures could increase significantly the transient permeability of the deep crust, and therefore influence metamorphic heat and mass transfer.

Notes: Abstract 40Ar/39Ar ages from white mica in rocks of the internal zone of the Brooks Range contractional orogen indicate that the Nanielik antiformal duplex developed at about 120 Ma and was remobilized on its southern boundary at c. 108 Ma. Blueschist facies metamorphism accompanied development of the antiform. The timing of the blueschist facies event and creation of the antiform overlap the period of shallow-seated deformation in the foreland fold and thrust belt and sedimentation in the foreland basin of the Brooks Range. Blueschist facies P-T conditions may therefore characterize the thicker parts of orogenic wedges in some orogenic systems; ancient blueschists need not necessarily be interpreted as indicators of active subduction or continent-continent collision.Microprobe analysis using quantitative wavelength-dispersive and electron backscattered electron imaging methods was used to characterize the composition of white micas in the dated samples. None of the samples was compositionally homogeneous; many contained 2-3 populations of white mica, including both potassic and sodic varieties. Samples which had undergone (in sequence) amphibolite, albite-epidote amphibolite and blueschist facies metamorphic events retained muscovites relict of the amphibolite facies event. Samples that had undergone only the blueschist facies event also contained multiple populations of mica, some probably from detrital sources.

Notes: Abstract Ductilely deformed amphibolite facies tectonites comprise two adjacent terranes in east-central Alaska. These terranes differ in protoliths, structural level and cooling ages. A structurally complex zone of gently north-dipping tectonites separates the two terranes. The northern, structurally higher Taylor Mountain terrane includes garnet amphibolite, biotite ± hornblende gneiss, marble, quartzite, metachert, pelitic schist and cross-cutting granitoids of intermediate composition (including the Late Triassic to Early Jurassic Taylor Mountain batholith). Lithological associations and isotopic data from the granitoids indicate an oceanic or marginal basin origin for the Taylor Mountain terrane. 40Ar/39Ar metamorphic cooling ages from the Taylor Mountain terrane are latest Triassic to earliest Middle Jurassic. The southern, structurally lower Lake George subterrane of the Yukon-Tanana terrane is made up of quartz-biotite schist and gneiss, augen gneiss, pelitic schist, garnet amphibolite and quartzite; we interpret it to comprise a continental margin and granitoid belt built on North American crust. Metamorphic cooling ages from the Lake George subterrane are almost entirely Early Cretaceous.Geothermobarometric analysis of garnet rims and adjacent phases in garnet amphibolite and pelitic schist from the Taylor Mountain terrane and Lake George subterrane indicate peak metamorphic conditions of 7.5-12 kbar at 555-715° C in the northern part of the Taylor Mountain terrane, in which NNE-vergent shear fabrics are preserved; 6.5-10.8 kbar at 520-670° C within the contact zone between the two terranes, in which NW-vergent shear fabrics predominate; and 6.8-11.8 kbar at 570-700° C in the Lake George subterrane of the Yukon-Tanana terrane, in which NW-vergent shear is recorded in the northern part of the study area and SE-vergent shear in the southern part. Where the two shear-sense directions occur together in the northern Lake George subterrane and, locally, in the contact zone, fabrics that record NW-vergent shear are more penetrative and preceded fabrics that record SE-vergent shear.We interpret the pressure, temperature, kinematic and age data to indicate that the metamorphism of the Taylor Mountain terrane and Lake George subterrane took place during different phases of a latest Palaeozoic through early Mesozoic shortening episode resulting from closure of an ocean basin now represented by klippen of the Seventymile-Slide Mountain terrane. High- to intermediate-pressure metamorphism of the Taylor Mountain terrane took place within a SW-dipping (present-day coordinates) subduction system. High- to intermediate-pressure metamorphism of the Lake George subterrane and the structural contact zone occurred during NW-directed overthrusting of the Taylor Mountain, Seventymile-Slide Mountain and Nisutlin terranes, and imbrication of the continental margin in Jurassic time. The difference in metamorphic cooling ages between the Taylor Mountain terrane and adjacent parts of the Lake George subterrane is best explained by Early Cretaceous unroofing of the Lake George subterrane caused by crustal extension, recorded in its younger top-to-the-SE fabric.

Notes: Abstract High-P/T metamorphic parageneses are preserved within two late Palaeozoic to early Mesozoic assemblages of the southern Klamath Mountains that show contrasting structural styles and mineral parageneses reflecting formation in different parts of a subduction-zone regime. Blueschist facies tectonites of the Stuart Fork terrane represent a coherent subduction complex formed at relatively deep crustal levels, whereas the chaotic metasedimentary mélange of the eastern Hayfork terrane contains a diverse range of metamorphic parageneses reflecting complex structural mixing of metamorphic components at shallower levels. The convergent-margin-type accretionary metamorphism evident in both terranes pre-dates Middle Jurassic low-P/T metamorphism resulting from regional tectonic contraction and magmatism.The epidote- to lawsonite-zone Stuart Fork blueschists (and eclogites locally) formed at pressures of about 6-11 kbar and temperatures of 250-400° C. Deformed matrix material of the eastern Hayfork mélange formed at similar temperatures but lower pressures, on the order of 3-6 kbar. The mélange contains a diverse assemblage of tectonic blocks that formed under a range of P-T conditions, including those of the blueschist, pumpellyite-actinolite, greenschist and upper greenschist to amphibolite facies.The succession of mineral parageneses and inferred P-T conditions of the eastern Hayfork blocks reflect those of igneous protolith formation, structural mixing, subduction-zone metamorphism, olistolith transport, and tectonic and erosional denudation. Although temporal relations are not well constrained, the evolution of these terranes is consistent with formation within a single convergent-margin system.

Notes: In a polymetamorphic, felsic, biotite-bearing gneiss, biotite has reacted to form magnetite and microcline. The resulting structure is a magnetite core surrounded by a mantle of feldspar and quartz normally not exceeding 20mm in diameter. Measurements of oxygen isotope ratios disclose disequilibrium between mantle microcline and mantle quartz and also between mantle and matrix minerals of the same species. A clustering of temperature estimates from the oxygen isotope distribution between magnetite and quartz and between magnetite and microcline in the interval 550 to 600°C suggests an approach to oxygen isotope equilibrium. No signs of a re-equilibriation of the reacting biotite can be found.

Notes: Abstract Clinopyroxenes and garnets from 11 blueschist-facies Fe-rich eclogite samples from the Voltri Group show a wide range of chemical compositions. Detailed analyses of single pyroxene and garnet grains show wide and scattered chemical inhomogeneity, the KD(KD= (Fe2+/Mg)Gt/(Fe2+/Mg)Cpx) ranges from 20 to 87 based on rim analyses only. The data obtained indicate that the mineral pairs never attained equilibrium under uniform P-T conditions and that the compositions of the metamorphic minerals were influenced mainly by the composition of the pre-metamorphic minerals and by topotactical reactions.

Notes: Abstract At Sulitjelma, Norway, there is a major inversion of metamorphic isograds beneath an inverted but undisrupted ophiolite. The flysch-like Furulund schist in which the inverted isograds occur is also inverted and the early folds in it are downward facing. The isograds cut across the axial surfaces of early folds and across the schistosity. These relationships are explained as the consequence of metamorphism during the progressive development of a large overfold. The inverted limb of the overfold is regarded as a major, thick, gently-dipping shear zone, separating the lower-grade, lower part of the Caledonian allochthon below from the higher-grade upper part of the allochthon above. The association between stratigraphical inversion, downward-facing of syn-schistosity folds and metamorphic inversion is explained by the progressive development of the shear zone. It is suggested that the presence of such shear zones is a common feature of orogenic belts formed by continental collision.

Notes: Abstract The Hercynian granitic basement which forms the Tenda Massif in NE Corsica represents part of the leading edge of the European Plate during middle-to-late Cretaceous (Eoalpine) high P metamorphism. The metamorphism of this basement, induced by the overthrusting of a blueschist facies (schistes lustrés) nappe, was confined to a major ductile shear zone (c. 1000m thick) within which deformation increases upwards towards the overlying nappe. Metamorphism within the basement mostly records lower blueschist facies conditions (crossite + epidote) except near the base of the shear zone where the greenschist facies assemblage albite + actinolitic amphibole has developed instead of crossite. Study of the primary mafic phase breakdown reactions within hornblende granodiorite reveals the following metamorphic zonation. Zone 1: biotite to chlorite. Towards zone 2: biotite to phengite. Zone 2: Hornblende to actinolitic Ca-amphibole + albite + sphene, and biotite to actinolitic Ca-amphibole + albite + phengite + Ti-ore + epidote. Zone 3: Hornblende to crossite + low Ti-biotite + phengite + sphene, and biotite to crossite + low Ti-biotite + phengite + Ti-ore + sphene ± epidote. P-T conditions at the base of the shear zone are estimated to have been 390-490°C at 600-900 M Pa (6-9kbar) and the Corsican basement is therefore deduced to have been buried to 20-30 km during metamorphism. This relatively shallow metamorphism contrasts with some other areas in the Western Alps where the Eoalpine event apparently buried the European continental crust to depths of 80 km or more. As there is no evidence for a long history of blueschist facies metamorphism prior to the involvement of the European continent, it is deduced that the Eoalpine blueschists were produced during the collision of the Insubric plate with Europe, rather than during Tethyan intraoceanic subduction. Coherent blueschist terrains such as the schistes lustres probably record buovant feature collision and obduction tectonics rather than any preceding oceanic subduction.

Notes: Abstract Three major blastomylonitic synmetamorphic (epidote amphibolite to mid amphibolite facies) shear zones are seen on the NW coast of the Mullet Peninsula in NW Mayo. These shear zones occur at the contacts of major structural units and in an imbricated slice where rocks of the Erris Complex are deformed and chemically modified. Chemical changes associated with individual shear zones have been deduced by comparing the compositions of various gneisses both within and adjacent to the shear zones. Compositional changes are different in the constituent rock-types within each unit and many elements normally considered immobile have been selectively mobilized within the shear zones. Little evidence of wholesale metasomatic introduction of components into these shear zones was found to accompany the selective mobilization.

Notes: Abstract In the Su-Lu ultrahigh-P terrane, eastern China, many coesite-bearing eclogite pods and layers within biotite gneiss occur together with interlayered metasediments now represented by garnet-quartz-jadeite rock and kyanite quartzite. In addition to garnet + omphacite + rutile + coesite, other peak-stage minerals in some eclogites include kyanite, phengite, epidote, zoisite, talc, nyböite and high-Al titanite. The garnet-quartz-jadeite rock and kyanite quartzite contain jadeite + quartz + garnet + rutile ± zoisite ± apatite and quartz + kyanite + garnet + epidote + phengite + rutile ± omphacite assemblages, respectively. Coesite and quartz pseudomorphs after coesite occur as inclusions in garnet, omphacite, jadeite, kyanite and epidote from both eclogites and metasediments. Study of major elements indicates that the protolith of the garnet-quartz jadeite rock and the kyanite quartzite was supracrustal sediments. Most eclogites have basaltic composition; some have experienced variable ‘crustal’contamination or metasomatism, and others may have had a basaltic tuff or pyroclastic rock protolith.The Su-Lu ultrahigh-P rocks have been subjected to multi-stage recrystallization and exhibit a clockwise P-T path. Inclusion assemblages within garnet record a pre-eclogite epidote amphibolite facies metamorphic event. Ultrahigh-P peak metamorphism took place at 700–890° C and P〉28 kbar at c. 210–230 Ma. The symplectitic assemblage plagioclase + hornblende ± epidote ± biotite + titanite implies amphibolite facies retrogressive metamorphism during exhumation at c. 180–200 Ma. Metasedimentary and metamafic lithologies have similar P-T paths. Several lines of evidence indicate that the supracrustal rocks were subducted to mantle depths and experienced in-situ ultrahigh-P metamorphism during the Triassic collision between the Sino-Korean and Yangtze cratons.

Notes: Abstract Blueschists occurring as layers in calcite marbles of the Meliata unit occur along the so-called Roznava tectonic line situated in the southern part of the Gemericum, Slovakia. Mineral assemblages and compositions from seven blueschists localities and one occurrence of amphibolite facies rocks overprinted by blueschist metamorphism were investigated. The most common minerals in the blueschists are blue amphibole, epidote and albite. Some Fe2+- and Al-rich rocks also contain garnet and chloritoid, respectively. Na-pyroxene with a maximum 50% jadeite component was also found. The blue amphiboles correspond mostly to crossite and also to glaucophane and ferroglaucophane in some samples. Almandine- and spessartine-rich garnet has very low MgO content (〈3 wt%). The Si content in phengite ranges between 3.3 and 3.5 pfu calculated on the basis of 11 oxygens. The zoning patterns of blue amphibole, garnet and chloritoid suggest their formation during a prograde stage of metamorphism. The P-T conditions of metamorphism are estimated to be about 380–460° C and 10–13 kbar. Pressures of 7.5–8.5 kbar and temperatures of 350–370° C were obtained for some actinolite- and aegirine-rich rocks. Apart from chlorite, other mafic minerals formed during retrograde metamorphism are biotite and occasionally also actinolite.

Notes: Abstract The Port aux Basques gneisses comprise three lithostratigraphic units separated by major fault zones: the Grand Bay Complex; the Port aux Basques Complex; and the Harbor le Cou Group. A similar regionally developed polyphase history of penetrative deformation characterizes each of these units. Thickening during D1 produced rare recumbent folds (F1) and an axial planar schistosity (S1), overprinted by D2 recumbent folds (F2), and transposed during development of a locally penetrative, differentiated crenulation cleavage (S2). In western sectors of the area, D2 was associated with NW-directed reverse shearing. The NE-trending structural grain reflects D3 transpression, partitioned into dextral transcurrent movement along major shear zones and development of upright-to-steeply inclined, periclinal folds (F3) and a variably penetrative schistosity (S3).Amphibolite facies metamorphism increases in grade from west to east across the area. Microstructures, including porphyroblast-matrix foliation relations and internal textural unconformities in garnet, indicate episodic porphyroblast nucleation and growth, which reflect a prograde traverse sequentially across univariant reactions during syntectonic metamorphism. Garnet, kyanite and staurolite porphyroblasts are wrapped by the S2 foliation, but each may contain trails of inclusions that define S1; commonly these trails preserve early stages of S2 crenulation cleavage development. Progressive and sequential reaction out of kyanite, staurolite and muscovite in favour of sillimanite, garnet, biotite and K-feldspar, and the development of an increasing volume of anatectic migmatite in south-eastern sectors of the area record syn- to late-D2 peak metamorphic conditions. Microstructural relationships and petrogenetic grid considerations indicate clockwise trajectories in P-T space for units of the Port aux Basques gneisses. Peak metamorphic conditions are estimated to have been 620–650° C at ≤8kbar in the west and 700–750° C at ≤8 kbar in the east.Titanite from an upper amphibolite facies calc-silicate gneiss yields U-Pb ages of c. 420 Ma, interpreted to date cooling shortly after the thermal peak in these gneisses. Variable D3 strain was associated with some recrystallization of hornblende and micas. 40Ar/39Ar hornblende plateau isotope correlation ages range from c. 419 to c. 393 Ma, from east to west across the area, and are interpreted to record cooling through c. 500° C coeval with or soon after D3 deformation. The range in ages may record the effects of heterogeneous D3 deformation and differential uplift from south-east to north-west associated with displacement on major shear zones. 40Ar/39Ar muscovite plateau ages cluster at c. 390 Ma, and date cooling through c. 375° C during regional exhumation. Cooling rates are moderate to fast and may indicate a component of tectonic exhumation.The Port aux Basques gneisses are a product of Silurian collisional tectonics. The higher grade of metamorphism in comparison with adjacent areas of the Canadian Appalachians is interpreted to reflect greater thickening due to juxtaposition of the St Lawrence promontory (Laurentian margin) with the Cabot promontory (Avalonian margin) during closure of the Iapetus Ocean.

Notes: Abstract Carbon isotope thermometry has been applied to coexisting calcite and graphite in marbles from throughout the Adirondack Mountains, New York. Eighty-nine calcite-graphite pairs from the amphibolite grade NW Adirondacks change systematically in temperature north-westwards from 680 to 640 to 670° C over a 30-km distance, reflecting transitions from amphibolite facies towards granulite facies to the north-west and to the south-east. Temperature contours based on calcite-graphite thermometry in the NW Adirondacks parallel mineral isograds, with the orthopyroxene isograd falling above 675° C, and indicate that regional metamorphic temperatures were up to 75° C higher than temperatures inferred from isotherms based on cation and solvus thermometry (Bohlen et al. 1985). Fifty-five calcite-graphite pairs from granulite grade marbles of the Central Adirondacks give regional metamorphic temperatures of 670–780° C, in general agreement with cation and solvus thermometry.Data for amphibolite and granulite grade marbles show a 12%oo range in δ13Ccal and δ13Cgr. A strong correlation between carbon isotopic composition and the abundance of graphite (Cgr/Crock) indicates that the large spread in isotopic compositions results largely from exchange between calcite and graphite during closed system metamorphism. The trends seen in δ13C vs. Cgr/Crock and δ13Ccal vs. δ13Cgr could not have been preserved if significant amounts of CO2-rich fluid had pervasively infiltrated the Adirondacks at any time. The close fit between natural data and calculated trends of δ13C vs. Cgr/Crock indicates a biogenic origin for Adirondack graphites, even though low δ13C values are not preserved in marble.Delamination of 17 graphite flakes perpendicular to the c-axis reveals isotopic zonation, with higher δ13C cores. These isotopic gradients are consistent with new graphite growth or recrystallization during a period of decreasing temperature, and could not have been produced by exchange with calcite on cooling due to the sluggish rate of diffusion in graphite. Samples located 〉2km from anorthosite show a decrease of 0.5-0.8%oo in the outer 100 μ of the grains, while samples at distances over 8 km show smaller core-to-rim decreases of c.0.2%oo. Correlation between the degree of zonation and distance to anorthosite suggests that the isotopic profiles reflect partial overprinting of higher temperature contact metamorphism by later granulite facies metamorphism. Core graphite compositions indicate contact metamorphic temperatures were 860–890° C within 1 km of the Marcy anorthosite massif. If samples with a significant contact metamorphic effect (Δ(cal-gr) 〈3.2%oo) are not included, then the remaining 38 granulite facies samples define the relation Δ13C(cal-gr) = 3.56 ± 106T-2 (K).

Notes: Abstract. The Skokloster and Stockholm Environmental Institute (SEI) approaches were applied to the assessment of 145 soils of the South African highveld region in terms of sensitivity to acid deposition. The critical load class calculated by variants of these methods was compared with the acid neutralizing capacity (ANC) determined by pH measurement of soil suspended in a dilute acetate buffer solution. This rapid index of ANC correlates well both with ANC determined by an established but more laborious method which involves titration and equilibration of soil with HCl, and with a number of soil properties related to base status. The correlation between Skokloster or SEI critical load classes and ANC was weak. It was concluded that ANC determination would be a preferable basis for classifying these soils in terms of their sensitivity to acid deposition, since the method is direct and integrates the contribution of various soil properties to acid sensitivity instead of requiring the relative contribution to be calculated according to somewhat arbitrary weightings given to broadly defined classes of soil properties.

Notes: Abstract. Emissions of ammonia (NH3) and nitrous oxide (N2O), and nitrate (NO-3) leaching were measured in two field experiments following application of pig slurry at rates corresponding to 83–96 kg NH4-N ha-1 before sowing. In spring and in autumn 1994, slurry was applied by four methods: trenching (T), shallow injection (S), band spreading immediately followed by harrowing (B/H) and band spreading (B). NH3 emission measurements were made during the first week after application in both experiments. In the spring experiment N2O emissions and NO-3 leaching were measured during 6 and 52 weeks after spreading respectively, and during 11 and 33 weeks after spreading in the autumn experiment. In spring, the increased N2O emissions (i.e. control subtracted) ranged from 0.27% (T) to 0.45% (B/H), and in the autumn study from 0.92% (T) to 1.14% (B/H), of applied NH4-N, although showing no statistically significant differences. In order to validate the chamber measurements, a ‘megachamber’(21 m2) was used together with an infrared spectrometer. The emissions agreed well for (B/H), while (B) resulted in lower emissions compared with the smaller chambers. Emissions of NH3 were about one order of magnitude higher. In spring, (B) gave the highest emission, reaching 19.5% of applied NH4-N, whereas (S), and (B/H) gave the lowest emissions, reaching 1.2 and 3.5% of applied NH4-N, respectively. NH3 emissions in autumn were 15–20% lower compared with spring. In spring the increased nitrate leaching ranged from 10.1 (T) to 24.9 kg ha-1 (B/H) and from 29.5 (B) to 37.8 kg ha-1 (T) in the autumn experiment, showing no statistically significant differences. Estimations of indirect N2O emissions due to ammonia deposition and nitrate leaching, suggested that the N2O contribution from NH3 deposition was relatively small, while the indirect N2O emissions from NO-3 leaching were of the same order of magnitude or higher than the direct N2O emissions.

Notes: Abstract. In a laboratory study, 15N ammonium fertilizer uptake and rice growth was determined in a non-acid sulphate marine soil (Typic Tropaquept) and an acid sulphate soil (Sulfic Tropaquept). Acid sulphate sensitive (IR 26) and acid sulphate tolerant (IR 46) rice varieties were grown in soil suspensions incubated at four Eh levels (+500, +250, +50, and -150 mV) in microcosms for three weeks. The results showed that rice grown in non-acid sulphate marine soils gave slightly better dry matter weight of 1.8g/pot, greater 15N uptake of 12.8 mg N/pot, and higher total N uptake of 38.4 mg N/pot than under acid sulphate soil conditions indicating the non-acid marine soil is more favourable to rice culture. Growth as measured by weight of dry matter was significantly reduced from 2.1g/pot under oxidized condition (+500 mV) to 0.8g/pot under highly reduced condition (-150 mV). N uptake by rice was significantly reduced from 16.9 mg/pot at + 500 mV to 4.5 mg N/pot at -150 mV Total N uptake also decreased with decreasing Eh. Growth, 15N uptake and total N uptake by acid sulphate tolerant rice, IR 46 were significantly higher than the acid sulphate sensitive rice variety, IR 26. Under highly reduced soil conditions (-150 mV), growing rice in acid sulphate soil would require additions of lime, intermittent irrigation and/or mid season drainage in order to increase soil redox potential and remove toxic substances.

Notes: Book reviewed in this articles: Take-all disease of cereals: a regional perspective. By Hornby, D., Bateman, G.L., Gutteridge, R.J., Lucas, P., Osbourn, A.E., Ward, E. & Yarham, D.H.

Notes: Abstract. This paper investigates the ways in which some bedouin in the Nubian Desert of southeastern Egypt take decisions about both the choice of site for cultivation and the subsequent management of their soils. It explores the complementarity of formal and informal sciences and how each might profitably inform the other. Results show that the bedouin understand the physical limitations and nutrient supply properties of soils, but not aspects such as pH. Decisions on the choice of cultivation site are often made with regard to other perceived risks, such as soil loss and intermittent inundation, rather than just soil quality. It is also apparent that there exists among bedouin a plurality of indigenous knowledge mediated by factors such as experience, wealth levels, household circumstances and production priorities. Understanding indigenous knowledge is essential in helping to develop better use of the soil in this area, about which little is known and which has only a short history of small scale cropping.

Notes: Book reviewed in this articles: Agroforestry for soil management. By A. Young.

Notes: Book reviewed in this articles: Biological indicators of soil health. Edited by C.E. Pankhurst, B.M. Doube & V.V.S.R. Gupta.

Notes: Abstract. Air- dried milled peat (0.45 g g-1 wet weight basis) is used as an electricity generating fuel in Ireland. It is stored on peat bogs in triangular section stockpiles. These stockpiles comprise of milled peat of varying types, classified by poured density, and are subject to rewetting during storage which incurs an economic penalty. A water applicator was constructed to study the nature of rewetting mechanisms in laboratory-scale milled peat stockpiles, and to assess some potential protection strategies. Results indicated that there were few consistent short-term mechanisms linked to milled peat type. Over a longer period, low density milled peat stored most water, high density milled peat generated most runoff from the stockpile surface, and under certain conditions, throughflow was very important in all milled peat types. The industrial implications of the findings conclude that protection strategies should focus on lower density stockpiles.

Notes: Abstract. We examined whether nitrogen (N) and phosphorus (P) export was enhanced from grassland receiving inorganic fertilizer and manures typical of intensive livestock production. Buffer strips were included in the study to determine if they could reduce nutrient export. Hillslope plots receiving granular inorganic fertilizer, liquid cattle slurry and solid cattle manure (FYM) were compared using rainfall simulation for 4 storms on consecutive days at 22 mm h-1 and 35 minutes duration. The plots were hydrologically isolated in a randomized block layout of 4 treatments × 3 replicates and measured 30 × 5m; the upper 20m received either fertilizer, slurry or FYM, while the lower 10 m acted as an unfertilized grass buffer strip. Nitrogen and P export in surface runoff from grassland receiving inorganic fertilizer exceeded that from FYM or slurry treatments; concentrations up to46mgN1--1 and 15 mgP1–1 were recorded.Sixty eight % and 62% of the N from FYM and slurry respectively, was exported in organic form. Seventy four % (FYM) and 39% (slurry) of the P was in particulate or dissolved organic form. The buffer strip reduced N export in surface runoff by 94% and P export by 98% from inorganic fertilizer plots. A 75% reduction in N export was recorded from the buffer zone below slurry plots but only a 10% reduction in P, with most P remaining in the particulate or dissolved organic fraction. There was no significant difference in N export from the buffer zone between the inorganic fertilizer treatment and the untreated control.

Notes: Abstract. The Representative Soil Sampling Scheme of England and Wales was started in 1969. During the 25 year period 1969–1993 mean soil pH values under arable and ley-arable cropping changed little, but under grassland they fell by 0.3 units. Under arable cropping, mean available phosphorus declined whilst available potassium increased fairly steeply for the first eight years then declined again to near the start value by 1993. No significant changes in soil phosphorus and potassium levels were found under ley-arable cropping, but the percentage of fields under ley-arable rotations declined from 33% to 14%. Under grassland, phosphorus and potassium levels both declined, although potassium levels rose initially before falling.For this report England and Wales were also divided into five regions. Wales and the West Country had the lowest pH values; these regions together with the Northern region were also found to have declining pH values. Phosphorus and potassium concentrations have been falling in the Northern region which stands out as having the lowest current potassium status with 20% of all fields deficient at index 0.Changes in pH and nutrients are discussed for soils sampled following the main arable crops. The largest changes found were the reductions in phosphorus and potassium in soils after potatoes. In 1993 almost one in ten sugarbeet crop soils had an excessive phosphorus level (index 5+) giving an increased risk of water pollution by this element.Under arable cropping the proportion of soils with low (〈 1.8%) organic matter declined. Average soil organic matter levels for all crops and grass have remained static during the period. It is concluded that agricultural soils in England and Wales have not contributed to an increase in the greenhouse gas carbon dioxide.

Notes: Book Reviewed in this article:Principles and Practice of Soil Science (Third Edition) By R. E. White.