ALBERT

Notes: Microstructures in slate belt rocks at the Elura Mine, near Cobar, south-eastern Australia, indicate that volume loss by syntectonic dissolution is coupled with mass accretion by reprecipitation of the dissolved material in dilational sites. The mass accretion is sustained primarily by repetitive tensile microfracturing at high pore-fluid pressures. Oriented growth in the inter- and intragranular microcracks is locally host-controlled, creating lattice- and shape-preferred orientations. The grain-scale crack-seal features throughout the rock reflect rhythmic fluid pressure fluctuations; a balance is achieved between the fracture-induced permeability (and consequent flushing rates), and the rate of fluid build-up in a relatively sealed environment.Instability in the balancing factors can lead to localization and intensification of tensile failure (and hence, tension vein formation) in the grain aggregate. Growth of veins by crack-seal also reflects a steady state, but with more localized fluctuations of fluid flow on the aggregate scale. Still larger imbalances between flushing and fluid accumulation (i.e. pressure variations) induce breccia veining. The larger pressure gradients over greater distances, associated with dilation localization (from pervasive microfracturing to spaced breccia domains), allow fluid channelling with an increased potential for chemical fluid/rock disequilibrium. Therefore, large breccia vein systems tend to be sites of extensive fluid/rock interaction and replacement, as spectacularly illustrated by the syntectonic sulphide orebodies at Elura. The huge amounts of silicate, carbonate and sulphide accumulated during folding at Elura illustrate the large scale of source and sink couples possible in solute mass transfer.

Notes: The Internal Zone of the Betic Cordilleras consists of several superimposed major thrust sheets with different P-T-t evolutions. On the basis of an integrated field, microscopic and laboratory study, the tectono-metamorphic history of the Mulhacen Complex and Almanzora Unit has been reconstructed in detail. The Mulhacen Complex has been affected by at least five phases of penetrative deformation, which have been labelled Dx-1, Dx, Dx+1, Dx+2 and Dx+3. Dx-1, and Dx are related to continent-continent collision, which is indicated by high pressure-low temperature (HP/LT) and subsequent intermediate P/T metamorphic conditions. Dx+1 is related to crustal thinning and heterogeneous extension. During this event the Almanzora Unit was juxtaposed against the Mulhacen Complex. This phase was succeeded by the establishment of low pressure-high temperature (LP/HT) conditions and at least two phases of folding and overthrusting. The Almanzora Unit shows a comparable tectono-metamorphic evolution post Dx+1. However, the P/T conditions prior to Dx+1 indicate a higher crustal position with respect to the Mulhacen Complex during the collisional event.

Notes: Following the Middle Devonian Acadian deformation an extensive belt of high grade metamorphism was formed in New England. In south-western Maine, at the northern end of this belt, there occurs a transition along the strike from regional low-pressure/high-temperature metamorphism to contact metamorphism in low-grade rocks. Petrological studies indicate that this transition occurs along a surface plunging to the north-east at about 3.5°, with respect to the Middle-to-Late Devonian erosion surface. In addition, detailed petrological mapping has defined a history of temporally separate, localized metamorphic events associated with plutonism and occurring at increasingly deeper levels to the south-west. Geochronological studies constrain ambient temperatures in the transition zone at the time of metamorphism to be less than 300° C in the north-east and between 350° C and 500° C in the south-west. They also establish a pattern of diachronous cooling due to differential uplift and erosion, with cooling occurring later and most rapidly to the south-west. Geophysical evidence suggests that along with this spatial variation in metamorphic style the shapes of the plutons in Maine undergo a transition from laterally extensive sheet-like bodies in the high grade terrane to more equant-shaped bodies in the low-grade terrane. Using the results of these petrological, geochronological and geophysical studies, as well as those of stratigraphical and structural studies we construct a thermal model for the transition zone. The model suggests that the Acadian metamorphism in south-western Maine is a result of deep-level contact metamorphism near laterally extensive granitic sills dipping to the north-east with respect to the present erosion surface. The plutons themselves are interpreted to be a result of lower crustal melting in response to crustal thickening in the presence of normal or slightly augmented mantle heat flux.

Notes: Abstract Regionally distributed pelitic granulites in the Wilson Lake region contain the assemblage sapphirine + hypersthene + sillimanite + quartz. Geochronology and geobarometry suggest it developed in early Proterozoic rocks at temperatures approaching 900°C and pressures above 10 kbar. Vein-like metasomatized rocks around a suite of mafic to ultramafic intrusions, emplaced near the peak of metamorphism about 1700 Ma ago, contain sapphirine, but these assemblages developed at temperatures near 750°C and pressures of 4.5 kbar. Both types of assemblage occur as relics in amphibolite-grade (biotite–sillimanite) migmatites. P–T determinations indicate rapid isothermal uplift of 20 km accompanied by mafic intrusion and hydration. The metamorphic history and tectonic setting suggest exposure of deep continental crust by thrusting during continental collision, followed by essentially isothermal decompression.

Notes: Abstract The effects of Tertiary Alpine metamorphism on pelitic Mesozoic cover rocks have been studied along a cross-section in the central Lepontine Alps in the Nufenen Pass area, Switzerland.Greenschist facies to amphibolite facies conditions are indicated by the formation of the index minerals chloritoid, garnet, staurolite and kyanite in pelitic rocks. Regional metamorphism reached maximum conditions during the interkinematic period between a main Alpine penetrative (D2) and a late Alpine (D3) crenulation type deformation phase or synchronous with the late Alpine deformation. Based on AFM phase relationships four different metamorphic zones can be distinguished: (1) chloritoid zone; (2) staurolite + chlorite zone; (3) staurolite + biotite zone; and, (4) kyanite zone.The isograds that separate these zones can be modelled by univariant reactions in the KFMASH system. The conditions of metamorphism calculated from geological ther-mobarometers for the maximum post-D2 por-phyroblast stage are from North to South: 500° C at 5-6 kbar and 600° C at 7-8 kbar.Detailed thermobarometry of garnet por-phyroblasts with complex textures suggests that maximum temperature was reached later than maximum pressure. Early garnet growth occurred along a prograde P-T-path, post-D2 rims grew with increasing temperature but decreasing pressure, and finally post-D3 garnet formed along a retrograde P-T-path.It may be concluded from the calculated pressure and temperature difference over a short distance (3 km) across the mapped area that the isogradic surfaces of the post-D2 metamorphism are steeply oriented. The data also suggest that isobaric and isothermal surfaces are parallel.Much of the observed metamorphic pattern can be explained as the result of a significant post-D2 differential uplift of the hot Pennine area relative to the Helvetic area along a tectonic contact zone. The closely spaced isograds (isotherms) in the North may then be interpreted as a thermal effect owing to the emplacement of the hot Pennine rocks against the Got-thard massif with its cover. Whereas, in the Pennine metasediments, post-D2 porphyroblast formation can be related to the decompression path which was steep enough for dehydration reactions to proceed. It is also remarkable that late kyanite porphyroblasts probably formed with decreasing pressure.The interpretation given here for the Nufenen Pass area may also apply to the Luk-manier Pass area where similar metamorphic patterns have been reported by Fox (1975). The formation of the ‘Northern Steep Belt’;, as denned by Milnes (1974b), and the associated late Alpine fold zones may, therefore, have significantly modified the metamorphic pattern of the Helvetic-Penninic contact zone.

Notes: Abstract Porphyroblast textures in a Karakorum phyllite reveal that porphyroblast growth was syn-tectonic with respect to a cleavage forming deformation. During and after porphyroblast growth it partitions the deformation such that zones of intensified cleavage are developed which wrap around the porphyroblast whilst the porphyroblast and its strain shadow undergo little deformation. Porphyroblast strain shadows comprise quartz, calcite and felspar with little mica, and are probably formed by solution transfer during deformation. Unless the deformation is so strongly partitioned that no deformation of the porphyroblasts and their immediate surrounds occurs, inequidimensional porphyroblasts will rotate. Porphyroblasts undergo some dissolution after they have finished growing.

Notes: Abstract New isotopic (Rb–Sr, U–Pb zircon and Sm–Nd) and petrological data are presented for part of an extensive Proterozoic mobile belt (locally known as the Rayner Complex) in East Antarctica. Much of the belt is the product of Mid-Proterozoic (∼ 1800–2000 Ma) juvenile crustal formation. Melting of this crust at about 1500 Ma ago produced the felsic magmas from which the dominant orthogneisses of this terrain were subsequently derived. Deformation and transitional granulite-amphibolite facies conditions (which peaked at 750 ± 50°C and 7–8 kbar (0.7–0.8 GPa) produced open to tight folding about E–W axes and syn-tectonic granitoids about 960 Ma ago. Subsequent felsic magmatism occurred at about 770 Ma and not, as has been widely advocated, at 500–550 Ma, which appears to have been a time of widespread upper greenschist facies (400–500°C) metamorphism, localized shearing and faulting.Sm-Nd model ages of 1.65–2.18 Ga disprove a previously favoured hypothesis that the Rayner Complex mostly represents reworked Archaean rocks from the neighbouring craton (Napier Complex). Models that involve rehydration of the Napier Complex are no longer required, since the Rayner Complex was its own source of water. Two episodes of Proterozoic crustal growth are identified, the later of which occurred between about 1200 Ma and 1000 Ma, and was relatively minor. Sedimentation took place only shortly before Late Proterozoic orogenesis.The multiphase history of the Rayner Complex has resulted in complex isotopic behaviour. Three temporally discrete episodes of Pb loss from zircon have been identified, the earliest two of which are responses to the c. 960 Ma and 540 Ma tectonothermal events. Fluid leaching was operative during the later event for there is a good correlation between degree of isotopic discordance and secondary mineral growth. Pb loss during the high-grade event was probably governed by the same process or by lattice annealing. Some zircon suites also document recent Pb loss. Most lower concordia intercepts have no direct geological meaning and are explicable as mixed ages produced by incomplete Pb loss during two or more secondary events. Whereas all zircon separates from the orthogneisses produce U–Pb isotopic alignments, zircons from the only analysed paragneiss produce scattered data, in part reflecting a range of provenance. The 960 Ma event was also associated with the growth of a characteristically low U zircon (∼ 300 μg/g) in rocks of inferred high Zr content.There is ubiquitous evidence for the resetting of Rb–Sr total-rock isochrons. Even samples separated by up to 10 km fail to produce igneous crystallization ages. Minor mineralogical changes produced by the 540 Ma upper greenschist-facies metamorphism were sufficient to almost completely reset some Rb–Sr isochrons and to produce open system conditions on outcrop scale, at least in one location.

Notes: Abstract The preserved array of pressures in the eastern Dalradian indicates that considerable syn- to post-metamorphic differential uplift has occurred. This inferred differential uplift suggests that Buchan sillimanite zone rocks originally lay at higher structural levels than presently adjacent cooler kyanite zone rocks to the west. A number of features are believed to coincide with the western margin of the sillimanite zone. These are a maximum in temperature, sharp thermal features, a high strain zone, and a train of metabasites. These features are explained by invoking syn-metamorphic movement between the Buchan sillimanite zone and the kyanite zone to its west, involving some horizontal component of movement. It is suggested that the lateral, now eroded, equivalents of the Buchan area once provided part of the required tectonic thickening for other parts of the Dalradian. Areas surrounding the Buchan area suffered tectonic burial followed by metamorphism during uplift relative to the Buchan area.

Notes: Abstract The upper Jurassic Nikoro greenstone complex of eastern Hokkaido suffered high-pressure intermediate type metamorphism. Characteristic minerals include lawsonite, aragonite, sodic pyroxene of the aegirinejadeite series, winchite. sodic amphibole of the glaucophane-riebeckite series, pumpellyite, epidote and actinolite.High-pressure metamorphism of the Nikoro greenstone complex is related to subduction of the Kula plate toward the Palaeo-Okhotsk Land during Cretaceous time.

Notes: Abstract The Berzosa fault is a major ductile shear zone, the Berzosa Shear Zone (BSZ), which separates the ‘Ollo de Sapo’anticline from the inner higher-grade crystalline axis of the Iberian Hercynian Belt. This shear zone is the site of abundant early kinematic quartz (± Al-silicates) segregations, rich in fluid inclusions. Host rocks are medium-grade staurolite schists and sillimanite gneisses.Fluid inclusions in selected quartz segregations across the Berzosa shear zone have been studied by microthermometric methods as well as, in some instances, by Raman analysis. The recorded fluid inclusion history begins at the end of an intense secondary recrystallization period during late-peak metamorphic conditions and lasts until late in the uplift history of the zone.Three types of inclusions have been found, which in a time sequence are: CO2± H2O; H2O+salt (B-type); and, N2+CH4. Three types of B inclusion may be distinguished in turn, depending on whether they were trapped during an earlier dynamic-recovery phase (B1-type), formed later as intergranular trails (B2-type), or were trapped apparently along with N2+CH4 in clusions from a heterogeneous fluid (B3-type).Considerations from isochores confirm that CO2± H2O inclusions were trapped during late-peak and high-T retrograde metamorphic conditions (in the range 650–500°C and 5–2 kbar), whilst N2+CH4 inclusions, along with the B3-type of inclusions, formed at low-pressures (〈1 kbar) and temperatures (± 300°C). B2-type inclusions were trapped chronologically between these two in a period in which strong inverse lateral thermal gradients developed in the zone. Inferred P-T paths for the area are convex to the T-axis.

Notes: Abstract The orthopyroxene-clinopyroxene, garnet-orthopyroxene and garnet-clinopyroxene geothermometers, and the garnet-orthopyroxene-plagioclase, garnet-clinopyroxene-plagioclase and anorthite-ferrosilite-grossular-almandine-quartz geobarometers are applied to metabasites and the garnetplagioclase-sillimanite-quartz geobarometer is applied to a metapelite from the Proterozoic Arendal granulite terrain, Bamble sector, Norway. P–T conditions of metamorphism were 7.3 ± 0.5 kbar and 800 ± 60°C.This terrain shows a regional gradation from the amphibolite facies, into normal LILE content granulite facies rocks and finally strongly LILE deficient granulite facies gneisses. Neither P nor T vary significantly across the entire transition zone. The change in ‘grade’parallels the increasing dominance of CO2 over H2O in the fluid phase.LILE-depletion is not a pre-condition of granulite facies metamorphism: granulites may have either ‘depleted’or ‘normal’chemistries. The results presented herein show that LILE-deficiency in granulite facies orthogneisses is not necessarily related to variations in either P or T. The important mechanisms in the Arendal terrain were (a) direct synmetamorphic crystallization from magma, with primary LILE-poor mineralogies imposed by the prevailing fluid regime, and (b) metamorphic depletion, involving scavenging of LILEs during flushing by mantle-derived CO2-rich fluids. The latter process is constrained by U–Pb and Rb–Sr isotopic work to have occurred no later than 50 Ma after intrusion of the acid-intermediate gneisses, and was probably associated with contemporary basic magmatism in a tectonic environment similar to a present day cordilleran continental margin.

Notes: Abstract Peridotite and infolded marble of the Seiad ultramafic complex were recrystallized in the upper amphibolite facies as part of the regional progressive metamorphism of the Rattlesnake Creek terrane. Field relations, including the occurrence of metarodingites, and metasomatic zones between dissimilar rock types, demonstrate that the metasediments and serpentinized ultramafic rocks were juxtaposed prior to regional, barrovian metamorphism. Temperatures are estimated to have reached 760–800°C at pressures of 7–8 kbar during the peak of metamorphism. Four low-variance parageneses have been identified within a small (3 km2) area of the complex, which may reasonably be assumed to have formed under the same P and T conditions. Isobaric T-Xco2 diagrams of appropriate equilibria are presented for three different internally consistent sets of thermodynamic data. Despite the seemingly small numerical differences between the standard state thermodynamic properties of the data sets, only one diagram allows the four observed assemblages to coexist within a reasonable temperature range. All three phase diagrams require differences in fluid composition on the scale of a thin section; strong evidence for effective control of pore fluid composition by local mineral reactions during metamorphism.

Notes: Abstract Chromian omphacite which contains up to 4 wt % Cr2O3 has been identified from low-grade metamorphic rocks in Nishisonogi, Kyushu, Japan. It occurs as aggregates, forming a thin horizon ([20 mm thick) in alayered metagabbro within a serpentinite melange zone, together with Cr-free omphacite, actino-lite, epidote and sphene. It may have been formed by the metasomatic introduction of Cr into the metagabbro from the serpentinite rather than by reaction with chromite. The structural formula, based on EPMA analyses, and the optical absorption spectrum of the chromian omphacite show that the Cr is positioned in the octahedral site.

Notes: Abstract A diagenctic through anchizone to epizone transition is demonstrated in pelitic rocks of the Lower Palaeozoic marginal basin of Wales by examination of variations in phyllo-silicate mineralogy, illite crystallinity and bo parameter of white micas. This transition represents a temperature range from ∼ 150°C to ∼ 400°C and the metamorphism is of a low-pressure facies series type, with a geothermal gradient of ∼ 40°Ckm-1. Variations in grade can be correlated largely with the original basin and shelf form, suggesting a depth-related metamorphism. However, in areas closer to the site of Caledonian plate collision an increasingly syn-tectonic metamorphic event is apparent.Correlation of pelite data with metabasite assemblages is variable, the most consistent relationship being between epizone crystallinity values andepidote-actinolite (greenschist facies) assemblages. Diagenetic clay mineral assemblages are found associated with prehnite-pumpellyite assemblages in metabasites and it is suggested that the latter represent non-buffered, and therefore non-diagnostic, assemblages.

Notes: Abstract Existing geochronological data are reviewed and new Rb-Sr, K-Ar and 39Ar–40Ar ages are presented, including a suite of 33 mica ages from a 20 km north–south tunnel section. These data are discussed in relation to the thermal history from the overthrusting of the Autroalpine nappes c. 65 Myr ago to the present. The earliest phase of metamorphism, involving lawsonite crystallization, is associated with emplacement of these nappes. Subsequently, temperatures in the rocks beneath rose, at a mean rate of 3–6°C/Myr, until the climax of metamorphism.At high structural levels, published data indicate an age 〉 35 Myr for the metamorphic climax. In contrast, a new 39Ar–40Ar step-heating age of 23.8 ± 0.8 Myr on amphibole, from near the base of Peripheral Schieferhülle, closely approximates the age of metamorphism and provides the first clear indication that the climax of metamorphism occurred later at deeper structure levels. Following the climax, near-isothermal uplift and erosion reduced pressure to c. 1 kbar before white mica closure at 19 Myr; this implies uplift at 〉3 mm/yr.Along the tunnel section, white mica K-Ar ages vary systematically from 24 Myr to 16.5 Myr with position relative to a late 4 km amplitude dome whereas biotite Rb-Sr ages are uniform at 16.5 Myr across the whole profile; doming is thus dated at 16.5 Myr with transient uplift rates 〉5 mm/yr. At other times uplift rates were 〈1 mm/yr.

Notes: Abstract Granulites at Fyfe Hills in Enderby Land, Antarctica, crystallized at temperatures in excess of 850°C, and possibly as high as 1000°C, and at pressures of 8-10kbar during the mid to late Archaean. A number of features, including repeated retrograde metamorphism at 5.5-8kbar, retrograde reaction textures, and rimward zoning in pressure sensitive systems, suggest that following peak metamorphism the granulites stabilized at a depth of 18-26 km. After stabilization, the granulites cooled near-isobarically to temperatures of 600-700°C. Assuming a total crustal thickness of 35-40 km during this late Archaean interval of isobaric cooling, the peak metamorphic crustal thickness is estimated at 35-56 km. This estimate is significantly less than the 60-70 km obtained by summing the depths of the present levels of exposure (26-34 km) and the thickness of the crust presently beneath Fyfe Hills (approxi-mately 35km) and is, therefore, consistent with independent evidence for extensive post-Archaean thickening of the Enderby Land crust.

Notes: Abstract A Hercynian charnockite occurs within high-grade gneisses in the Agly Massif, French Pyrenees. Its thermal history has been evaluated using the Fe-Mg distribution coefticient (KD) between garnet and biotite. These minerals have different origins but similar compositions in the charnockites and host gneisses. In the charnockite, the Bi–Ga pairs are the retrograde products of Opx alteration. This Opx reaction with feldspar can be written. Opx + PI + Fluid 1(H2O + Al + K + Fe + Ti) = Bi + Ga + Q + Fluid 2(H2O + Na). The garnets are relatively Ca poor (4–2.5% grossular); they are automorphic and zoned in the gneisses and poikiloblastic in the charnockites. Both types show a retrograde rim (of few hundred microns’width) across which Fe and Mn increase as Mg decreases. The biotites show a good correlation between the octahedral cations (Ti4++ Fe2+) and (Mg2++ Al3+VI); Ti and Fe both increase, whereas Mg and AlVI decrease. There is an inverse linear correlation between Fe2+ and Mg2+ and the Fe/Mg ratio increases as Ti increases. The relation between Ti and KGa-BiDFe-Mg is less clear: it seems that KD slightly decreases as Ti increases. The equilibration temperatures of Ga–Bi pairs are discussed: the charnockite Ga-Bi pairs have equilibrated between 550°C and 600°C; whereas those of the gneisses have equilibrated between 550°C and 650°C. Two main thermal steps appear: one in the gneisses between 600-650°C and a second one in both the gneisses and the charnockites between 550°C and 600°C.

Notes: All along the Himalayan chain an axis of crystalline rocks has been preserved, made of the Higher Himalaya crystalline and the crystalline nappes of the Lesser Himalaya. The salient points of the metamorphism, as deduced from data collected in central Himalaya (central Nepal and Kumaun), are:〈list xml:id="l1" style="custom"〉1The Higher Himalaya crystalline, also called the Tibetan Slab, displays a polymetamorphic history with a first stage of Barrovian type overprinted by a lower pressure and/or higher temperature type metamorphism. The metamorphism is due to quick and quasi-adiabatic uplift of the Tibetan Slab by transport along an MCT ramp, accompanied by thermal refraction effects in the contact zone between the gneisses and their sedimentary cover. The resulting metamorphic pattern is an apparent (diachronic) inverse zonation, with the sillimanite zone above the kyanite zone.2Conversely, the famous inverted zonation of the Lesser Himalaya is basically a primary pattern, acquired during a one-stage prograde metamorphism. Its origin must be related to the thrusting along the MCT, with heat supplied from the overlying hot Tibetan Slab, as shown by synmetamorphic microstructures and the close geometrical relationships between the metamorphic isograds and the thrust.3Thermal equilibrium is reached between units above and below the MCT. Far behind the thrust tip there is good agreement between the maximum temperature attained in the hanging wall and the temperature of the Tibetan Slab during the second metamorphic stage; but closer to the MCT front, the thermal accordance between both sides of the thrust is due to a retrogressive metamorphic episode in the basal part of the Tibetan Slab.

Notes: South of the Main Mantle Thrust in north Pakistan, rocks of the northern edge of the Indian plate were deformed and metamorphosed during the main southward thrusting phase of the Himalayan orogeny. In the Hazara region, between the Indus and Kaghan Valleys, metamorphic grade increases northwards from chlorite zone to sillimanite zone rocks in a typically Barrovian sequence. Metamorphism was largely synchronous with early phases of the deformation. The metamorphic rocks were subsequently imbricated by late north-dipping thrusts, each with higher grade rocks in the hanging wall than in the footwall, such that the metamorphic profile shows an overall tectonic inversion. The rocks of the Hazara region form one of a number of internally imbricated metamorphic blocks stacked, after the metamorphic peak, on top of each other during the late thrusting. This imbrication and stacking represents an early period of post-Himalayan uplift.

Notes: Crustal thickening along the northern margin of the Indian plate, following the 50 Ma collision along the Indus Suture Zone in Ladakh, caused widespread high-temperature, medium-pressure Barrovian facies series metamorphism and anatexis. In the Zanskar Himalaya metamorphic isograds are inverted and structurally telescoped along the Main Central Thrust (MCT) Zone at the base of the High Himalayan slab. Along the Zanskar valley at the top of the slab, isograds are the right way-up and are also telescoped along northeast-dipping normal faults of the Zanskar Shear Zone (ZSZ), which are related to culmination collapse behind the Miocene Himalayan thrust front. Between the MCT and the ZSZ a metamorphic-anatectic core within sillimanite grade rocks contains abundant leucogranite-granite crustal melts of probable Himalayan age. A thermal model based on a crustal-scale cross-section across the Zanskar Himalaya suggests that M1 isograds, developed during early Himalayan Barrovian metamorphism, were overprinted during high-grade MCT-related anatexis and folded around a large-scale recumbent fold developed in the hanging wall of the MCT.

Notes: Abstract In the contact metamorphic aureole of the Tinaroo Batholith (north Queensland, Australia), mylonitic rocks were metamorphosed during a regional folding/crenulation event (D2) synchronous with the emplacement of muscovite-bearing granitoids. Prismatic and skeletal andalusite porphyoblasts grew in carbonaceous schists, mainly from the dissolution of staurolite. Muscovite, quartz and biotite played a dual role in this reaction, acting in a catalytic capacity as well as reactants or products. Staurolite was replaced by coarse-grained muscovite ± biotite, whereas andalusite locally replaced quartz ± muscovite ± biotite, with diffusion of H, Al, Si, Mg, Fe and K ionic species linking sites of dissolution and growth.Graphite contributed to the reaction mechanism in a number of ways. Accumulations of graphite in front of advancing andalusite crystal faces led to skeletal growth and the formation of chiastolite structure, where incremental growth occurred on adjacent {110} faces, with subsequent filling in and inclusion of graphite along the diagonal zones. The presence of graphite in some layers in the schist matrix prevented recrystallization of strained muscovite grains. The muscovite grains in these layers, in contrast to adjacent thin non-graphitic layers, were preferentially replaced by quartz. This resulted in muscovite-depletion haloes in graphitic layers around andalusite porphyroblasts. Somewhat arcuate zones of graphite, concentrated during dissolution of quartz along a crenulation cleavage, occur on some andalusite faces. Reactivation of the mylonitic foliation during the formation of D2 crenulations led to a preferential dissolution of quartz in zones of progressive shearing localized near andalusite porphyroblasts and hence the accumulation of graphite.Lack of deflection of the pre-existing mylonitic foliation and anastomosing of the axial planes of D2 crenulations around andalusite porphyroblasts demonstrate not only the timing of growth, but also that growing porphyroblasts do not push aside existing foliations.

Notes: THE YOUNG EARTH: AN INTRODUCTION TO ARCHAEAN GEOLOGY. By E.G. Nisbet. Allen and Unwin, Boston, 1987. pp. 402.

Notes: Abstract Mica porphyroblasts in schists from several regions show nearly planar inclusion trails that are parallel over areas much larger than the wavelengths of later folds. This indicates that the porphyroblasts have not rotated, with respect to geographical co-ordinates, during deformation. Instead, the matrix has rotated, as suggested by Ramsay (1962). Even in zones of marked shortening in the matrix adjacent to large rigid porphyroblasts (e.g. of cordierite or staurolite), small biotite porphyroblasts have not rotated, but have become thinned by solution, as indicated by parallelism of inclusion trails in separate biotite grains and by evidence of truncation of inclusion trails by the matrix foliation. Less common are biotite porphyroblasts that have single asymmetrical microfolds in the matrix adjacent to the porphyroblasts and so appear to have rotated; these porphyroblasts are characterized by kinking.

Notes: Abstract Two major problems which exist in the use of illite crystallinity to define low-grade metamorphic zones are the variety of values chosen for the zone boundaries and the persistent use of three different indices of crystallinity. Although measurement techniques, which cause much of the interlaboratory variation, can be standardized, it is shown that there is, nevertheless, significant additional variation which demands calibration on standards. The greatest variations are due to choices of different absolute values of crystallinity to define zone boundaries. The problem of relating measurements between different indices is approached by fitting mathematical relationships to pairs of measurements from the same sample. A power–law relationship is a satisfactory fit to the Kubler–Weaver and Weaver–Weber pairs, while the Kubler–Weber indices are linearly related. These relationships are used to transform definitions of the diagenetic zone, anchizone and epizone from one index to the others, although they apply strictly only to the data set from which they are derived. This results in compatibility between the three zones and shows that previous definitions to the anchizone in different indices have been chosen at incompatible values. The boundaries of Kubler's anchizone (0.42 and 0.25 Δ2θ) are 0.4 and 0.215 Δ2θ in this study, which become 5.1 and 14.6 in the Weaver index and 278 and 149 in the Weber index. An error analysis shows that percentage errors in both Kubler and Weaver indices increase with crystallinity; the Kubler measurements are marginally preferred at all grades.

Notes: Abstract Migmatites in the Quetico Metasedimentary Belt contain two types of leucosome: (1) Layer-parallel leucosomes that grew during deformation and prograde metamorphism. These are enriched in SiO2, Sr, and Eu, but depleted in TiO2, Fe2O3, MgO, Cs, Rb, REE, Sc, Th, Zr, and Hf relative to the Quetico metasediments. (2) Discordant leucosomes that formed after the regional folding events when metamorphic temperatures were at their peak. These are enriched in Rb, Ba, Sr and Eu, but display a wide range of LREE, Th, Zr, and Hf contents relative to the Quetico metasediments.Layer-parallel leucosomes formed by a subsolidus process termed tectonic segregation. This stress-induced mass transfer process began when the Quetico sediments were deformed during burial, and continued whilst the rocks were both stressed and heterogeneous. Subsolidus leucosome compositions are consistent with the mobilization of quartz and feldspar from the host rocks by pressure solution. The discordant leucosomes formed by partial melting of the Quetico metasediments, possibly during uplift of the belt. The range of composition displayed by the anatectic leucosomes arises from crystal fractionation during leucosome emplacement. Some anatectic leucosomes preserve primary melt compositions and have smooth REE patterns, but those with negative Eu anomalies represent fractionated melts, and others with positive Eu anomalies represent accumulations of feldspar plus trapped melt.

Notes: Abstract The effects of varying amounts of partial melt on the deformation of granitic aggregates have been tested experimentally at conditions (900°C, 1500 MPa, 10-4 to 10-6/s) where melt-free samples deform by dislocation creep, with microstructures approximately equivalent to those of upper greenschist facies. Experiments were performed on samples of various grain sizes, including an aplite (150 μm) and sintered aggregates of quartz-albitemicrocline (10–50 and 2–10 μm). Water was added to the samples to obtain various amounts of melt (1–15% in the aplite, 1–5% in the sintered aggregates). Optical and TEM observations of the melt distribution in hydrostatically annealed samples show that the melt in the sintered aggregates is homogeneously distributed along an interconnected network of triple junction channels, while the melt in the aplites is inhomogeneously distributed.The effect of partial melt on deformation depends an melt amount and distribution, grain size and strain rate. For samples deformed with ˜ 1% melt, all grain sizes exhibit microstructures indicative of dislocation creep. For samples deformed with 3–5% melt, the 150 μm and 10–50 μm grain size samples also exhibit dislocation creep microstructures, but the 2–10 μm grain size samples exhibit abundant TEM-scale evidence of dissolution-precipitation and little evidence of dislocation activity, suggesting a switch in deformation mechanism to predominantly melt-enhanced diffusion creep. At natural strain rates melt-enhanced diffusion creep would predominate at larger grain sizes, although probably not for most coarse-grained granites.The effects of melt percentage and strain rate have been studied for the 150 μm aplites. For samples with ˜ 5 and 10% melt, deformation at 10–6/s squeezes excess melt out of the central compressed region allowing predominantly dislocation creep. Conversely, deformation at 10-5/s produces considerable cataclasis presumably because the excess melt cannot flow laterally fast enough and a high pore fluid pressure results. For samples with 15% melt, deformation at both strain rates produces cataclasis, presumably because the inhomogeneous melt distribution resulted in regions of decoupled grains, which would produce high stress concentrations at point contacts. At natural strain rates there should be little or no cataclasis if an equilibrium melt texture exists and if the melt can flow as fast as the imposed strain rate. However, if the melt is confined and cannot migrate, a high pore fluid pressure should promote brittle deformation.

Notes: Abstract Crystal-chemical relationships between coexisting sodic and calcic amphiboles have been studied in eclogitic metagabbros from the Aosta Valley, Western Alps. Textural analysis gives evidence of three successive high-pressure parageneses:1. Pre-kinematic high-grade blueschist assemblages, preserved as polymineralic inclusions in garnet cores and made of glaucophane and actinolite (stage A).2. Synkinematic eclogite assemblages, composed of garnet + omphacite + glaucophane ± actinolite ± white mica ° Clinozoisite + quartz + rutile (stage B).3. Post-kinematic epitactic overgrowths of barroisitic amphibole on glaucophane and actinolite (stage C). P–T conditions of the eclogitic metamorphism have been estimated at around 500–550°C, 16 kbar.Glaucophane and actinolite coexist as discrete grains in stage A and B assemblages. This texture and the chemistry of the amphiboles unambiguously denotes the existence of a miscibility gap between sodic and calcic amphiboles (from NaM4= 0.80 in actinolite to NaM4= 1.70 in glaucophane at T= 500–550°C). A comparison with published analyses allows a new solvus along the glaucophane–actinolite join to be drawn.The later barroisitic amphibole (stage C) exhibits strong chemical zonation indicating disequilibrium growth. This amphibole cannot either be used to define a miscibility gap with glaucophane or actinolite or be considered as an intermediate stage between these two end-members.

Notes: Abstract Garnet granulites from Sri Lanka preserve textural and chemical evidence for prograde equilibration at temperatures of at least 700–750°C and pressures in the vicinity of 6–8 kbar. Associated strain patterns suggest prograde metamorphism occurred during and immediately following an episode of crustal thickening, with the prograde P–T conditions probably reflecting a combination of the conductive and advective transport of heat at the mid-levels of tectonically thickened crust. The occurrence of prograde wollastonite provides evidence for internally buffered fluid compositions, or fluid absent conditions, during peak metamorphism and precludes pervasive advection of a CO2-rich fluid. The advective heat component is therefore likely to have been provided by the transport of silicate melt. Intricate symplectitic textures record partial re-equilibration of the garnet granulites to lower pressures (˜ 4–6 kbar) at high temperatures (600–750°C), and testify either to the erosional denudation of the overthick crust prior to significant cooling (i.e. quasi-isothermal decompression) or to a subsequent static heating possibly of early Palaeozoic age (Pan-African). The metamorphic history of the Sri Lankan granulites is compared with high grade terrains in the neighbouring fragments of Gondwana, with the emphasis on similarities with Proterozoic granulites of the East Antarctic craton.

Notes: TECTONIC SETTINGS OF REGIONAL METAMORPHISM. Edited by E.R. Oxburgh, B.W.D. Yardley and P.C. England. Royal Society of London. 1987.

Notes: Abstract This paper provides methods and a description of a Pascal computer program, thermocalc, for various thermodynamic calculations using the thermodynamic dataset presented in earlier papers in this series (Holland & Powell, 1985; Powell & Holland, 1985). The dataset involves uncertainties on the thermodynamic parameters and therefore allows uncertainties to be calculated on results, for example in geothermometry and geobarometry. Recommendations are made for the uncertainties on activities to be used in calculations on rocks, particular emphasis being placed on preventing underestimates of these uncertainties at small mole fractions. Apposite examples of phase diagram and rock calculations are presented with ouput from thermocalc, demonstrating the utility of the program. Of the rock calculations, the most valuable are considered to be those involving simultaneous combination ‘least squares’of calculated conditions for a set of reactions applicable to a rock. This set of reactions involves the independent reactions which can be written between the end-members in the minerals in a rock and in the thermodynamic dataset. In contrast to an approach based on specific geothermometers and geobarometers, this approach maximizes the benefit of having an internally consistent thermodynamic dataset. thermocalc is available in IBM PC and Mac versions, from Roger Powell for A$25 or Tim Holland for £10 per version.

Notes: Abstract An inverted metamorphic gradient associated with the northern mylonite zone of the Cheyenne belt, a deeply eroded Precambrian suture in southern Wyoming, has been documented within metasedimentary rocks of the Early Proterozoic Snowy Pass Supergroup. Metamorphic grade in the steeply dipping supracrustal sequence increases from the chlorite through the biotite, garnet, and staurolite zones both stratigraphically and structurally upward toward the northern mylonite zone. A minimum temperature increase of approximately 100° C over a km-wide zone is required for this transition. Parallelism of inverted isograds with the trace of the northern mylonite zone implies a genetic relationship between deformation associated with that zone and the inverted metamorphic gradient within the Snowy Pass Supergroup.Field evidence together with microstructural and petrofabric analysis indicate northward thrusting of amphibolite-grade rocks over rocks of the Snowy Pass Supergroup along the northern mylonite zone. Mineral equilibria and garnet-biotite geothermometry on synkinematic mineral assemblages within the Snowy Pass metasedimentary rocks indicate deformation at minimum temperatures of 480° C and pressures of 350–400 MPa (3°5–4°0 kbar). This implies tectonic burial or upper plate thickness of 13–15 km.The narrow character of metamorphic zonation and microtextures within the Snowy Pass Supergroup which indicate late synkine-matic growth of garnet and staurolite, preclude rotation of pre-existing isograds by folding as a mechanism for development of the inverted gradient. Conductive transport of heat from the upper into the lower plate across the originally low-angle thrust is insufficient to produce the necessary temperatures in the lower plate. Shear heating is considered insufficient to produce the observed metamorphic transition unless high shear stresses are postulated. Up-dip advection of metamorphic fluids is a feasible, but unproven, mechanism for heat transport. The possibility that rapid uplift due to stacking of several thrust sheets may have played a role in preserving the inverted metamorphic gradient cannot be evaluated at present.

Notes: Abstract Partial melting of tonalitic gneisses in the 2.7 Ga Badcallian granulite facies metamorphic episode in the Scourian complex of north-west Scotland produced a suite of granitic to trondhjemitic liquids. On cooling and excavation of the complex, these melts underwent fractional crystallization and the residual liquids eventually became water saturated. Comparison with experimental data suggests that water saturation would have occurred in these melts at around 620–700°C. From the retrograde P–T-time path followed by the complex it is estimated that H2O-dominated fluids were exsolved from these melts at c. 2.5 Ga. It is proposed that these fluids were the cause of the 2.5 Ga Inverian retrogression of the Scourian complex and that water-saturated melts formed during the crystallization of the leucogneisses were intruded as a suite of pegmatites. The timing of pegmatite intrusion is consistent with this proposition as are the temperature estimates, timing, distribution and nature of the Inverian phase of metamorphism. It is likely that the crystallization of melts is an important process in bringing about hydrous retrogressive metamorphic episodes in a number of other basement terrains, such as West Greenland and Australia.

Notes: Abstract The Cretaceous-Eocene basic to intermediate marine volcanic rocks of the Mucuchi Formation constitute the Western Cordillera in northern Ecuador. Their chemical features mostly correspond to those of tholeiitic basalts with some calc-alkaline affinities and suggest an oceanic island arc setting. The Macuchi rocks are affected by low-grade, non-deformative metamorphism, characterized by zeolite, prehnite-pumpellyite and lower greenschist facies assemblages. Depth-zonation is suggested by the downward mineral sequence: (i) laumontite+ (pistacitic epidote, pumpellyite + prehnite); (ii) pumpellyite+ prehnite + pistacitic epidote; (iii) actinolite+biotite+ pistacitic epidote + chlorite. This broad zonation and the chemistry of individual minerals point to an interaction between the volcanic rocks and sea-water under a moderate to high thermal gradient (= 75° C/km?). Alteration appears to have been dependent primarily on fluid control (volume, pressure, composition), temperature and reaction kinetics which together partly overshadow the role of load-pressure. Compositional variations of a mineral species at the scale of a contiguous flow or even at the scale of a thin section show that intensity of alteration was spatially uneven depending on rock permeability and consequently, metastable equilibrium commonly exists. However, a progressive approximation to equilibrium as a result of P–T control is shown by the mineralogy. A high fo2 of the fluid phase is evident from the mineral chemistry. The metamorphism of the Macuchi volcanics is similar to the hydrothermal-burial type produced during the development of a volcanic arc where lavas and volcanoclastics accumulated in a shallow marine environment. However, some of its characteristics point to a transition toward systems defined by a higher T/P ratio such as those found in ocean-floor metamorphism.A model is proposed in which the Macuchi volcanics are assigned to an oceanic island arc generated contemporaneously with a marginal basin which has opened as the outcome of progressive north-south attenuation of the continental crust due to mantle diapirism.

Notes: Abstract Partitioning of Fe and Mg between garnet and phengitic muscovite was calibrated as a geothermometer by Green & Hellman (1982) using experimental data at 25–30 kbar. When the thermometer is applied to pelites regionally metamorphosed at pressures of between 3 and 7 kbar it yields temperatures much higher than those from the garnet–biotite thermometer. A new empirical calibration is proposed for use with such rocks, with particular application where garnet occurs at lower grades than biotite. The new calibration is where K is given by: In K= In Kd and Xii are mole fractions in the garnets.The calibration was derived from comparison with the garnet–biotite thermometer of Ferry & Spear (1978), assuming no pressure-dependence for the partitioning between garnet and muscovite, no ferric iron partitioning, ideal mixing in muscovite, and the garnet mixing model of Ganguly & Saxena (1984) modified for a non-linear Ca effect. This latter garnet mixing model was selected because it gave the geologically most reasonable results. It has not proved possible to distinguish a pressure effect from a ferric-iron effect.Despite the simplifying assumptions used to derive the calibration, it yields temperatures generally within 15°C of those given by the garnet–biotite thermometer, and has been used to supply thermometric data in a low-grade region of the Canadian Rockies.

Notes: Abstract Mineral assemblages in different samples of amphibolite facies pelitic schists collected from two separate outcrops in the Moosilauke area, NH, record differences in the chemical potential of water during metamorphism. Mineralogical, petrological, and field relations indicate that mineral assemblages at both outcrops equilibrated at 520°C and 3.5–4.0 kbar. Thermodynamic analysis of the mineral assemblages demonstrates that maximum chemical potential differences at each outcrop were of the order of 150 calories, over distances of 10–20 m.The differences in the chemical potential of water recorded in both bed-to-bed and outcrop-to-outcrop relations are consistent with the following conclusions: (1) mineral assemblages on a specific outcrop did not equilibrate with an external reservoir of fluid of fixed composition, (2) the relatively small magnitude of the chemical potential differences suggests little or no infiltration of externally derived fluid, (3) these differences on the outcrop scale are probably related to initial compositional variations and the buffer capacity of the mineral assemblage, and (4) the different values of the chemical potential of water exhibited by the various mineral assemblages permits an understanding of the effects of variable μH2O for amphibolite facies pelitic schists.

Notes: Abstract A detailed study of garnet–chloritoid micaschists fom the Sesia zone (Western Alps) is used to constrain phase relations in high pressure (HP) metapelitic rocks. In addition to quartz, phengite, paragonite and rutile, the micaschists display two distinct parageneses, namely garnet + chloritoid + chlorite and garnet + chloritoid + kyanite. Talc has never been observed. Garnet and chloritoid are more magnesian when chlorite is present instead of kyanite. The distinction of the two equilibria results from different bulk rock chemistries, not from P–T conditions or redox state. Estimated P–T conditions for the eclogitic metamorphism are 550–600°C, 15–18 kbar.The presence of primary chlorite in association with garnet and chloritoid leads us to construct two possible AFM topologies for the Sesia metapelites. The paper describes a KFMASH multisystem for HP pelitic rocks, which extends the grid of Harte & Hudson (1979) towards higher pressures and adds the phase talc. Observed parageneses in HP metapelites are consistent with predicted phase relations. Critical associations are Gt–Ctd–Chl and Gt–Ctd–Ky at relatively low temperatures and Gl–Chl–Ky and Gt–Tc–Ky at relatively high temperatures.

Notes: Abstract Low-pressure prograde metamorphism of pelitic rocks in the Cooma Complex, south-east Australia, has produced cordierite-andalusite schists at intermediate grades. The first foliation (S1) is preserved largely as inclusion trails in cordierite porphyroblasts. Microstructural evidence indicates that the cordierite porphyroblasts grew during the early stages of development of a crenulation-foliation (S2) and that andalusite porphyroblasts grew during the development of a later crenulation-foliation (S3). Microstructural evidence also indicates that the andalusite was a product of the prograde reaction: cordierite + muscovite ± andalusite + biotite + quartz. The occurrence of the products of this reaction in ‘beard’structures between cordierite microboudins formed by extension in S3 confirms that the andalusite grew during the development of S3. The investigation shows that porphyroblast-matrix relationships can preserve the orientation of an early S-surface that has been largely obliterated from the matrix, as well as providing relatively direct evidence of sequential mineral growth and metamorphic reactions.

Notes: Abstract Several small bodies of metabasite (maximum dimensions of 1000 m x 500 m) are included in the metamorphic rocks of the Nevado-Filabride Complex in the Betic Cordilleras (Almeria Region). The body of 400 m x 100 m, located 200 m due west of the Lubrin village, contains troctolitic gabbro with well-preserved igneous textures and mineral compositions, wholly amphibolitized gabbro, garnet-bearing metagabbro eclogite. Along with the textural and mineral changes, sensible and regular geochemical variations can be observed, where the content of MgO decreases from 24% to 11%, while that of CaO and Na2O increases from 7% to 11% and from 2% to 3%, respectively. In addition, the content of some minor elements such as Sr, Y, Nb, Zr and Sc increases while that of Ni and Cr decreases from troctolitic gabbro to the eclogite. The amphibolitized gabbro shows values scattered around those of the troctolitic gabbro. These geochemical variations are ascribed to inherited differences in the pre-metamorphic protolith, i.e. a fractionated gabbro which varies from olivine-rich to clinopyroxene-rich gabbro. Nevertheless, some metasomatism affected the Lubrin body without changing the main chemical trends, as documented by the significantly different 87Sr/86Sr ratios of each rock-type. This points to a metasomatism which involved the introduction of crustal radiogenic strontium. The petrographical and mineral chemical features are interpreted to be the result of syn-metamorphic fluid circulation possibly combined with deformation by shearing. The igneous texture and mineral chemistry have been retained wherever both fluid circulation and shearing were ineffective. On the contrary, where both events were effective, the formation of eclogite occurred. Later, the entire body underwent a retrogressive amphi-bolitic stage under greenschist facies conditions, which was probably responsible for the formation of the amphibolitized gabbro portion and for the retrogression of the eclogite.

Notes: Abstract Scapolite, wollastonite, calcite, diopside, grossular-andradite garnet and sphene occur in calc-silicate rocks in the granulite terrain of the Arunta Block, central Australia. This assemblage buffers the CO2 activity at a low value, so that any coexisting fluid phase must be H2O rich and CO2 poor (Xco2= 0.2-0.3). In contrast, the H2O activity in the surrounding felsic and mafic granulites was low. Thus fluid activities during granulite facies metamorphism were locally buffered in various rock units and fluid flow appears to have been restricted or fluid may have been absent. Late retrograde rims of garnet and garnet-quartz separate phases formed in the high-grade stage. Formation of these rims would have required either an influx of water-rich fluid or a decrease in pressure. Evidence from the surrounding granulites shows that in one locality, the calc-silicate rocks had undergone late isobaric hydration; in another locality, minor uplift had occurred soon after peak P-T conditions. In both, scapolite had partly broken down to plagioclase-calite. A calc silicate rock from the granulite terrain of Enderby Land, Antarctica, contains scapolite, wollastonite, calcite, diopside, quartz and sphene; this assemblage also indicates low CO2 activities. In this rock, wollastonite has broken down to calcite-quartz, to indicate isobaric cooling without influx of hydrous fluid.

Notes: Abstract Chloritoid-bearing metasedimentary rocks occur in close proximity to blueschists and eclogites in the Tertiary high-pressure metamorphic belt of northern New Caledonia. The typical assemblage of chloritoid-bearing rocks in the epidote zone is quartzchlorite-muscovite-garnet-chloritoid. In the omphacite zone, epidote is an additional member of the chloritoid-bearing assemblage. Paragonite is rare, plagioclase was not detected, and rutile and ilmenite are the Fe-Ti oxide phases. Chloritoid-glaucophane is not a common assemblage. Chloritoid-bearing rocks have relatively low (Ca+K+Na)/Al ratios and the chloritoids are relatively Mg-rich with Mg/ (Mg+Fe) up to about 0.4. A comparison of the mineral assemblages and mineral chemistry with experimental and computed phase equilibria suggest an upper temperature limit near 560° C in the omphacite zone and a minimum temperature limit near 450° C at 10 kbar. An empirical garnet-chlorite Fe-Mg exchange thermometer does not yield consistent results for the higher-grade rocks, suggesting Ts ranging from 390 to 535° C in the omphacite zone and 420–465° C in the epidote zone. The distribution coefficient KD= (Fe/Mg)ctd/(Fe/Mg)chl for chloritoid and chlorite ranges from 3.9 to 6.4, values which are lower than those (=10) from lower greenschist facies rocks, but are near those of upper greenschist facies and albite-epidote amphibolite facies.

Notes: Abstract Considering the minerals cordierite (Cd), sapphirine (Sa), hypersthene (Hy), garnet (Ga), spinel (Sp), sillimanite (Si) and corundum (Co) in the system FeO-MgO-Al2O3-SiO2 (FMAS), the stable invariant points are [Co], [Ga], [Cd] and [Sa]. Constraints imposed by experimental data for the system MAS indicate that under low PH2o conditions the invariant points occur at high temperature (〉 900° C) and intermediate pressure (7-10 kbar). This temperature is higher than that commonly advocated for granulite facies metamorphism. In granulites Fe-Mg exchange geothermometers may yield temperatures of 100–150° C below peak metamorphic conditions and evidence for peak temperatures is best preserved by relict high-temperature assemblages and by Al-rich cores in orthopyroxene. Application of the FMAS grid to some well-documented granulite occurrences introduces important constraints on their P-T histories. Rocks of different bulk compositions, occurring in close proximity in the field, may record distinct segments of their P-T paths. This applies particularly to rocks with evidence for reaction in the form of coronas, symplectites and zoned minerals. Consideration of curved reaction boundaries and XMs isopleths may explain apparently contradictory results for the stability of cordierite obtained from low-temperature experiments and thermochemical calculations on the one hand and hightemperature experimental data on the other.

Notes: Abstract The Bergen-Jotun kindred rocks of this study, the Storådalen Complex (SCX), Svartdalen Gneiss (SG) and Mjølkedøla Purple Gabbro (MPG), have been shown to be a co-magmatic series with calc-alkaline affinities. The analyses of Ba, Nb, Y, and Zr presented here show no variation in these elements between the three rock units and are consistent with the calc-alkaline character of the rocks. The lithophile elements Ba, K, and Sr are enriched relative to MORB and the high field strength elements Nb, Y, and Zr are depleted relative to MORB, Zr especially so.The SCX contains rocks with low (〉30) differentiation indices which are interpreted as plagioclase + pyroxene ± olivine ± amphibole cumulates. The remainder of the SCX, together with the MPG and SG, is regarded as the congealed liquid in equilibrium with these cumulates. The distribution of trace elements between these two components of the SCX can be adequately modelled using a Rayleigh fractionation process, measured ‘liquid’compositions, and calculated bulk distribution coefficients. It is thus concluded that the trace element geochemistry of the rocks of this study is consistent with subduction-related, mantle-derived magmas that fractionate within a continental or mature island arc environment. Subsequent high-grade metamorphism and deformation of Sveconorwegian age have been essentially isochemical.

Notes: Abstract High-pressure granulite-facies gneisses in the NE Ox inlier in NW Ireland have undergone extensive Caledonian retrogression. In the local area of Slishwood, however, reworking was negligible and the gneisses (psammites, semipelites, pelites, metabasites and ultramafites) preserve evidence of P–T changes at high grade which mainly post-date pre-Caledonian polyphase deformation. Temperatures reached 850–900°C (based on garnet-clinopyroxene geothermometry and the presence of mesoperthite) during and after decompression from earlier eclogite-facies conditions (inferred from textural evidence of plagioclase release in sieve-textured augite). Subsequent cooling at high pressure is inferred from the unequivocal replacement of sillimanite by kyanite.A Sm–Nd mineral isochron (gt–cpx–plag–WR) of 605 ± 37 Ma is taken to date a point on the cooling path, and confirms the hitherto suspected pre-Caledonian age of the high-grade metamorphism. Geochemical and Sm–Nd isotopic data indicate that the protoliths were probably late Proterozoic arkosic sediments and tholeiites. Following metamorphism they apparently came to reside near the base of the crust where they slowly cooled. The eventual exhumation of these gneisses is attributed to Caledonian crustal imbrication, followed by rapid isostatic recovery.

Notes: Abstract Sphalerite geobarometry has long been known to give poor results when applied to regional metamorphic terranes. Application of the sphalerite geobarometer to three low-to medium-grade sulphide deposits—the Moke Creek and Waitahuna deposits, Otago, and the Goose Cove deposit, Newfoundland—yields pressures up to 9 kbar, which appear to be too high when compared with other geological data. Textural and mineralogical relationships suggest that the Goose Cove and, possibly, the Moke Creek deposits lacked the required equilibrium assemblage (pyrite + hexagonal pyrrhotite + sphalerite) during peak metamorphic conditions, rendering the geobarometer inapplicable. In addition, all three deposits show evidence of re-equilibration at T 〈 300°C, which has resulted in decreased FeS contents and high apparent pressures. Analyses of sphalerites from very low-grade metachert from South Georgia Island, which contains the assemblage sphalerite + pyrite + monoclinic pyrrhotite + chalcopyrite, confirm that low-temperature equilibration of this assemblage results in approximately 10–11 mol. % FeS in sphalerite. Comparison of these results with published descriptions of other deposits suggests that lack of the appropriate assemblage and retrograde re-equilibration of sphalerite probably account for most anomalously high-pressure estimates. Erratic compositions of sphalerites containing chalcopyrite inclusions may result from replacement of high-temperature intermediate solid-solution by chalcopyrite during cooling. Strain may enhance retrograde re-equilibration of sphalerite by grain-size reduction or dislocation-assisted diffusion and/or nucleation. Re-evaluation of the data from Moke Creek suggests that the sulphides experienced pervasive greenschist facies re-equilibration at pressures of about 4.5 kbar, with late stage mobilization at about 2.8 kbar, and thus sphalerite compositions are not likely to reflect blueschist facies conditions. Pressure estimates based on sphalerite geobarometry should take into account at what stage in the history of a metamorphic terrane the sphalerite composition equilibrated.

Notes: Abstract The magnitudes of plastic strains of 104 metacherts were determined from the deformed shape of initially spherical radiolarians in the Sambagawa high-P type metamorphic belt of Western Shikoku, Japan. The strain magnitude increases with increasing metamorphic temperature from several per cent to 250%. The a2/a3 ratio of strain ellipsoids in the higher metamorphic grades decreases with increasing metamorphic grade while the a1/a2 ratio increases rapidly. The long axis of the strain ellipsoid for every grade is nearly parallel to the length of the metamorphic belt, suggesting that the flow direction of the synmetamorphic deformation was uniform along the belt. A map of strain zones within the Sambagawa high-P type metamorphic belt reveals that the metamorphic belt underwent a progressive bulk inhomogeneous shear deformation and that the high-grade zones represent a deep-seated boundary shear zone on the accretionary wedge between a subducting oceanic plate and the immobile rigid continental plate.

Notes: Abstract Compositions of actinolite, hornblende and cummingtonite, together with pyroxene and plagioclase, are studied in basic intrusions in the Dalradian of north-east Scotland, and the Glen Scaddle complex in the West Moine. Amphibolitization is due to influx of water from the country rocks. Pyroxene compositions are found to have adjusted to the regional metamorphic environment. Owing to the difficulty of diffusion of Al and Si, calcic amphiboles are zoned and commonly contain quartz blebs. Discontinuities in zoning give rise to actinolite-hornblende pairs. Compared with north-east Scotland, disequilibrium is less strong in the Glen Scaddle area: in the latter, plagioclase compositions have been greatly changed, Na partition between hornblende and plagioclase is close to equilibrium, the maximum Al content of hornblende is lower and zoning patterns are more consistent. The Fe/Mg ratio in calcic amphiboles varies with Al content, while approaching equilibrium partition with other minerals. Both zoning patterns and Fe/Mg partition with cummingtonite suggest that Fe/Mg of the calcic amphiboles increases more strongly with increasing (Alvi+Fe3+) than can be explained simply by substitution of Al,Fe3+ for Mg on M2. Model reactions for amphibole formation are constructed. Cummingtonite formed at lower chemical potential of CaO than actinolite: Ca was exchanged for Mg,Fe between orthopyroxene-derived and clinopyroxene-derived local systems. Both cummingtonite and actinolite were formed because of kinetic constraints, as intermediate reaction products: actinolite-hornblende pairs represent disequilibrium. This work suggests that many occurrences of actinolite with hornblende, where the minerals are zoned, may also be due to diffusion kinetics.

Notes: Abstract Lewisian grey gneisses from Gruinard Bay, North-west Scotland retain mineralogical and geochemical evidence for Scourian horn-blende-granulite facies metamorphism, and they may be used to assess current models of elemental depletion at granulite grade. Their ‘immobile’major and trace element geochemistry is indistinguishable from that of Lewisian amphibolite and pyroxene-granulite facies counterparts. The K, Rb, Th and U contents of the Gruinard Bay gneisses are depleted relative to amphibolite facies gneisses, but generally the abundances of these elements are above those of comparable pyroxene granulites. U and Th have reached an advanced stage of depletion, but allanite appears to be crucial in maintaining significantly higher U and Th abundances at Gruinard Bay than in pyroxene granulites. K and Rb loss is less extreme, and depends on the stability of the rock-forming minerals: K-feldspar; biotite; and, amphibole. Early removal of K and Rb has resulted in a small rise in K/Rb, but further preferential Rb loss would have been required to generate the characteristically high K/Rb ratios of Lewisian pyroxene granulites.The residence of U and Th in the accessory minerals of granulite facies gneisses, which are often correlated with the residua of intracrustal partial melting, renders unlikely their extreme incompatibility required by such models. Even if such phases are ignored, high mineral-melt partition coefficients for silicic melts argue against partial fusion as an efficient depletion mechanism. On the other hand, the advanced stage of U and Th depletion reached in Gruinard Bay gneisses, which were still partly hydrous, severely restricts the role played by CO2-dominated fluids and a hydrous medium is preferred.

Notes: Abstract Oxygen and hydrogen isotope analyses have been made of coexisting quartz, ilmenite, muscovite, and biotite from Late Precambrian metapelitic rocks, staurolite-kyanite to K-feldspar-muscovite-sillimanite zones, from Mica Creek, British Columbia. The δ18O and †D values of these minerals are generally uniform and do not decrease significantly with increasing metamorphic grade. This implies that there has not been significant infiltration of deep crustal, possibly magmatic, fluids into the metapelites that has been suggested for other high-grade metamorphic terranes. The uniformity of oxygen isotope compositions of the Mica Creek metapelite rocks may reflect isotopic uniformity in the sedimentary protolith rather than widespread exchange with an isotopically homogeneous metamorphic pore fluid.Temperature estimates based upon 18O exchange thermometry for samples below the sillimanite zone are in reasonable agreement with the results of garnet-biotite Fe–Mg exchange thermometry. In the higher grade rocks, the oxygen isotope and garnet-biotite thermometry yield results which disagree by about 100°C. The highest temperatures recorded by oxygen isotope thermometry, 595°C, are at least 60°C below the minimum temperatures required by phase equilibria. These discrepancies appear to result from pervasive equilibrium retrograde exchange of oxygen isotopes between coexisting minerals. In addition, there are problems with calibration of garnet-biotite thermometry at higher temperatures. Retrograde oxygen isotope exchange may be a general characteristic of high-grade metamorphic rocks and oxygen isotope thermometry may not usually record peak metamorphic temperatures if they significantly exceed 600°C.

Notes: Abstract Mctamorphic rocks of the St Anthony Complex of north-western Newfoundland are best interpreted in terms of a high-temperature shear zone formed between down-going continental margin rocks and overriding oceanic lithosphere in a subduction zone. High-grade rocks, immediately beneath the oceanic lithosphere peridotite, display retrograde meta-morphism in high-strain zones, whereas lower grade rocks, near the base of the metamorphic complex, display prograde metamorphism in high-strain zones. Mylonite zones in meta-basitcs at all levels in the complex contain the assemblage epidote-hornblende-albite-sodic oligoclase. These observations suggest that the ‘inverted metamorphic gradient’within the St Anthony Complex results from the fortuitous preservation of residual metamorphic assemblages from different crustal levels within an epidote amphibolite facies shear zone. The degree of re-equilibration is strongly dependent on the degree of strain, and is best achieved in synmetamorphic mylonite zones. This interpretation of the St Anthony Complex can be extended to other sub-ophiolite metamorphic sheets, which show very similar relationships. It is proposed that most metamorphic sheets beneath ophiolites are high temperature shear zones, the P-T paths of which preserve records of burial and exhumation in subduction zones.

Notes: The equilibrium thermodynamics of the reaction:And the equilibrium constant is composed of activities formulated using ideal mixing on sites. Consideration is given to the evaluation of uncertainties in pressures calculated using the geobarometer. Preliminary testing suggests that the geobarometer has considerable potential. Much wider testing is now required.

Notes: Abstract. Pink piemontite-spessartine-bearing and grey-green spessartine-bearing manganiferous quartzose schists derived from siliceous pelagites, and green quartzofeldspathic schists, are described from the greenschist facies of the Haast Schist terrane, near Arrow Junction, western Otago. Electron microprobe data are reported for sphene, spessartine-rich garnet, manganoan epidote, piemontite, tourmaline, phengitic muscovite, chlorite, albite, haematite, rutile, manganoan calcite and chalcopyrite.Metamorphism occurred at about 6.4kbar, 400°C. Xco2 was above the quartz-rutile-calcite-sphene buffer (Xco2± 0.02) throughout the recorded metamorphic history of the piemontite schists. It dropped from above to below this critical buffering value in a spessartine-rich schist and it was close to or below the buffering value in the quartzofeldspathic schists. Production of piemontite required high fO2, believed to be inherited from MnOx in the parent pelagite. Substantial loss of O2 (e.g. minimum of 0.19% by weight in one rock) during diagenesis and/or metamorphism is inferred. In the grey-green schists this inhibited piemontite formation. Slight loss of O2 and Ca2+ accompanied minor late-stage replacement of piemontite by second generation spessartine. Observed zoning and mineral replacements indicate rise of temperature, drop in pressure, or invasion by solutions of lower fO2 and XCO2 equilibrated with surrounding schists.The detailed chemistry of the minerals studied correlates with available Mn and with bulk-rock (Fe3+ x 100)/(Fe2++ Fe3+). The oxidation ratio ranges from 24 in average green quartzofeldspathic schist, through 78 in average grey-green manganiferous quartzose schist, to almost 100 in some piemontite-bearing schists. As Fe2+ gives way to Fe3+, Mg/Fe ratios tend to rise in chlorite, phengite, tourmaline, spessartine, and calcite, Mn increases and Ti decreases in haematite, Mn increases in spessartine and calcite, and Fe increases in rutile. Available divalent cations are depleted relative to Al; chlorite is more aluminous, and phengite more paragonitic than in typical Haast schists.

Notes: Andalusite porphyroblasts are totally pseudomorphosed by margarite–paragonite aggregates in aluminous pelites containing the peak mineral assemblage andalusite, chlorite, chloritoid, margarite, paragonite, quartz ± garnet, in a NW Iberia contact area. Equilibria at low P–T are investigated using new KFMASH and (mainly) MnCNKFMASH grids constructed with Thermocalc 3.21. P–T and T–X pseudosections with phase modal volume isopleths are constructed for compositions relatively richer and poorer in andalusite to model the assemblages in an andalusite-bearing rock that contains a thin andalusite-rich band (ARB) during retrogression. Their compositions, prior to retrogression, are used in the modelling, and have been retrieved by restoring the pseudomorph-forming elements into the current-depleted matrix, except for Al2O3 which is assumed to be immobile. Compositional differences between the thin band and the rest of the rock have not resulted in differences in andalusite porphyroblast retrogression. The absence of chloritoid resorbtion implies either a pressure increase at constant reacting-system composition, or that its composition changed during retrogression at constant pressure, by becoming enriched in the progressively replaced andalusite porphyroblasts. T–X pseudosections at 1 kbar model this latter process using as end-members in X, first, the restored original rock and ARB compositions, and, then the same process, taking into account the change in composition of both as retrogression proceeded. The MnNCKFMASH pseudosections of rocks with different Al contents facilitate making further deductions on the rock-composition control of the resulting assemblages upon retrogression. Andalusite eventually disappears in relatively Al-poor rocks, resulting, as in this study, in a rock formed by chloritoid–chlorite as the only FM minerals, plus margarite–paragonite pseudomorphs of andalusite. In rocks richer in Al, chlorite would progressively disappear and a kyanite/andalusite–chloritoid assemblage would eventually be stable at retrograde conditions. The Al-silicate, stable during retrogression in Al-rich rocks, indicates pressure conditions and hence the tectonic context under which retrogression took place.

Notes: In statistically optimised P–T estimation, the contributions to overall uncertainty from different sources are represented by ellipses. One source, for a diffusion-controlled reaction at non-equilibrium, is diffusion modelling of the reaction texture. This modelling is used to estimate ratios, Q, between free-energy differences, ΔG, of reactions among mineral end-members, to replace the equilibrium condition ΔG = 0. The associated uncertainty is compared with those already inherent in the equilibrium case (from end-member data, activity models and mineral compositions). A compact matrix formulation is introduced for activity coefficients, and their partial derivatives governing error propagation. The non-equilibrium example studied is a corona reaction with the assemblage Grt–Opx–Cpx–Pl–Qtz. Two garnet compositions are used, from opposite sides of the corona. In one of them, affected by post-reaction Fe, Mg exchange with pyroxene, the problem of reconstructing the original composition is overcome by direct use of ratios between chemical-potential differences, given by the diffusion modelling. The number of geothermobarometers in the optimisation is limited by near-degeneracies. Their weightings are affected by strong correlations among Q ratios. Uncertainty from diffusion modelling is not large in comparison with other sources. Overall precision is limited mainly by uncertainties in activity models. Hypothetical equilibrium P–T are also estimated for both garnet compositions. By this approach, departure from equilibrium can be measured, with statistical uncertainties. For the example, the result for difference from equilibrium pressure is 1.2 ± 0.7 kbar.

Notes: Field, petrographic and microprobe investigations of metaclastic rocks, calcareous schists, marbles, chloritic calcareous meta-volcanic units and schists/paragneisses which crop out along the eastern portion of the Central East-West Cross Island Highway in Taiwan demonstrate that metamorphic intensity gradually increases eastward. The lower greenschist facies Slate Formation on the W contains completely recrystallized, pure albitic plagioclase, but at least some of the white micas (± chlorites) probably represent relict detrital flakes. Neo-blastic biotite and epidote occur sporadically in the Pihou(?) Formation, and increase dramatically eastward; concomitantly the abundance of carbonaceous matter decreases to zero in the eastern Tailuko zone, and the amount of chlorite + white mica diminishes somewhat. Epidote becomes more aluminous at higher metamorphic grade. Eastward, phengites change progressively to more muscovitic compositions as the proportion of biotite increases.A close approach to chemical equilibrium for the pre-Cenozoic, complexly deformed metamorphic basement assemblages is suggested by regular, systematic, major and minor element partitioning between analysed coexisting phases. Fractionation is less pronounced on the E, reflecting higher temperatures. Estimated physical conditions of recrystallization with αH2O and αCO2 moderate, are: T 〉 325 ± 75°C, P 〉 3 kbar (W); T 〉 425 ± 75°C, P 〉 4kbar(E).The gradual eastward increase in metamorphic intensity from the Slate Formation through the Pihou(?) Formation and the three Tailuko zones, as well as the relict precursor textures in the pre-Cenozoic layered basement rocks indicate that the observed paragenetic sequence could represent a synchronous Neogene recrystallization event, probably accompanying the Plio-Pleistocene collision of the Asiatic continental margin and the Luzon (Coastal Range) andesitic arc.

Notes: This paper characterizes the metamorphic thermal structure of the Higo Metamorphic Complex (HMC) and presents the results of a numerical simulation of a geotherm with melt migration and solidification. Reconstruction of the geological and metamorphic structure shows that the HMC initially had a simple thermal structure where metamorphic temperatures and pressures increased towards apparent lower structural levels. Subsequently, this initial thermal structure has been collapsed by E–W and NNE–SSW trending high-angle faults. Pressure and temperature conditions using the analysis of mineral assemblages and thermobarometry define a metamorphic field P–T array that may be divided into two segments: the array at apparent higher structural levels has a low-dP/dT slope, whereas that at apparent lower structural levels has a high-dP/dT slope. This composite array cannot be explained by heat conduction in subsolidus rocks alone. Migmatite is exposed pervasively at apparent lower structural levels, but large syn-metamorphic plutons are absent at the levels exposed in the HMC. Transport and solidification of melt within migmatite is a potential mechanism to generate the composite array. Thermal modelling of a geotherm with melt migration and solidification shows that the composite thermal structure may be formed by a change of the dominant heat transfer from an advective regime to a conduction regime with decreasing depth. The model also predicts that strata beneath the crossing point will consist of high-grade solid metamorphic rocks and solidified melt products, such as migmatite. This prediction is consistent with the observation that migmatite was associated with the very high-dP/dT slope. The melt migration model is able to generate the very high-dP/dT segment due to the high rate of heat transfer by advection.

Notes: The structure, microstructure and petrology of a small area close to the village of Bard in Val d'Aosta (Italy) has been studied in detail. The area lies across the contact between the Gneiss Minuti (GM) and the Eclogitic Micaschist (EMS) Complexes of the Lower element of the Sesia portion of the Sesia-Lanzo Zone (Western Alps). Both complexes have undergone high-pressure metamorphism, but the metamorphic assemblages indicate a sudden increase in pressure in going across the contact from the GM to the EMS. Therefore, we interpret the contact as a thrust dividing the lower element of the Sesia into two sub-elements. This interpretation is supported by structural evidence.The early Alpine (90-70 Ma) metamorphic history is best preserved in the EMS and is one of increasing pressure associated with thrusting. The maximum P/T recorded in the EMS is 〉1500 MPa (〉15kbar) and 550°C and in the GM is 〈 1500-1300 MPa (〈 15-13 kbar) and 500-550°C. We suggest that the rocks were probably in an active Benioff zone during this time.From then on the histories of the GM and EMS are the same. Deformation continued and the thrust and thrust slices were folded during decreasing pressure. We interpret the first postthrusting deformation in terms of uplift associated with continued shortening of the crust and underplating after the Benioff zone had become inactive and a new Benioff zone had developed further to the north-west.A still later deformation and the Lepontine metamorphism (38 Ma) are related to continued uplift. Much of this deformation is characterized by structures indicative of vertical shortening and lateral spreading as the mountains rose above the general level of the surface.

Notes: In the Boi Massif of Western Timor the Mutis Complex, which is equivalent to the Lolotoi Complex of East Timor, is composed of two lithostratigraphical components: various basement schists and gneisses; and the dismembered remnants of an ophiolite. Cordierite-bearing pelitic schists and gneisses carry an early mineral assemblage of biotite + garnet + plagioclase + Al-silicate, but contain no prograde muscovite; sillimanite occurs in a textural mode which suggests that it replaced and pseudomorphed kyanite at an early stage and some specimens of pelitic schist contain tiny kyanite relics in plagioclase. Textural relations between, and mineral chemistries of, ferro-magnesian phases in these pelitic chists and gneisses suggest that two discontinuous reactions and additional continuous compositional changes have been overstepped, possibly with concomitant anatexis, as a result of decrease in Pload during high temperature metamorphism. The simplified reactions are: garnet and/or biotite + sillimanite + quartz + cordierite + hercynite + ilmenite + excess components. P-T conditions during the development of the early mineral assemblage in the pelitic gneisses are estimated to have been P + 10 kbar and T 〉 750°C, based upon the plagioclase-garnet-Al-silicate-quartz geobarometer and the garnet-biotite geothermometer. P-T conditions during the subsequent development of cordierite-bearing mineral assemblages in the pelitic gneisses are estimated to have been P + 5 kbar and T + 700°C with XH2O 〈 0.5, based upon the Fe content of cordierite occurring in the assemblage quartz + plagioclase + sillimanite + biotite + garnet + cordierite coexisting with melt.Final equilibration between some of the phases suggests that conditions dropped to P 〉 2.3 kbar and T 〉 600°C. A similar exhumation P-T path is suggested for the pelitic schists with early metamorphic conditions of P 〉 6.2 kbar and T 〉 745°C and subsequent development of cordierite under conditions in the range P = 3-4 kbar and T = 600-700°C. The tectonic implications of these P-T estimates are discussed and it is concluded that the P-T path followed by these rocks was caused by decompression during rifting and synmetamorphic ophiolite emplacement resulting from processes during the initiation and development of a convergent plate junction located in Southeast Asia during late Jurassic to Cretaceous time.

Notes: Plagioclase compositions vary from An0.1–2.5 to An32 with increasing grade in chlorite zone to oligoclase zone quartzofeldspathic schists, Franz Josef-Fox Glacier area, Southern Alps, New Zealand. This change is interrupted by the peristerite composition gap in rocks transitional between greenschist and amphibolite facies grade. Oligoclase (An20-24) and albite (An0.1–0.5) are found in biotite zone schists below the garnet isograd. With increasing grade, the plagioclase compositions outline the peristerite gap, which is asymmetric and narrows to compositions of An12 and An6 near the top of the garnet zone. In any one sample, oligoclase is the stable mineral in mica-rich layers above the garnet isograd, whereas albite and oligoclase exist in apparent textural equilibrium in adjacent quartz-plagioclase layers. The initial appearance of oligoclase in both layers results from the breakdown of epidote and possibly sphene. Carbonate is restricted to the quartz-plagioclase rich layers and probably accounts for the more sodic composition of oligoclase in these layers. The formation of more Ca-rich albite and more Na-rich oligoclase near the upper limit of the garnet zone coincides with the disappearance of carbonate and closure of the peristerite gap. Garnet appears to have only a localized effect on Ca-enrichment of plagioclase in mica-rich layers within the garnet zone. The Na-content of white mica increases sympathetically with increasing Ca-content of oligoclase and metamorphic grade.Comparison of the peristerite gap in the Franz Josef-Fox Glacier schists and schists of the same bulk composition in the Haast River area, 80 km to the S, indicates that oligoclase appears and epidote disappears at lower temperatures, and that the composition gap between coexisting albite and oligoclase is narrower in the Franz Josef-Fox Glacier area. It is suggested that a higher thermal gradient (38-40°C/km) and variations in Si/Al ordering during growth of the plagioclases between the two areas may account for these differences. In the Alpine schists the peristerite gap exists over a temperature and pressure interval of about 370-515°C and 5.5-7 kbar (550-700 MPa) PH2O.

Notes: The abundance and morphology of microdiamond in dolomite marble from Kumdy-kol in the Kokchetav Massif, are unusual; a previous study estimated the maximum content of diamonds in dolomite marble to be about 2700 carat ton−1. Microdiamond is included primarily in garnet, and occasionally in diopside and phlogopite pseudomorphs after garnet. They are classified into three types on the basis of their morphology: (1) S-type: star-shaped diamond consisting of translucent cores and transparent subhedral to euhedral very fine-grained outer parts; (2) R-type: translucent crystals with rugged surfaces; and (3) T-type: transparent, very fine-grained crystals. The S-type is the most abundant.Micro-Laue diffraction using a 1.6-µm X-ray beam-size demonstrated that the cores of the star-shaped microdiamond represent single crystals. In contrast, the most fine-grained outer parts usually have different orientations compared to the core. Laser–Raman studies indicate that the FWHM (Full Width at Half Maximum) of the Raman band of the core of the S-type diamond is slightly larger than that for the outer parts. Differences in morphology, crystal orientations, and in the FWHM of the Raman band between the core and the fine-grained outer-parts of S-type microdiamond suggest that the star-shaped microdiamond was formed discontinuously in two distinct stages.

Notes: Oscillatory zoning in low δ18O skarn garnet from the Willsboro wollastonite deposit, NE Adirondack Mts, NY, USA, preserves a record of the temporal evolution of mixing hydrothermal fluids from different sources. Garnet with oscillatory zoning are large (1–3 cm diameter) euhedral crystals that grew in formerly fluid filled cavities. They contain millimetre-scale oscillatory zoning of varying grossular–andradite composition (XAdr = 0.13–0.36). The δ18O values of the garnet zones vary from 0.80 to 6.26‰ VSMOW and correlate with XAdr. The shape, pattern and number of garnet zones varies from crystal to crystal, as does the magnitude of the correlated chemistry changes, suggesting fluid system variability, temporal and/or spatial, over the time of garnet growth. The zones of correlated Fe content and δ18O indicate that a high Fe3+/Al, high δ18O fluid mixed with a lower Fe3+/Al and δ18O fluid. The high δ18O, Fe enriched fluids were likely magmatic fluids expelled from crystallizing anorthosite. The low δ18O fluids were meteoric in origin.These are the first skarn garnet with oscillatory zoning reported from granulite facies rocks. Geochronologic, stable isotope, petrologic and field evidence indicates that the Adirondacks are a polymetamorphic terrane, where localized contact metamorphism around shallowly intruded anorthosite was followed by a regional granulite facies overprint. The growth of these garnet in equilibrium with meteoric and magmatic fluids indicates an origin in the shallow contact aureole of the anorthosite prior to regional metamorphism. The zoning was preserved due to the slow diffusion of oxygen and cations in the large garnet and protection from deformation and recrystallization in zones of low strain in thick, rigid, garnetite layers. The garnet provide new information about the hydrothermal system adjacent to the shallowly intruded massif anorthosite that predates regional metamorphism in this geologically complex, polymetamorphic terrane.

Notes: The blueschist and greenschist units on the island of Sifnos, Cyclades were affected by Eocene high-pressure (HP) metamorphism. Using conventional geothermobarometry, the HP peak metamorphic stage was determined at 550–600 °C and 20 kbar, close to the blueschist and the eclogite facies transition. The retrograde P–T paths are inferred with phase diagrams. Pseudosections based on a quantitative petrogenetic grid in the model system Na2O–CaO–FeO–MgO–Al2O3–SiO2–H2O reveal coeval decompression and cooling for both the blueschist and the greenschist unit. The conditions of the metamorphic peak and those of the retrograde stages conform to a similar metamorphic gradient of 10–12 °C km−1 for both units. The retrograde overprint can be assigned to low-pressure blueschist to HP greenschist facies conditions. This result cannot be reconciled with the (prograde) Barrovian-type event, which affected parts of the Cyclades during the Oligocene to Miocene. Instead, the retrograde overprint is interpreted in terms of exhumation, directly after the HP stage, without a separate metamorphic event. Constraints on the exhumation mechanism are given by decompression-cooling paths, which can be explained by exhumation in a fore-arc setting during on-going subduction and associated crustal shortening. Back-arc extension is only responsible for the final stage of exhumation of the HP units.

Notes: Metabasic rocks from the Adula Nappe in the Central Alps record a regional high-pressure metamorphic event during the Eocene, and display a regional variation in high-pressure mineral assemblages from barroisite, or glaucophane, bearing garnet amphibolites in the north to kyanite eclogites in the central part of the nappe. High-pressure rocks from all parts of the nappe show the same metamorphic evolution of assemblages consistent with prograde blueschist, high-pressure amphibolite or eclogite facies conditions followed by peak-pressure eclogite facies conditions and decompression to the greenschist or amphibolite facies. Average PT calculations (using thermocalc) quantitatively establish nested, clockwise P–T paths for different parts of the Adula Nappe that are displaced to higher pressure and temperature from north to south. Metamorphic conditions at peak pressure increase from about 17 kbar, 640 °C in the north to 22 kbar, 750 °C in the centre and 25 kbar, 750 °C in the south. The northern and central Adula Nappe behaved as a coherent tectonic unit at peak pressures and during decompression, and thermobarometric results are interpreted in terms of a metamorphic field gradient of 9.6 ± 2.0 °C km−1 and 0.20 ± 0.05 kbar km−1. These results constrain the peak-pressure position and orientation of the nappe to a depth of 55–75 km, dipping at an angle of approximately 45° towards the south. Results from the southern Adula Nappe are not consistent with the metamorphic field gradient determined for the northern and central parts, which suggests that the southern Adula Nappe may have been separated from central and northern parts at peak pressure.

Notes: The Priest pluton contact aureole in the Manzano Mountains, central New Mexico preserves evidence for upper amphibolite contact metamorphism and localized retrograde hydrothermal alteration associated with intrusion of the 1.42 Ga Priest pluton. Quartz–garnet and quartz–sillimanite oxygen isotope fractionations in pelitic schist document an increase in the temperatures of metamorphism from 540 °C, at a distance of 1 km from the pluton, to 690 °C at the contact with the pluton. Comparison of calculated temperature estimates with one-dimensional thermal modelling suggests that background temperatures between 300 and 350 °C existed at the time of intrusion of the Priest pluton. Fibrolite is found within 300 m of the Priest pluton in pelitic and aluminous schist metamorphosed at temperatures 〉580 °C. Coexisting fibrolite and garnet in pelitic schist are in oxygen isotope equilibrium, suggesting these minerals were stable reaction products during peak metamorphism. The fibrolite-in isograd is coincident with the staurolite-out isograd in pelitic schist, and K-feldspar is not observed with the first occurrence of fibrolite. This suggests that the breakdown of staurolite and not the second sillimanite reaction controls fibrolite growth in staurolite-bearing pelitic schist. Muscovite-rich aluminous schist locally preserves the Al2SiO5 polymorph triple-point assemblage – kyanite, andalusite and fibrolite. Andalusite and fibrolite, but not kyanite, are in isotopic equilibrium in the aluminous schist. Co-nucleation of fibrolite and andalusite at 580 °C in the presence of muscovite and absence of K-feldspar suggests that univariant growth of andalusite and fibrolite occurred. Kyanite growth occurred during an earlier regional metamorphic event at a temperature nearly 80 °C lower than andalusite and fibrolite growth. Quartz–muscovite fractionations in hydrothermally altered pelitic schist and quartzite are small or negative, suggesting that late isotopic exchange between externally derived fluids and muscovite, but not quartz, occurred after peak contact metamorphism and that hydrothermal alteration in pelitic schist and quartzite occurred below the closure temperature of oxygen self diffusion in quartz (〈500 °C).

Notes: Abstract Concordant U–Pb ages of c. 530–510 Ma and c. 470–420 Ma on titanite from calcsilicate, orthogneiss and amphibolite rocks constrain the age of high-T metamorphism in the Early Palaeozoic mobile belt at the western margin of Proterozoic Gondwana (Argentina, 26–29°S). The U–Pb ages document the time of titanite formation at high-T conditions according to the stable mineral paragenesis and occurrence of titanite in the metamorphic fabric. The presence of migmatite at all sample sites indicates temperatures were 〉 c. 650 °C. Titanite formed at similar metamorphic conditions at different times on the regional and on the outcrop scale. The titanite crystals preserved their U–Pb isotopic signatures and chemical composition under ongoing upper amphibolite to granulite facies temperatures. Different thermal peaks or deformations are only detected by the different U–Pb ages and not by changes in the mineral paragenesis or metamorphic fabric of the samples. The range of U–Pb ages, e.g. in the Ordovician and Silurian (c. 470, 460, 440, 430, 420 Ma), is interpreted as the effect polyphase deformation with deformation-enhanced recrystallization of titanite and/or different thermal peaks during a long-standing, geographically fixed, high-T regime in the mid-crust of a continental magmatic arc. A clear correlation of the different ages with distinct tectonic events, e.g. collision of terranes, is not possible based on the present knowledge of the region.

Notes: Abstract The garnet blueschists from the Ile de Groix (Armorican Massif, France) contain millimetre- to centimetre-sized pseudomorphs consisting of an aggregate of chlorite, epidote and paragonite. The pseudomorphed phase developed at a late stage of the deformation history, because it overgrows a glaucophane–epidote–titanite foliation. Garnet growth occurred earlier than the beginning of the ductile deformation, and thus garnet is also included in the pseudomorphs. Microprobe analyses show that garnet is strongly zoned, with decreasing spessartine and increasing almandine and pyrope contents from core to rim. Grossular content is higher in garnet cores (about 35 mole%) compared to garnet rims (about 30 mole%). Blue amphibole has glaucophane compositions with a low Fe3+ content and become more magnesian when inclusions in garnet (XMg = 0.62–0.65) are compared with matrix grains (XMg = 0.67–0.70). Matrix epidote has a pistacite content of about 50 mole%. On the basis of their shape and the nature of the breakdown products, the pseudomorphs are attributed to lawsonite. A numerical model (using Thermocalc) has been developed in order to understand the reactions controlling both the growth and the breakdown of lawsonite. Lawsonite growth could have taken place through the continuous hydration reaction Chl + Ep + Pg + Qtz + Vap = Gln + Lws, followed by the fluid-absent reaction Chl + Ep + Pg = Grt + Gln + Lws. Peak P–T conditions are estimated at about 18–20 kbar, 450 °C. This indicates that lawsonite growth took place at increasing P and T, hence can be used as a geobarometer in the buffering assemblage garnet–glaucophane–epidote. The final part of the history is recorded by lawsonite breakdown, after cessation of the ductile deformation, and recording the earliest stages of the exhumation.

Notes: Previous models of hydrodynamics in contact metamorphic aureoles assumed flow of aqueous fluids, whereas CO2 and other species are also common fluid components in contact metamorphic aureoles. We investigated flow of mixed CO2–H2O fluid and kinetically controlled progress of calc-silicate reactions using a two-dimensional, finite-element model constrained by the geological relations in the Notch Peak aureole, Utah. Results show that CO2 strongly affects fluid-flow patterns in contact aureoles. Infiltration of magmatic water into a homogeneous aureole containing CO2–H2O sedimentary fluid facilitates upward, thermally driven flow in the inner aureole and causes downward flow of the relatively dense CO2-poor fluid in the outer aureole. Metamorphic CO2-rich fluid tends to promote upward flow in the inner aureole and the progress of devolatilization reactions causes local fluid expulsion at reacting fronts. We also tracked the temporal evolution of P-T-XCO2conditions of calc-silicate reactions. The progress of low- to medium-grade (phlogopite- to diopside-forming) reactions is mainly driven by heat as the CO2 concentration and fluid pressure and temperature increase simultaneously. In contrast, the progress of the high-grade wollastonite-forming reaction is mainly driven by infiltration of chemically out-of-equilibrium, CO2-poor fluid during late-stage heating and early cooling of the inner aureole and thus it is significantly enhanced when magmatic water is involved. CO2-rich fluid dominates in the inner aureole during early heating, whereas CO2-poor fluid prevails at or after peak temperature is reached. Low-grade metamorphic rocks are predicted to record the presence of CO2-rich fluid, and high-grade rocks reflect the presence of CO2-poor fluid, consistent with geological observations in many calc-silicate aureoles. The distribution of mineral assemblages predicted by our model matches those observed in the Notch Peak aureole.

Notes: The Swakane Gneiss and the overlying Napeequa Complex in the North Cascade range, Washington, were metamorphosed and deformed during development of a Cretaceous-Paleogene continental arc, and are among the structurally deepest exposed rocks within the Cordilleran arcs of North America. Peak metamorphic conditions in both the Swakane Gneiss and Napeequa Complex were c. 640–750 °C, 9–12 kbar. Clockwise paths and widespread evidence for high-P metamorphism in meta-supracrustal rocks (burial to 〉40 km) document major vertical tectonic motion during arc construction and unroofing.These and other moderately high-pressure rocks in the North Cascades-Coast Mountains experienced a dramatically different tectonometamorphic history than metamorphic rocks within other Cordilleran arcs. The exhumed arc complexes of the Sierra Nevada and Peninsular Ranges are dominated by relatively low-P metamorphic and plutonic rocks (typically 〈6 kbar). There is no evidence that the northern Cordillera was thickened to a greater degree than these other belts, suggesting that the greater magnitude of vertical motion in the Cascades may have been related to exhumation mechanisms: Eocene extension in the northern Cordillera vs. erosional unroofing in the central and southern Cordillera.

Notes: High-pressure (HP) granulites and eclogitized metagabbro are exposed along an orogen-parallel high-P belt that was developed at c. 1050–1020 Ma in the NE Grenville Province. Among these rocks, mafic granulites derived from a Labradorian anorthosite suite of the Lelukuau terrane contain garnet, Al-Na diopside, and, depending on bulk composition, plagioclase and kyanite. Moreover, the distribution of phases is influenced by the original igneous texture. For instance, in high XMgO leucocratic varieties, garnet porphyroblasts nucleated together with kyanite in An-rich cores of plagioclase domains whereas in low XMgO rocks garnet occurs together with clinopyroxene within formerly igneous ferromagnesian domains and kyanite is missing. In contrast, garnet pseudomorphs after igneous plagioclase in melanocratic varieties display evidence of earlier corona development. Metamorphic textures are consistent with a two stage evolution: (a) development of garnet and Al-Na-diopside (Cpx1) under high-P metamorphic conditions, concomitant with elimination of plagioclase in the mesocratic to melanocratic varieties; and (b) partial loss of Al-Na from Cpx1 resulting in production of new andesitic plagioclase, and growth of new clinopyroxene (Cpx2) after garnet and quartz in leucocratic to mesocratic rocks consistent with decompression. Widespread equilibrium textures between garnet-Pl2-Cpx2 and/or reset Cpx1 are consistent with development at the thermal peak. Estimated P–T conditions for the presumed thermal peak fall in the range 1500–1800 MPa and 800–900 °C and are comparable to those recorded by eclogitized gabbros from other parts of the high-P belt of the NE Grenville province. Low jadeite content of clinopyroxene from the HP granulites is attributed to the low bulk Na2O/(Na2O + CaO) of these rocks relative to common basaltic compositions. Scarcity of apparent retrograde textural overprint in both the HP granulites and the eclogites suggests fast subsequent cooling, consistent with extrusion of the high-P belt towards the foreland shortly after the metamorphic peak.

Notes: The upper deck of the East Athabasca mylonite triangle (EAmt), northern Saskatchewan, Canada, contains mafic granulites that have undergone high P–T metamorphism at conditions ranging from 1.3 to 1.9 GPa, 890–960 °C. Coronitic textures in these mafic granulites indicate a near-isothermal decompression path to 0.9 GPa, 800 °C. The Godfrey granite occurs to the north adjacent to the upper deck high P–T domain. Well-preserved corona textures in the Godfrey granite constrain igneous crystallization and early metamorphism in the intermediate-pressure granulite field (Opx + Pl) at 1.0 GPa, 775 °C followed by metamorphism in the high pressure granulite field (Grt + Cpx + Pl) at 1.2 GPa, 860 °C. U–Pb geochronology of zircon in upper deck mafic granulite yields evidence for events at both c. 2.5 Ga and c. 1.9 Ga. The oldest zircon dates are interpreted to constrain a minimum age for crystallization or early metamorphism of the protolith. A population of 1.9 Ga zircon in one mafic granulite is interpreted to constrain the timing of high P–T metamorphism. Titanite from the mafic granulites yields dates ranging from 1900 to 1894 Ma, and is interpreted to have grown along the decompression path, but still above its closure temperature, indicating cooling following the high P–T metamorphism from c. 960–650 °C in 4–10 Myr. Zircon dates from the Godfrey granite indicate a minimum crystallization age of 2.61 Ga, without any evidence for 1.9 Ga overgrowths. The data indicate that an early granulite facies event occurred at c. 2.55–2.52 Ga in the lower crust (c. 1.0 GPa), but at 1.9 Ga the upper deck underwent high P–T metamorphism, then decompressed to 0.9–1.0 GPa. Juxtaposition of the upper deck and Godfrey granite would have occurred after or been related to this decompression. In this model, the high P–T rocks are exhumed quickly following the high pressure metamorphism. This type of metamorphism is typically associated with collisional orogenesis, which has important implications for the Snowbird tectonic zone as a fundamental boundary in the Canadian Shield.

Notes: Orthopyroxene-free garnet + clinopyroxene + plagioclase ± quartz-bearing mineral assemblages represent the paragenetic link between plagioclase-free eclogite facies metabasites and orthopyroxene-bearing granulite facies metabasites. Although these assemblages are most commonly developed under P–T conditions consistent with high pressure granulite facies, they sometimes occur at lower grade in the amphibolite facies. Thus, these assemblages are characteristic but not definitive of high pressure granulite facies. Compositional factors favouring their development at amphibolite grade include Fe-rich mineral compositions, Ca-rich garnet and plagioclase, and Ti-poor hornblende. The generalized reaction that accounts for the prograde development of garnet + clinopyroxene + plagioclase ± quartz from a hornblende + plagioclase + quartz-bearing (amphibolite) precursor is Hbl + Pl + Qtz=Grt + Cpx + liquid or vapour, depending on whether the reaction occurs above or below the solidus. There are significant discrepancies between experimental and natural constraints on the P–T conditions of orthopyroxene-free garnet + clinopyroxene + plagioclase ± quartz-bearing mineral assemblages and therefore on the P–T position of this reaction. Semi-quantitative thermodynamic modelling of this reaction is hampered by the lack of a melt model and gives results that are only moderately successful in rationalizing the natural and experimental data.

Notes: This study analyses the mineralogical and chemical transformations associated with an Alpine shear zone in polymetamorphic metapelites from the Monte Rosa nappe in the upper Val Loranco (N-Italy). In the shear zone, the pre-Alpine assemblage plagioclase + biotite + kyanite is replaced by the assemblage garnet + phengite + paragonite at eclogite facies conditions of about 650 °C at 12.5 kbar. Outside the shear zone, only minute progress of the same metamorphic reaction was attained during the Alpine metamorphic overprint and the pre-Alpine mineral assemblage is largely preserved. Textures of incomplete reaction, such as garnet rims at former grain contacts between pre-existing plagioclase and biotite, are preserved in the country rocks of the shear zone. Reaction textures and phase relations indicate that the Alpine metamorphic overprint occurred under largely anhydrous conditions in low strain domains. In contrast, the mineralogical changes and phase equilibrium diagrams indicate water saturation within the Alpine shear zones. Shear zone formation occurred at approximately constant volume but was associated with substantial gains in silica and losses in aluminium and potassium. Changes in mineral modes associated with chemical alteration and progressive deformation indicate that plagioclase, biotite and kyanite were not only consumed in the course of the garnet-and phengite-producing reactions, but were also dissolved ‘congruently’ during shear zone formation. A large fraction of the silica liberated by plagioclase, biotite and kyanite dissolution was immediately re-precipitated to form quartz, but the dissolved aluminium- and potassium-bearing species appear to have been stable in solution and were removed via the pore fluid. The reaction causes the localization of deformation by producing fine-grained white mica, which forms a mechanically weak aggregate.

Notes: Low-pressure crystal-liquid equilibria in pelitic compositions are important in the formation of low-pressure, high-temperature migmatites and in the crystallization of peraluminous leucogranites and S-type granites and their volcanic equivalents. This paper provides data from vapour-present melting of cordierite-bearing pelitic assemblages and augments published data from vapour-present and vapour-absent melting of peraluminous compositions, much of which is at higher pressures. Starting material for the experiments was a pelitic rock from Morton Pass, Wyoming, with the major assemblage quartz-K feldspar-biotite-cordierite, approximately in the system KFMASH. A greater range in starting materials was obtained by addition of quartz and sillimanite to aliquots of this rock. Sixty-one experiments were carried out in cold-seal apparatus at pressures of 1–3.5 kbar (particularly 2 kbar) and temperatures from 700 to 840 °C, with and without the addition of water. In the vapour-present liquidus relations at 2 kbar near the beginning of melting, the sequence of reactions with increasing temperature is: Qtz + Kfs + Crd + Sil + Spl + V = L; Qtz + Kfs + Crd + Spl + Ilm + V = Bt + L; and Qtz + Bt + V = Crd + Opx + Ilm + L. Vapour-absent melting starts at about 800 °C with a reaction of the form Qtz + Bt = Kfs + Crd + Opx + Ilm + L. Between approximately 1–3 kbar the congruent melting reaction is biotite-absent, and biotite is produced by incongruent melting, in contrast to higher-pressure equilibria. Low pressure melts from pelitic compositions are dominated by Qtz-Kfs-Crd. Glasses at 820–840 °C have calculated modes of approximately Qtz42Kfs46Crd12. Granites or granitic leucosomes with more than 10–15% cordierite should be suspected of containing residual cordierite. The low-pressure glasses are quite similar to the higher-pressure glasses from the literature. However, XMg increases from about 0.1–0.3 with increasing pressure from 1 to 10 kbar, and the low-temperature low-pressure glasses are the most Fe-rich of all the experimental glasses from pelitic compositions.

Notes: Quartz-rich veins in metapelitic schists of the Sanandaj-Sirjan belt, Hamadan region, Iran, commonly contain two Al2SiO5 polymorphs, and, more rarely, three coexisting Al2SiO5 polymorphs. In most andalusite and sillimanite schists, the types of polymorphs in veins correlate with Al2SiO5 polymorph(s) in the host rocks, although vein polymorphs are texturally and compositionally distinct from those in adjacent host rocks; e.g. vein andalusite is enriched in Fe2O3 relative to host rock andalusite. Low-grade rocks contain andalusite + quartz veins, medium-grade rocks contain andalusite + sillimanite + quartz ± plagioclase veins, and high-grade rocks contain sillimanite + quartz + plagioclase veins/leucosomes. Although most andalusite and sillimanite-bearing veins occur in host rocks that also contain Al2SiO5, kyanite-quartz veins crosscut rocks that lack Al2SiO5 (e.g. staurolite schist, granite). A quartz vein containing andalusite + kyanite + sillimanite + staurolite + muscovite occurs in andalusite–sillimanite host rocks. Textural relationships in this vein indicate the crystallization sequence andalusite to kyanite to sillimanite. This crystallization sequence conflicts with the observation that kyanite-quartz veins post-date andalusite–sillimanite veins and at least one intrusive phase of a granite that produced a low-pressure–high-temperature contact aureole; these relationships imply a sequence of andalusite to sillimanite to kyanite. Varying crystallization sequences for rocks in a largely coherent metamorphic belt can be explained by P–T paths of different rocks passing near (slightly above, slightly below) the Al2SiO5 triple point, and by overprinting of multiple metamorphic events in a terrane that evolved from a continental arc to a collisional orogen.

Notes: Zircon fission track dating and track length analysis in the high-grade part of the Asemigawa region of the Sanbagawa belt demonstrates a simple cooling history passing through the partial annealing zone at 63.2 ± 5.8 (2 σ) Ma. Combining this age with previous results of phengite and amphibole K–Ar and 40Ar/39Ar dating gives a cooling rate of between 6 and 13 °C Myr−1, which can be converted to a maximum exhumation rate of 0.7 mm year−1 using the known shape of the P–T path. This is an order of magnitude lower than the early part of the exhumation history. In contrast, zircon fission track analyses in the low-grade Oboke region show that this area has undergone a complex thermal history probably related to post-orogenic secondary reheating younger than c. 30 Ma. This event may correlate with the widespread igneous activity in south-west Japan around 15 Ma. The age of subduction-related metamorphism in the Oboke area is probably considerably older than the generally accepted range of 77–70 Ma.

Notes: Garnet-bearing ultramafic rocks including clinopyroxenite, wehrlite and websterite locally crop out in the Higashi-akaishi peridotite of the Besshi region in the Cretaceous Sanbagawa metamorphic belt. These rock types occur within dunite as lenses, boudins or layers with a thickness ranging from a few centimetres to 1 metre. The wide and systematic variation of bulk-rock composition and the overall layered structure imply that the ultramafic complex originated as a cumulate sequence. Garnet and other major silicates contain rare inclusions of edenitic amphibole, chlorite and magnetite, implying equilibrium at relatively low P–T conditions during prograde metamorphism. Orthopyroxene coexisting with garnet shows bell-shaped Al zoning with a continuous decrease of Al from the core towards the rim, consistent with rims recording peak metamorphic conditions. Estimated P–T conditions using core and rim compositions of orthopyroxene are 1.5–2.4 GPa/700–800 °C and 2.9–3.8 GPa/700–810 °C, respectively, implying a high P/T gradient (〉 3.1 GPa/100 °C) during prograde metamorphism. The presence of relatively low P–T conditions at an early stage of metamorphism and the steep P/T gradient together trace a concave upwards P–T path that shows increasing P/T with higher T, similar to P–T paths reported from other UHP metamorphic terranes. These results suggest either (1) down dragging of hydrated mantle cumulate parallel to the slab–wedge interface in the subduction zone by mechanical coupling with the subducting slab or (2) ocean floor metamorphism and/or serpentinization at early stage of subduction of oceanic lithosphere and ensuing HP–UHP prograde metamorphism.

Notes: During emplacement and cooling, the layered mafic–ultramafic Kettara intrusion (Jebilet, Morocco) underwent coeval effects of deformation and pervasive fluid infiltration at the scale of the intrusion. In the zones not affected by deformation, primary minerals (olivine, plagioclase, clinopyroxene) were partially or totally altered into Ca-amphibole, Mg-chlorite and CaAl-silicates. In the zones of active deformation (centimetre-scale shear zones), focused fluid flow transformed the metacumulates (peridotites and leucogabbros) into ultramylonites where insoluble primary minerals (ilmenite, spinel and apatite) persist in a Ca-amphibole-rich matrix. Mass-balance calculations indicate that shearing was accompanied by up to 200% volume gain; the ultramylonites being enriched in Si, Ca, Mg, and Fe, and depleted in Na and K. The gains in Ca and Mg and losses in Na and K are consistent with fluid flow in the direction of increasing temperature.When the intrusion had cooled to temperatures prevailing in the country rock (lower greenschist facies), deformation was still active along the shear zones. Intense intragranular fracturing in the shear zone walls and subsequent fluid infiltration allowed shear zones to thicken to metre-scale shear zones with time. The inner parts of the shear zones were transformed into chlorite-rich ultramylonites. In the shear zone walls, muscovite crystallized at the expense of Ca–Al silicates, while calcite and quartz were deposited in ‘en echelon’ veins. Mass-balance calculations indicate that formation of the chlorite-rich shear zones was accompanied by up to 60% volume loss near the centre of the shear zones; the ultramylonites being enriched in Fe and depleted in Si, Ca, Mg, Na and K while the shear zones walls are enriched in K and depleted in Ca and Si. The alteration observed in, and adjacent to the chlorite shear zones is consistent with an upward migrating regional fluid which flows laterally into the shear zone walls. Isotopic (Sr, O) signatures inferred for the fluid indicate it was deeply equilibrated with host lithologies.