ALBERT

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 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 In the northeastern part of the Grenville Province, along the gulf of St Lawrence, cordierite is widespread in the migmatites of Baie Jacques Cartier (BJC) and Baie des Ha! Ha! (BHH). In the BJC area, rafts of mesosome occur in a pervasive network of leucosome consisting of cordierite-bearing pegmatite. In BHH, however, the mesosome and leucosome are well segregated and locally separated by thin biotite –hornblende melanosomes.Leucosomes in the BJC area record the highest temperatures (oxide thermometry = 900°C), whereas leucosomes of BHH and mesosomes of both areas indicate peak temperatures around 800°C (oxide thermometry; biotite–garnet thermometry with fluorine-rich biotite). Peak pressures were constrained at 720 MPa using the Ilm-Sil–Qtz–Grt–Rt (GRAIL) equilibrium.The area is thought to have undergone extensive melting under relatively modest pressures. The highest temperatures recorded in the BJC area are probably related to a pervasive impregnation of this terrane by aluminous granitic melts.Most post-peak P–T estimates for the mesosomes fall on a nearly isobaric, clockwise, P–T path (0.6 MPa/°C) with the exception of the high-temperature leucosomes of BJC, which fall about 100°C away from this path; this is additional evidence for the external origin of these leucosomes. The ultimate source of heat that generated the migmatites is thus though to be an underlying plutonic complex (anorthosite?).

Notes: Abstract The introduction of externally derived fluids into rocks of the Zermatt–Saas zone of the Swiss Alps gave rise to the simultaneous formation of shear and hydraulic fractures. These fractures are now filled with albite-rich assemblages and surrounded by alteration halos up to c. 2 m wide. The alteration assemblages are zoned and an examination of reactions in P–T–aH2O space implies that the parageneses developed by the hydration of fluid-absent eclogites. A mechanical analysis of the veins (after Sibson, 1981) shows that Pfluid/Pload must have been at least 0.96. Fluid migration into the country rocks must have been driven by excess hydraulic head either derived from the vertical extent of the veins or due to their connection to a deeper, external reservoir, possibly tapped along thrust surface(s). Diffusive and capillary transport were insignificant. The fluids may have been derived from underlying metasediments that were dehydrating during the quasi-isothermal uplift of this part of the Alps, or they may have originated during the prograde mesoalpine metamorphism documented in the area.

Notes: Abstract The Catalina Schist of southern California is a subduction zone metamorphic terrane. It consists of three tectonic units of amphibolite-, high-P greenschist- and blueschist-facies rocks that are structurally juxtaposed across faults, forming an apparent inverted metamorphic gradient. Migmatitic and non-migmatitic metabasite blocks surrounded by a meta-ultramafic matrix comprise the upper part of the Catalina amphibolite unit. Fluid-rock interaction at high-P, high-T conditions caused partial melting of migmatitic blocks, metasomatic exchange between metabasite blocks and ultramafic rocks, infiltration of silica into ultramafic rocks, and loss of an albitic component from nonmigmatitic, clinopyroxene-bearing metabasite blocks.Partial melting took place at an estimated P=˜8–11 kbar and T=˜640–750°C at high H2O activity. The melting reaction probably involved plagioclase + quartz. Trondhjemitic melts were produced and are preserved as leucocratic regions in migmatitic blocks and as pegmatitic dikes that cut ultramafic rocks.The metasomatic and melting processes reflected in these rocks could be analogous to those proposed for fluid and melt transfer of components from a subducting slab to the mantle wedge. Aqueous fluids rather than melts seem to have accomplished the bulk of mass transfer within the mafic and ultramafic complex.

Notes: Abstract Migmatite structures in the Coast Plutonic-Metamorphic Complex are well exposed in the inlet of Boca de Quadra, southeast Alaska. Two types of anatectic migmatites are present. Patch migmatites formed by in situ melting and subsequent crystallization of melt. Diktyonitic migmatites comprise a discontinuous veined network of leucocratic material, in which leucosomes enclose boudins of host rock. The margins of these boudins show the development of both melanosomes and shear band fabrics.Strain analysis of diktyonitic melanosomes indicates that these regions have undergone volume decreases of 20-27%. This volume decrease is attributed to melt extraction into the adjacent fracture-filling leucosomes. Thus, diktyonitic migmatites formed by shear-induced segregation of partial melt, whereas in patch migmatites the lack of shear stresses inhibited melt segregation. The variable structural style of anatectic migmatites in Boca de Quadra is not related to host-rock composition, but may be due to differences in the amount of differential stress during migmatization. These in turn may be controlled by host-rock strength and/or diachroneity of migmatization and deformation.Determination of volume changes during migmatization using strain analysis is potentially capable of discriminating intrusive and anatectic migmatites and consequently of documenting melt segregation and subsequent migration across crustal levels.

Notes: Abstract Chloritoid and pyrophyllite occur together in all major goldfields of the Witwatersrand Basin and are widespread in virtually all rock types of the upper Witwatersrand Supergroup, including metaconglomeratic reefs and altered mafic rocks. Both minerals are particularly characteristic of the pelitic horizons intimately associated with reef packages, but they are also developed locally in the regionally persistent metapelites that have basin-wide extent. Pyrophyllite is particularly common in foliated zones, adjacent to quartz veins, and near unconformably overlying auriferous conglomerates.The wide distribution of chloritoid and pyrophyllite in metapelites of the Witwatersrand Basin is attributed to alteration of chlorite-rich shales, rather than to unusual premetamorphic starting materials. This alteration event involved the redistribution of many elements, with up to 40% volume loss, mainly due to removal of silica. Removal of most of the Mg and some Fe accounts for the stabilization of chloritoid and pyrophyllite. Relatively immobile elements included Al, Ti, Nb, Cr, V, P, La and Ce, whereas Si, Fe, Mn, Zn, Co, Ni, Cu, Mg and Ca were lost, and K, Rb and Ba were introduced by an infiltrating fluid.The alteration event is inferred to have been within the chloritoid and pyrophyllite stability field (and thus syn-metamorphic) as bulk chemical changes in metapelites are from chlorite directly towards chloritoid and then pyrophyllite, rather than to lower grade minerals such as kaolinite. Muscovite–chlorite–chloritoid and muscovite–chloritoid–pyrophyllite assemblages are attributed to fluid buffering along appropriate curves, as their production by metamorphism of lower grade mineral mixes is considered unlikely, based on the present bulk rock compositional data. A metamorphic timing for the alteration accounts for the correlation of strongly foliated areas with greater degrees of inferred alteration. The transitions from chlorite to chloritoid to pyrophyllite define zones of increasing alteration.Widespread infiltration as part of peak metamorphism is suggested by the distribution of chloritoid and pyrophyllite, quartz veining and textures. Fluid:rock ratios calculated from a silica budget in one metapelitic horizon exceed 100:1 over many square kilometres. These values need not imply multi-pass fluid flow, as much of the silica migration may be redistribution on a scale of a few metres, from source rocks into veins. Although infiltration during metamorphism may have affected much of the upper Witwatersrand succession, channelized fluid flow within reef packages, along faults and unconformities and in certain metaconglomerates and metapelites is inferred.

Notes: Abstract Biotite and cordierite occur in a 1-km wide zone of pelitic hornfelses around the McGerrigle pluton. These phases display systematic changes in XFe that can be attributed to continuous reactions involving chlorite or andalusite in the system KFMASH. Through much of the zone biotite and cordierite were products of the ‘breakdown’of chlorite. Close to the pluton this continuous reaction was terminated by a discontinuous reaction that introduced andalusite. Pelites which interdigitate with apophyses of the intrusive at the pluton margin contain assemblages that record a continuous reaction between biotite, cordierite, andalusite, muscovite, and quartz or, alternatively, the discontinuous breakdown of muscovite and quartz to K-feldspar and andalusite.The mole fraction of Fe in biotite and cordierite increased significantly with the progress of the first continuous reaction and apparently decreased during the second continuous reaction. The KD of Fe-Mg between the minerals decreased and apparently increased, respectively, during the two reactions.Biotite-cordierite-chlorite assemblages are interpreted to have been stable at temperatures between 525° C and 615° C and biotite-cordierite-andalusite assemblages stable at temperatures between 615° C and 635° C. The confining pressure was estimated to have been 〈 2 kbar.The results of this study suggest that the KD of Fe-Mg between biotite and cordierite is a function of temperature, the Fe-Mg exchange characteristics of the controlling continuous reaction and non-ideal mixing of Fe and Mg.

Notes: Abstract The textural and compositional features of phlogopites in a contact-metamorphic dolomite marble inclusion in the Bergell intrusion (central Alps) and in a metasomatic reaction vein cutting through this marble suggest different origins for vein phlogopites:(a) High-Al vein phlogopite represents former marble phlogopite which has been compositionally modified by reaction with the vein forming fluid.(b) Low-Al vein phlogopite represents phlogopite precipitated from the vein forming fluid.As both types of vein phlogopite were in contact with the same vein forming fluid at the same time, low-Al phlogopite most likely represents an equilibrium phlogopite composition, whereas high-Al phlogopite does not. High-Al vein phlogopite retained its Al-content from the contact-metamorphic marble parent phlogopite and only underwent Fe-Mg exchange with the metasomatic fluid.All the vein phlogopites studied are strongly enriched in Fe relative to marble phlogopite. The data may suggest in general that phlogopite Al/Si ratios may be retained from the conditions under which the phlogopites first formed, whereas the Mg/Fe-ratios may be substantially modified by exchange with other ferromagnesian solid phases and/or a metamorphic fluid at later stages in their metamorphic history. This may have significant effects on calculated pressures and temperatures from thermobarometers involving biotite.

Notes: Abstract The Saint-Martin des Noyers Formation is interpreted as a slice of an island-arc system of Lower to Middle Palaeozoic age, located in the internal part of the Variscan orogen in Vendée (Armorican Massif, France). Metamorphosed igneous rocks range in composition from ultramafic to rhyolitic. The regular increase in the FeO/(FeO+MgO) ratio, from mafic to silicic samples, results in a systematic variability in the nature and composition of the metamorphic phases. In basaltic samples, the occurrence of relict garnet-barroisite assemblages suggests relatively high-pressure conditions for the peak of metamorphism. During a subsequent retrograde evolution, the primary barroisitic hornblendes recrystallized to texturally complex mixtures of actinolite and hornblende. Despite this complication, it is possible to decipher a P–T-t path based on amphibole chemistry. The P–T trajectory deduced is dominated by the effect of pressure and consistent with early underthrusting and subsequent tectonic uplift of the ancient arc of Saint-Martin des Noyers.

Notes: Abstract In order to study the thermal structure of active thrust belts, we have developed a numerical model of conductive heat transfer between thrust sheets during deformation. Our finite difference approach alternates small, instantaneous increments of displacement and isotherm translation with conductive relaxation of perturbed isotherms. In each step, conduction occurs for a length of time equal to the displacement increment divided by the thrust velocity. Computer simulations demonstrate that conductive heat transfer is significant during deformation and that temperatures in hanging-wall rocks decrease while temperatures in foot-wall rocks increase over distances of up to 10 km from the thrust surface. When the effects of internal heat production are also calculated, heating of foot-wall rocks exceeds cooling of hanging-wall rocks. Rocks located between two thrusts may experience a complicated temperature–time path of early heating followed by cooling. These models help to explain the rapid metamorphism of rocks in the Taconian thrust belt in the northern Appalachians of New England soon after deposition of the youngest sediments.

Notes: Abstract 40Ar/39Ar age spectrum analysis of phengite separates from Naxos, part of the Attic Cycladic Metamorphic Belt in Greece, indicates that cooling following high-pressure, low- to medium-temperature metamorphism, M1, occurred about 50 Ma ago. Phengite has 40Ar* gradients that suggest that part of the scatter observed in conventional K–Ar ages was caused by diffusion of radiogenic argon from the minerals during a younger metamorphism, M2. In central Naxos, this metamorphism (M2) has overprinted the original mineral assemblages completely, and is associated with development of a thermal dome. Excellent 40Ar/39Ar plateaus at 15.0 ± 0.1 Ma, 11.8 ± 0.1 Ma, and 11.4 ± 0.1 Ma, obtained on hornblende, muscovite and biotite, respectively, from the migmatite zone, indicate that relatively rapid cooling followed the M2 event, and that no significant thermal overprinting occurred subsequent to M2. Toward lower M2 metamorphic grade, 40Ar/39Ar plateau ages of hornblendes increase to 19.8 ± 0.1 Ma; concomitantly the proportion of excess 40Ar in the spectra increases as well. We propose that the peak of M2 metamorphism occurred beween 15.0 and 19.8 Ma ago. K–Ar ages of biotites from a granodiorite on the west coast are indistinguishable from those found in the metamorphic complex, and hornblende K–Ar ages from the same samples are in the range 12.1–13.6 Ma. As the latter ages are somewhat younger than most ages obtained from the metamorphic complex, intrusion of the granodiorite most likely followed the peak of the M2 metamorphism.The metamorphic evolution of Naxos is consistent with rapid crustal thickening during the Cretaceous or early Tertiary, causing conditions at which supracrustal rocks experienced pressures in the range 900–1500 MPa. Transition to normal crustal thicknesses ended the M1 metamorphism about 50 Ma ago. The M2 metamorphism and granodiorite intrusion occurred during a period of heat input into the crust, possibly related to the migration of the Hellenic volcanic ar°C in a southerly direction through the area.

Notes: Abstract The layers of six stromatic migmatites from Northern, Western, and Central Europe display small but systematic chemical and mineralogical differences. At least five of these migmatites do not show any signs of largescale metamorphic differentiation, metasomatism, or segregation of melts. It is concluded, therefore, that the compositional layering observed in most of the investigated migmatites is due to compositional differences inherited from the parent rocks. Almost isochemical partial melting seems to be the most probable process transforming layered paragneisses, metavolcanics, or schists into migmatites.The formation of neosomes is believed to be caused by higher amounts of partial melts formed due to higher amounts of water moving into these layers. The neosomes have less biotite and more K-feldspar, if K-feldspar is present at all, than the adjacent mesosomes. These differences are small but systematic and seem to control the access of different amounts of water to the various rock portions. Petrographical observations, chemical data, and theoretical considerations indicate a close relationship between rock composition, rock deformation, transport of water, partial melting, and formation of layered migmatites.

Notes: Abstract Anatectic migmatites of contrasting structural style are found adjacent to the contacts of the Ballachulish Igneous Complex, Argyllshire, Scotland. On the east flank, evidence for migmatization is largely restricted to the local development of millimetre-centimetre scale Kfs + Qtz-rich leucocratic segregations, which accompany fragmentation of brittle hornfels layers and ductile deformation of mm-cm scale semipelitic layers. Large volumes of semipelitic rock rich in feldspar and quartz on the east flank show no migmatitic features, and bedding is usually preserved undisturbed right up to the contact. On the west flank, in contrast, similar semipelitic rocks show widespread migmatitic features and disruption of layering is substantial and widespread over a 400 m wide zone. Within the west-flank migmatites, 1–100 cm scale rigid bedding fragments (schollen) may be suspended and disoriented in a semipelitic matrix that underwent ductile deformation. The P-T conditions on both flanks are in the same range: 3 kbar and 650–700°C.The contrast in gross structural style is believed to result from differences in the volumes of melt produced and differences in the proportion of rock in which the critical melt fraction of the rocks was exceeded. On the east flank, only on a mm-cm scale was enough melt locally accumulated to cause disruption of some layers and segregation of melt. On the west flank, melting proceeded substantially in a broad tract of semipelitic rocks, resulting in larger scale contrasts in rheology that led to the present chaotic structures in this zone.Because migmatization occurred at a pressure too low for muscovite dehydration melting, and at temperatures too low for substantial biotite dehydration melting, the different amounts of melting on the east and west flanks most probably resulted from the introduction of differing amounts of externally derived water. On the east flank, and throughout most of the aureole, the absence of melting even in quartzofeldspathic protoliths indicates that there was no substantial movement of fluid towards or away from the igneous complex during migmatization. The contrasting situation on the west flank may have resulted from devolatilization of underlying quartz diorite magma (˜ 690–710°C), which released heat and fluids into the overlying quartz- and feldspar-rich semipelites (solidus temperature ˜ 650–680°C).

Notes: Abstract The sedimentary and igneous rocks comprising the lower Proterozoic Olary Block, South Australia, were deformed and metamorphosed during the mid-Proterozoic ‘Olarian’Orogeny. The area is divided into three zones on the basis of assemblages in metapelitic rocks, higher grade conditions occurring in the south-east. Mineral assemblages developed during peak metamorphism, which accompanied recumbent folding, include andalusite in Zones I and II and sillimanite in Zone III. Upright folding and overprinting of mineral assemblages occurred during further compression, the new mineral assemblages including kyanite in Zone II and kyanite and sillimanite in Zone III. The timing relationships of the aluminosilicate polymorphs, together with the peak metamorphic and overprinting parageneses, imply an anticlockwise P–T path for the ‘Olarian’Orogeny, pressure increasing with cooling from the metamorphic peak.

Notes: Abstract Sodic amphiboles are common in Franciscan type II and type III metabasites from Cazadero, California. They occur as (1) vein-fillings, (2) overgrowths on relict augites, (3) discrete tiny crystals in the groundmass, and (4) composite crystals with metamorphic Ca–Na pyroxenes in low-grade rocks. They become coarse-grained and show strong preferred orientation in schistose high-grade rocks. In the lowest grade, only riebeckite to crossite appears; with increasing grade, sodic amphibole becomes, first, enriched in glaucophane component, later coexists with actinolite, and finally, at even higher grade, becomes winchite. Actinolite first appears in foliated blueschists of the upper pumpellyite zone. It occurs (1) interlayered on a millimetre scale with glaucophane prisms and (2) as segments of composite amphibole crystals. Actinolite is considered to be in equilibrium with other high-pressure phases on the basis of its restricted occurrence in higher grade rocks, textural and compositional characteristics, and Fe/Mg distribution coefficient between actinolite and chlorite. Detailed analyses delineate a compositional gap for coexisting sodic and calcic amphiboles. At the highest grade, winchite appears at the expense of the actinolite–glaucophane pair.Compositional characteristics of Franciscan amphiboles from Ward Creek are compared with those of other high P/T facies series. The amphibole trend in terms of major components is very sensitive to the metamorphic field gradient. Na-amphibole appears at lower grade than actinolite along the higher P/T facies series (e.g. Franciscan and New Caledonia), whereas reverse relations occur in the lower P/T facies series (e.g. Sanbagawa and New Zealand). Available data also indicate that at low-temperature conditions, such as those of the blueschist and pumpellyite–actinolite facies, large compositional gaps exist between Ca- and Na-amphiboles, and between actinolite and hornblende, whereas at higher temperatures such as in the epidote–amphibolite, greenschist and eclogite facies, the gaps become very restricted.Common occurrence of both sodic and calcic amphiboles and Ca–Na pyroxene together with albite + quartz in the Ward Creek metabasites and their compositional trends are characteristic of the jadeite–glaucophane type facies series. In New Caledonia blueschists, Ca–Na pyroxenes are also common; Na-amphiboles do not appear alone at low grade in metabasites, instead, Na-amphiboles coexist with Ca-amphiboles throughout the progressive sequence. However, for metabasites of the intermediate pressure facies series, such as those of the Sanbagawa belt, Japan and South Island, New Zealand, Ca–Na pyroxene and glaucophane are not common; sodic amphiboles are restricted to crossite and riebeckite in composition and clinopyroxenes to acmite and sodic augite, and occur only in Fe2O3-rich metabasites.The glaucophane component of Na-amphibole systematically decreases from Ward Creek, New Caledonia, through Sanbagawa to New Zealand. This relation is consistent with estimated pressure decrease employing the geobarometer of Maruyama et al. (1986). Similarly, the decrease in tschermakite content and increase in NaM4 of Ca-amphiboles from New Zealand, through Sanbagawa to New Caledonia is consistent with the geobarometry of Brown (1977b). Therefore, the difference in compositional trends of amphiboles can be used as a guide for P–T detail within the metamorphic facies series.

Notes: Abstract Edenite/tremolite and edenite/magnesio-hornblende in equilibrium with plagioclase, chlorite, epidote, quartz and vapour involve several types of reactions for which KD can be related to T and P. Thermodynamic calculation of these equilibria leads to isopleth systems. Given knowledge of the progressive changes of end-member activities in zoned Ca–Mg amphiboles (based on microprobe analyses), it is possible to construct precise pressure–temperature–time paths (P–T–t paths) which have been followed by metabasites during polyphase metamorphism. When applied to basic rocks from the River Vilaine area, this method allows us to construct a P–T–t path that can be compared directly to the P–T–t path constructed from interbedded acid rocks (aluminous micaschists) in the same structural unit. Through time, both basic and acid rocks underwent the same complex deformation history that can be described conveniently in the L–S fabric system of Flinn. This allows us to construct a P–T–t deformation path for this structural unit.These paths are interpreted in terms of an under/overthrusting continental collision belt (the Hercynian belt), and represent an illustration of the time delay caused by stacking of more than two crustal units.

Notes: Abstract The Western Baja terrane (WBt) of west-central Baja California is an uplifted subduction complex that is divided into smaller ‘subterranes’on the basis of bounding faults and petrological differences. Each subterrane contains coherent Early Jurassic to Early Cretaceous sedimentary and mafic volcanic rocks (not melange) that have been metamorphosed under blueschist facies conditions. Key phases in metabasites and metaturbidites include jadeitic to acmitic clinopyroxene, sodic amphibole, lawsonite, aragonite, chlorite, titanite and white mica. Pressure indicators include the jadeite content of clinopyroxene and the presence of aragonite. Temperature indicators include the presence of lawsonite, the absence of greenschist facies minerals and results from vitrinite reflectance studies. Conditions at the peak of metamorphism were 〉8 kbar, 225–325°C for subterrane 1, 7–8 kbar, 170–220°C for subterrane 2, and 5–6 kbar, 175–200°C for subterrane 3; these correspond to cold geothermal gradients (6–9/km). Vein assemblages that include aegerine–jadeite and aegerine, albite, aragonite, lawsonite and sodic amphibole indicate uplift during continued cold conditions, probably during steady-state subduction.

Notes: LOW TEMPERATURE METAMORPHISM. Edited by M. Frey. Blackie & Son Limited, Glasgow and London. 1987. pp. 364.

Notes: Abstract Contact metamorphism adjacent to a porphyritic quartz-monzodiorite at Kentucky, New South Wales, Australia has produced hornfelses in porphyritic leucogranite at a peak temperature of about 650–700° C and a maximum confining pressure of about 2 kbar (200 MPa). A gradation appears to exist from normal slightly peraluminous to modified strongly peraluminous metagranite hornfelses, which have also been enriched in sulphur. The strongly peraluminous hornfelses, containing cordierite, andalusite, sillimanite, biotite, pyrite and pyrrhotite, retain residual porphyritic igneous microstructures. These rocks appear to have been formed by leaching of base cations, during and possibly just before the contact metamorphism. Folia of fibrous sillimanite anastomose between lenticular grains of quartz and feldspar and truncate igneous zoning in plagioclase grains, suggesting that cation leaching and solution transfer occurred during growth of the sillimanite. Fibrous sillimanite also grew in grain boundaries of polygonal aggregates formed by the contact metamorphism. Therefore, at least some of the cation leaching appears to have occurred at the highest metamorphic grade. Metasandstones that are locally strongly peraluminous adjacent to the monzodiorite stock also, have probably undergone similar leaching.

Notes: Abstract The Paikon Series is considered to be a volcanic arc sequence with a mainly neritic sedimentary sequence and bimodal tholeiitic volcanism of early Mesozoic age. The metamorphic assemblages are syn- to post-kinematic with respect to a pre-Tithonian tectonic phase and range from the lawsonite-chlorite-albite facies through transitional Na-amphibole-greenschist facies to the chlorite sub-zone of the greenschist facies. The metamorphic imprint of the Paikon Series corresponds to a temperature range from less than 330° C to ± 450° C under a total pressure from 3 kbar to 6–7 kbar. The overprinting of these facies on an earlier blueschist assemblage, related either to a subduction zone or to a tectonic overpressure caused by thrusting, is suspected.

Notes: Abstract Textural evolution and compositional variation of clinopyroxenes in Ward Creek metabasites are described. Pyroxenes change, with increasing grade, from finegrained aggregates through fan-shaped medium-grained prisms to blocky coarse crystals. Characteristic features of metamorphic pyroxenes include: (1) the occurrence of coexisting pyroxene pairs, the compositions of which are used to delineate compositional gaps; (2) the existence of large compositional variations of pyroxenes, within a single specimen, which record a considerable span of P and/or T for crystallization; and, (3) the development of compositional trends in single specimens and in three metamorphic zones which are progressive in nature.The first formed clinopyroxene (Jd20Aug65Ac15) in the lower lawsonite zone mimics the composition of relict igneous augite. It changes continuously, with increasing grade, at nearly constant low XJd content towards acmite. At a composition around Jd20Aug30Ac50, the trend turns towards jadeite and intersects a solvus to form two coexisting clinopyroxenes in the middle lawsonite zone. At higher grade, the compositional gap becomes restricted towards the jadeite-omphacite join and clinopyroxene increases in XJd toward jadeite. A reversed compositional trend occurs at higher grade; clinopyroxenes decrease in jadeite component at nearly constant Aug/Ac ratio of 50/50 and finally become omphacite in the uppermost pumpellyite and epidote zones. The Na–Ca pyroxenes, close to the binary join Jd–Ac, occur in the lawsonite- and pumpellyite-zones, ranging from XJd= 1.0–0.30 together with Ab and Qz. The ubiquitous occurrence of aragonite at temperature estimates of 170–240° C by Taylor & Coleman (1968) for these zones does not support the low-temperature extrapolation of the Jd–Ab–Qz curve by Holland (1980).The estimated metamorphic field gradient indicates an inflection point at 7 kbar, 200° C. Below this, blueschist facies metamorphism proceeded under dominant pressure-increase from 4 to 7 kbar at nearly constant temperature, about 150–200° C, whereas at higher grade recrystallization, above the inflection point, the metamorphic temperature increased from 200 to 350° C at nearly constant pressure, about 7–8 kbar. Such an inflection point suggests the depth of underplating of either seamounts or accretionary packages in a subduction zone.

Notes: Abstract Sapphirine-bearing rocks occur in three conformable, metre-size lenses in intrusive quartzo-feldspathic orthogneisses in the Curaçà valley of the Archaean Caraiba complex of Brazil. In the lenses there are six different sapphirine-bearing rock types, which have the following phases (each containing phlogopite in addition):A: Sapphirine, orthopyroxene;B: Sapphirine, cordierite, orthopyroxene, spinel;C: Sapphirine, cordierite;D: Sapphirine, cordierite, orthopyroxene, quartz;E: Sapphirine, cordierite, orthopyroxene, sillimanite, quartz;F: Sapphirine, cordierite, K-feldspar, quartz.Neither sapphirine and quartz nor orthopyroxene and sillimanite have been found in contact, however. During mylonitization, introduction of silica into the three quartz-free rocks (which represent relict protolith material) gave rise to the three cordierite and quartz-bearing rocks. Stable parageneses in the more magnesian rocks were sapphirine–orthopyroxene and sapphirine–cordierite. In more iron-rich rocks, sapphirine–cordierite, sapphirine-cordierite–sillimanite, cordierite–sillimanite, sapphirine–cordierite–spinel–magnetite and quartz–cordierite–orthopyroxene were stable. The iron oxide content in sapphirine of the six rocks increases from an average of 2.0 to 10.5 wt % (total Fe as FeO) in the order: C,F–A,D–B,E. With increase in Fe there is an increase in recalculated Fe2O3 in sapphirine.The four rock types associated with the sapphirine-bearing lenses are:I: Orthopyroxene, cordierite, biotite, quartz, feldspar tonalitic to grandioritic gneiss;II: Biotite, quartz, feldspar gneiss;III: Orthopyroxene, clinopyroxene, hornblende, plagioclase meta-norite;IV: Biotite, orthopyroxene, quartz, feldspar, garnet, cordierite, sillimanite granulite gneiss.The stable parageneses in type IV are orthopyroxene–cordierite–quartz, garnet–sillimanite–quartz and garnet–cordierite–sillimanite.Geothermobarometry suggests that the associated host rocks equilibrated at 720–750°C and 5.5–6.5 kbar. Petrogenetic grids for the FMASH and FMAFSH (FeO–MgO–Al2O3–Fe2O3–SiO2–H2O) model systems indicate that sapphirine-bearing assemblages without garnet were stabilized by a high Fe3+ content and a high XMg= (Mg/ (Mg+Fe2+)) under these P–T conditions.

Notes: Abstract Chloritoid–hornblende occurs in quartz–muscovite pelitic schist derived from sediment in a volcaniclastic sequence of the Grenville Supergroup and from reworked sedimentary and regolithic material above the unconformity at the base of the Flinton Group. Comparison of these samples with other pelitic rocks on triangular composition diagrams and in the ACNF and ACFM tetrahedra indicates that the presence of hornblende cannot be explained by unusually high CaO content. The rare assemblage is attributed to a combination of relatively low Al2O3 and high K2O with high CaO/(CaO+Na2O) and FeO/(FeO+MgO).On two qualitative reaction grids derived from AFM diagrams projected through CaO and plagioclase, respectively, the P–T stability field of chloritoid–hornblende overlaps the first appearance of staurolite–biotite in normal pelitic rocks in the kyanite field. Staurolite–hornblende overlaps chloritoid–hornblende and extends to the higher temperatures and pressures of the kyanite–hornblende field.The phase relations in these rocks provide a link between the conventional hornblende-absent grids for pelitic rocks and those for K2O-poor (muscovite-absent) pelitic and mafic amphibolitic rocks.

Notes: Abstract The effect of ductile deformation (dislocation creep) on the kinetics of the aragonite-calcite transformation has been studied at 1 atm (330° C and 360° C) and 900-1500 MPa (500° C) using undeformed and either previously or simultaneously deformed samples (500° C and a strain rate of 10-6 s). Deformation enhances the rate of the transformation of calcite to aragonite, but decreases the rate of transformation of aragonite to calcite. The difference results from a dependence of transformation rate on grain size, coupled with a difference in the accommodation mechanisms, climb versus recry-stallization, of these minerals during dislocation creep. Dislocation climb is relatively easy in calcite and thus plastic strain results in high dislocation densities without significant grain size reduction. The rate of transformation to aragonite is enhanced primarily because of the increase in nucleation sites at dislocations and subgrain boundaries. In aragonite, on the other hand, dislocation climb is difficult and thus plastic strain produces extensive dynamic recry-stallization resulting in a substantial grain size reduction. The transformation of aragonite is inhibited because the increase in calcite nucleation sites at dislocations and/or new grain boundaries is more than offset by the inability of calcite to grow across high angle grain boundaries. Thus the net effect of ductile deformation by dislocation creep on the kinetics of polymorphic phase transformations depends on the details of the accommodation mechanism.

Notes: Abstract Endogreisens which replace K-feld-spar-quartz dykes in a Devonian (360 Ma) tin deposit at Mt Bischoff, north-west Tasmania, formed from the interaction of unusual solutions, probably derived from an underlying leucogranite pluton, porphyry dykes and limited quantities of local dolomitic country rock components. The intensity of greisenization and pH of the solutions increase inward to the greisenized dykes’cores and downward. The following types of greisen assemblages indicate increasing degrees of greisenization: ‘sericite’muscovite + quartz ± tourmaline ± fluorite, topaz + quartz ± tourmaline ± fluorite, weberite, prosopite, ralstonite, Ca-ralstonite; and quartz ± topaz ± fluorite. Where the solutions interacted with dolomite, exogreisens consisting of topaz- or tourmaline-bearing assemblages were formed. The greisens were subsequently overprinted to varying degrees by siderite, sulphides and hydrous silicates (talc, serpentine, chlorite, micas).The temperature during greisenization ranged from 180 to 414°C, based on fluid inclusions in topaz, quartz, fluorite, sellaite and cassiterite. The main greisen-forming event occurred at temperatures of 360±20°C. The fluids boiled intermittently. Their salinities ranged from 31.5 to 38.9 wt% total dissolved salts, consisting of Ca–K–Na–Fe–Cl±hydrocarbon species. Fluid inclusion data indicate that only 0.5–1.5 km of cover were present above this deposit at the time of formation.The greisenized dykes were intruded by and intrude different stages of breccias. The breccias consist mainly of country rock and greisenized dyke fragments, with rock-flour and later tourmaline alteration. The Mt Bischoff greisen system is possibly part of a ‘porphyry tin’style deposit formed at near-surface conditions (0.5–1.0 km).

Notes: Abstract Dehydration-melting reactions, in which water from a hydrous phase enters the melt, leaving an anhydrous solid assemblage, are the dominant mechanism of partial melting of high-grade rocks in the absence of externally derived vapour. Equilibria involving melt and solid phases are effective buffers of aH2,o. The element-partitioning observed in natural rocks suggests that dehydration melting occurs over a temperature interval during which, for most cases, aH2o is driven to lower values. The mass balance of dehydration melting in typical biotite gneiss and metapelite shows that the proportion of melt in the product assemblage at T± 850°C is relatively small (10–20%), and probably insufficient to mobilize a partially melted rock body.Granulite facies metapelite, biotite gneiss and metabasic gneiss in Namaqualand contain coarse-grained, discordant, unfoliated, anhydrous segregations, surrounded by a finer grained, foliated matrix that commonly includes hydrous minerals. The segregations have modes consistent with the hypothesis that they are the solid and liquid products of the dehydration-melting reactions: Bt + Sil + Qtz + PI = Grt ° Crd + Kfs + L (metapelite), Bt + Qtz + Pl = Opx + Kfs + L (biotite gneiss), and Hbl + Qtz = Opx + Cpx + Pl + L (metabasic gneiss). The size, shape, distribution and modes of segregations suggest only limited migration and extraction of melt. Growth of anhydrous poikiloblasts in matrix regions, development of anhydrous haloes around segregations and formation of dehydrated margins on metabasic layers enclosed in migmatitic metapelites all imply local gradients in water activity. Also, they suggest that individual segregations and bodies of partially melted rock acted as sinks for soluble volatiles. The preservation of anhydrous assemblages and the restricted distribution of late hydrous minerals suggest that retrograde reaction between hydrous melt and solids did not occur and that H2O in the melt was released as vapour on crystallization.This model, combined with the natural observations, suggests that it is possible to form granulite facies assemblages without participation of external fluid and without major extraction of silicate melt.

Notes: Abstract Nearly pure CO2 fluid inclusions are abundant in migmatites although H2O-rich fluids are predicted from the phase equilibria. Processes which may play a role in this observation include (1) the effects of decompression on melt, (2) generation of a CO2-bearing volatile phase by the reaction graphite + quartz + biotite + plagioclase = melt + orthopyroxene + CO2-rich vapour, (3) selective leakage of H2O from CO2+ H2O inclusions when the pressure in the inclusion exceeds the confining pressure during decompression, and (4) enrichment of grain-boundary vapour in CO2 by subsolidus retrograde hydration reactions.

Notes: Abstract The Hidaka metamorphic terrane in the Meguro-Shoya area, Hokkaido, Japan is divided into four progressive metamorphic zones: A—biotite zone; B—cordierite zone; C—cordierite–K-feldspar zone; and, D—sillimanite–K-feldspar zone of the andalusite–sillimanite facies series type of metamorphism. The metamorphic grade ranges from the higher temperature part of the greenschist facies (zone A) through the amphibolite facies (zones B and C) to the lower temperature part of the granulite facies (zone D). The zone boundaries intersect the bedding planes at high angles. P–T conditions estimated are 450–550°C and 2 kbar for zone A, 550–600°C and 2–2.5 kbar for zone B, 600–650°C and 2.5–3 kbar for zone C and 650–750°C and 3–4 kbar for zone D. The metapelites of zone D were partially melted.At the later stage of the regional metamorphism which is early Oligocene to early Miocene in age, cordierite tonalite and biotite tonalite intrusives associated with segments of the highest grade rocks (zone D) were emplaced into the lower temperature part of the regional metamorphic rocks, giving rise to a contact metamorphic aureole. The thermally metamorphosed terrain (zone C') belongs to the amphibolite facies and its P–T conditions are estimated to have been 550–700°C and 2 kbar.The P–T–t paths of the Hidaka metamorphism show a thickening–heating–uplifting process. The metamorphism is inferred to have taken place beneath an active island arc accompanied by partial melting of the crust.

Notes: Abstract Textural relationships between porphyroblasts of biotite and garnet in metasediments in the Nordkinn Peninsula area of the Finnmarkian Caledonides of North Norway are apparently complex. There is evidence for two textural zones in both mineral phases and superficially the development of these appears to have overlapped, at least in part, in time and space. This apparently complex porphyroblast growth history can be considerably simplified if only one period of garnet growth occurred and if different inclusion fabrics developed where garnet replaced biotite porphyroblasts and where it overgrew the matrix foliation. The possibility that porphyroblasts with textural evidence for multiphase growth histories actually grew during a single crystallization event is of importance in the interpretation and elucidation of tectonometamorphic relationships.

Notes: Abstract Retrograde and prograde mineral assemblages from metapelitic and metabasic rocks of the Iforas Granulitic Unit (Mali) were generated by the superimposition of two granulite facies metamorphic events. They clearly result from a polycyclic evolution and can be related to a late Eburnean unroofing followed by a Pan-African burial.Thermobarometry on Pan-African garnet-bearing assemblages yields (P, T) estimates of 620±50°C and 5± Ikbar. The nearly anhydrous conditions produced in the Eburnean appear to be the direct cause of the unusually lowtemperature granulite-facies metamorphism in the Pan-African. These P, T estimates are compared with those obtained on the underlying unit (Kidal Assemblage) upon which the Iforas Granulitic Unit was thrust. A P-T-t path, during the Pan-African orogeny, is proposed and discussed for both the Iforas Granulites and Kidal Assemblage.

Notes: Abstract Fluids, some of which are CO2-rich (up to 40 mol.% CO2) and some of which are highly saline (up to 18 wt% NaCl equivalent), are trapped as fluid inclusions in quartz-calcite (∼ metallic minerals) veins which cross-cut the pumpellyite-actinolite to amphibolite facies rocks of the Alpine Schist. Fluids were commonly trapped as immiscible liquid-vapour mixes in quartz and calcite showing open-space growth textures. Fluid entrapment occurred at fluid pressures near 500 bars (possibly as low as 150 bars) at temperatures ranging from 260 to 330° C. Saline fluids may have formed by partitioning of dissolved salts into an aqueous phase on segregation of immiscible fluids from a low-density CO2-rich fluid. Calcite deposited by these fluids has δ13C ranging from – 8.4 to – 11.5 and δ18O from + 4 to + 13. Isotopic data, fluid compositions and mode of occurrence suggest that the fluids are derived from high-grade metamorphic rocks. Fluid interaction with wall-rock has caused biotite crystallization and/or recrystallization in some rocks and retrogression of biotite to chlorite in other rocks.Fluid penetration through the rock is almost pervasive in many areas where permeability, probably related to Alpine Fault activity, has focussed fluids on a regional scale into fractured rocks. The fluid flow process is made possible by high uplift-rates (in excess of 10 mm/year) bringing hot rocks near to the surface.

Notes: Abstract Cordierite-anthophyllite rocks and related cordierite-rich, talc-rich and chlorite-rich rocks occur in the Rosebud Syncline, north-west Queensland, Australia, as part of a Proterozoic metasedimentary sequence. Field relations and rock compositions attest the sedimentary origin of these rather unusual metamorphic rocks. Their chemical composition is comparable to that of unmetamorphosed, alkali- and Ca-poor pelites, which are associated with some evaporite deposits.Other occurrences of cordierite-anthophyllite rocks have commonly been interpreted as metamorphosed chloritic alteration products derived from mafic or felsic volcanics. A comparative chemical study, using analyses of cordierite-anthophyllite rocks from such alteration zones and analyses of unmetamorphosed magnesian pelites, demonstrates the general chemical similarity between these two rock groups of entirely different origin. However, distinct differences in major element relations help to distinguish these two genetic groups. Particularly useful are Al2O3–FeO–MgO plots, in which evaporitic pelites occupy the Fe-poor side.The highly magnesian metamorphic rocks from the Rosebud Syncline fall entirely into the compositional field of evaporitic clays and shales. Furthermore, analyses of relatively immobile trace elements give supporting evidence for the sedimentary origin of these cordierite-anthophyllite rocks. The correlation with trace element ranges of clays and shales is very good. However, the correlation with trace element ranges of mafic and felsic volcanics is poor, and major discrepancies occur with Cr, Ni, Co, Nb, Sc, Th and Ti.Thus, the magnesian metamorphics of the Rosebud Syncline appear to be derived from evaporitic clays rich in magnesian clay minerals, such as palygorskite, sepiolite, chlorite or corrensite. The complete metamorphic rock assemblage of interlayered calcareous, aluminous and magnesian rocks is interpreted as a metamorphosed carbonate-evaporite-pelite sequence.

Notes: Abstract A suite of granulites including a meta-ironstone, pyroxenites, and spinel-lherzolites from East Tonagh Island, Enderby Land, Antarctica, preserve exsolution-recry-stallization features consistent with a shared metamorphic evolution that involves marked cooling from initial metamorphic temperatures of nearly 1000°C. Reintegrated pre-exsolution and pre-reaction grain compositions in the meta-ironstone indicate the former coexistence of metamorphic pigeonite (Wo12En38Fs50) and ferroaugite (Wo35En31Fs34) at temperatures in excess of 980°C for pressures of 7 kbar (0.7 GPa) using pyroxene quadrilateral thermometry (Lindsley, 1983). Intra-grain lamellae relationships indicate the exsolution of a second pigeonite (Wo12En35Fs53) from the ferroaugite at temperatures in the range 930–970°C, prior to the c. 720–600°C exsolution of orthopyroxene and clinopyroxene (100) lamellae and later partial recrystallization at similar temperatures. Although pyroxenitic and iherzolitic granulites preserve a much less complete history, reintegrated porphyroclast compositions in these yield temperature estimates which approach those inferred from the metaironstone. Pyroxene thermometry based on neoblast compositions suggests that recrystallization post-dating a late, low intensity, deformation phase (D3) occurred at temperatures greater than 600°C. These results are consistent with the independent evidence obtained from studies of metapelitic and felsic rock types for very high temperature metamorphism throughout the Napier Complex followed by near-isobaric cooling and later deformation under lower-grade granulite facies conditions. Comparison with similar pyroxene data from Fyfe Hills (Sandiford & Powell, 1986) demonstrates further the regional significance of these high temperatures, and implies broadly isothermal metamorphic conditions over a large area (∼ 5000 km2) and thickness (6–9 km) of lower crust at c. 3070 Ma.

Notes: Abstract A detailed field and petrological study of rocks from nappes cut by the Valle dell'Orco (Italian Western Alps), in particular the Sesia–Lanzo composite unit, has revealed geological and metamorphic histories which started in pre-alpine times and lasted up to the alpine subduction-collisional processes. During these processes the nappes sustained an early high P–low T stage and a later low P greenschist facies stage, but followed partly distinctive P–T–time trajectories. This paper discusses the kinematic evolution and the thermal history of the alpine belt from the early subduction/underthrust to the later exhumation stage. The metamorphic crystallization is often governed by incomplete and/or local equilibrium, and the pervasive syn-metamorphic deformation and the composition of the syn-metamorphic fluid phase (if present) have exerted an effective local control on reaction kinetics.

Notes: Abstract Optical and X-ray studies of carbonaceous material in the Tono contact metamorphic aureole, Kitakami Mountains, northeast Japan, have revealed that metamorphic graphitization proceeded through two discontinuous changes: first, optically anisotropic domains develop within the coaly phytoclast, forming transitional material, and then, ordered graphite crystallizes by the decomposition of pre-existing carbonaceous materials. Coaly material disappears in the uppermost chlorite zone. Transitional material appears in the middle of the lower chlorite zone. Graphite appears in the upper chlorite zone and its modal abundance increases across the andalusite iso-grad to the cordierite isograd where all the carbonaceous materials have converted to graphite. The apparently continuous variation in the crystallographic parameters of the bulk carbonaceous material during graphitization is largely due to variation in the modal proportions of three types of carbonaceous materials. The temperature of graphitization in the present area is at least 100°C higher than the temperature in the Sanbagawa and New Caledonia high-pressure metamorphic terrains, probably due to the slow reaction rate of metamorphic graphitization and to the short duration of contact metamorphism.

Notes: Abstract Dissolution of quartz and the recryst-allization and re-equilibration of phyllosilicates produce the dark anastomosing seams that dominate microstructures of tectonic melange which occur in a low-grade, imbricated and multiply deformed, mid-Palaeozoic, intracra-tonic fiysch sequence in northeastern Australia. Seams are composed of very closely spaced or coalesced cleavage lamellae, which are very thin layers of extremely fine-grained phyllosilicates. Cleavage seams enclose lenses of silt-stone or greywacke, which formerly occurred in continuous sedimentary layers, indicating extremely heterogeneous and disruptive deformation. Microphacoids enclosed by cleavage seams have subtle shape asymmetries analogous to those of porphyroclasts. Phyllosilicate-preferred orientations within microphacoids commonly lie at a low angle to enclosing seams, and asymmetric relationships occur within seams. The shape and fabric asymmetries appear to be constant, and are regarded as analgous to S and C planes. The number and extent of seams, the amount of dissolution they indicate, and the efficiency of deformation partitioning imply some enhancement of chemical activity and substantial silica loss from the system. This, in turn, suggests the passage of large amount of silica-undersaturated fluid, and melanges may be zones of high fluid flow. However, the microstructures and the disruptive nature of the fabrics may also reflect the influence of high bulk shear strains and suggest some relationship between the shearing component of deformation and the development of cleavages and foliations.

Notes: Abstract In the Bois de Cené area, blueschist facies rocks, characterized by glaucophane and/ or chloritoid, provide evidence for a suture zone in the Variscan. This terrain is considered to be the eastern equivalent of the Ile de Groix high-pressure metamorphic terrain. Petrological study of the two characteristic types of rocks found in the area shows that the primary high-pressure paragenesis was modified during a retrogression which followed substantial decompression, probably at constant or decreasing temperature. The simplest interpretation is that this retrogression followed tectonic emplacement within a nappe pile.

Notes: Abstract In the Fleur de Lys Supergroup, western Newfoundland, inclusion trails in garnet and albite porphyroblasts indicate that porphyroblasts overgrew a crenulation foliation, without rotation, probably during the deformation event that produced the crenulations. Further deformation of the matrix resulted in strong re-orientation and retrograde metamorphism of the matrix foliation, which is consequently highly oblique to the crenulation foliation preserved in the porphyroblasts. The resulting matrix foliation locally preserves relics of the early crenulations, and also has itself been crenulated later in places. Thus the porphyroblasts grew before the later stages of deformation, rather than during the final stage, as had been suggested previously. The new interpretation is consistent with available 40Ar/39Ar cooling ages which indicate a late Ordovician-early Silurian metamorphic peak, rather than the Devonian peak suggested by previous workers. The inclusion patterns and microprobe data indicate normal outward growth of garnet porphyroblasts from a central nucleus, rather than as a series of veins as proposed by de Wit (1976a, b). However, the observations presented here support growth of porphyroblasts without rotation, which is implied by the de Wit model.

Notes: Abstract In foliated K-feldspar-sillimanite metapelites, fibrous sillimanite is commonly concentrated in folia that anastomose between lenticular pods of coarser-grained aggregates rich in quartz, feldspar and biotite, with or without garnet, cordierite and residual andalusite. Many of the folia appear to be limbs of crenulation microfolds. The sillimanite concentrations may be due largely to the ability of fibrous sillimanite aggregates to undergo strong non-coaxial deformation by grain-boundary sliding (‘fibre sliding’;) without appreciable build-up of dislocations, whereas other minerals are unstable in these zones and so concentrate in lower-strain interfolial zones. Initiation, and especially concentration of the sillimanite in folia, may be assisted by fluid flow and local base-cation leaching, whereby minerals unstable in zones of strong non-coaxial strain are dissolved and removed from these zones.

Notes: Abstract The Tal y Fan Intrusion is a 110 m thick sub-concordant metabasite sheet intruded into volcaniclastic and pyroclastic rocks of Ordovician age in North Wales. Despite low grade metamorphism, primary textural zones resulting from initial cooling of the sheet are preserved and retain primary mineralogical and chemical variations which influenced the nature and extent of metamorphic recrystallization. This has resulted in a vertical sequence of secondary mineral assemblages through the intrusion. During early hydrothermal alteration K-feldspar replaced plagioclase micropheno-crysts in the marginal and contact zones, and olivine in the central zone was replaced by saponite. Subsequent regional metamorphism resulted in the development of (metastable) prehnite-pumpellyite-epidote assemblages in two sub-zones characterized by high Fe2O3. Elsewhere the assemblage prehnite-actinolite-epidote developed except in the contact and marginal zones where activity of CO2 suppressed both prehnite and pumpellyite. Both assemblages contain excess albite, quartz and chlorite and, on the basis of uniform mineral compositions over the area of an individual thin section, are considered to represent buffered equilibrium assemblages indicative of prehnite-pumpellyite and prehnite-actinolite facies conditions. A metamorphic temperature of 310° C at 1.85 kbar is obtained using the P-T-X grid of Liou, Maruyama & Cho (1985), which implies a field gradient of ∼ 44° C km-1. Assuming that metamorphism relates to burial, an overburden thickness of ∼ 7 km is indicated. Total maximum thicknesses, however, of Ordovician, Silurian and Lower Devonian strata, in the area, do not exceed 6 km indicating a field gradient of 52° C km-1. These relatively high gradients may possibly be related to concealed late Caledonian intrusions, or alternatively may result from high heat flow as a consequence of crustal thinning, rapid sedimentation and intense magmatic activity in a marginal basin setting.

Notes: Abstract Mineralogical and geochemical evidence indicates that partial melting and desulphidation have occurred in the Big Bell gold deposit. Decarbonation may also have occurred, to account for the lack of a carbonate alteration halo; this is compatible with the present data, but difficult to test.The Big Bell deposit consists of auriferous sulphide-bearing (‘lode’;) schists with muscovite and K-feldspar, and surrounding biotite schists, all derived by intense premetamorphic alteration of rocks of mafic composition. Assemblages which include cordierite-sillimanite-K-feldspar-garnet-biotite-quartz suggest peak metamorphic conditions of 4–5 kbar, and 650–700° C, based on phase relations, geobaro-meters and garnet-biotite Fe-Mg exchange partitioning. Partial melting occurred at peak metamorphism, particularly in the altered mafic rocks in and around the deposit, and its occurrence may have been essential to the preservation of the deposit. Melting greatly limited the importance of devolatilization reactions, resulting in negligible aqueous fluids and no means of removing appreciable gold. Minor gold loss may have accompanied desulphidation. A diversity of complex metamorphic assemblages occurs around the mine, compared to the assemblages developed regionally; variable bulk rock composition influences this contrast, but there is no evidence of higher metamorphic grades at the mine, nor that this might have been the prime control on the different assemblages in this narrow belt.It is suggested that the Big Bell and Hemlo deposits are the higher metamorphic grade equivalents of the more abundant greenschist facies gold deposits within Archaean greenstone belts. This interpretation is favoured by the host rock setting and geochemical characteristics of Big Bell. Alternative models that suggest that this class of deposit is a new type must account for the absence of high-grade equivalents of the greenschist facies deposits and also the lack of low-grade equivalents of the Big Bell/Hemlo type.Archaean gold deposits in high-grade metamorphic terrains have undergone a series of processes that are not recorded in the more typical gold deposits of the greenschist facies.

Notes: Abstract Comparison of the stability relations of Fe-piemontite, Pm17Ps17Cz66 (Ca2Al2Mn0.5 Fe0.5Si3O12(OH)), with similar experimentally determined results on Pm33Cz67 suggests that, at moderate fo2, addition of Fe to piemontite raises its thermal stability and slightly decreases its sensitivity to oxygen fugacity. Reversal study of the reaction: Fe-piemontite + quartz = garnet (Gr33Sp27And40) + anorthite + fluid at Pfluid= 2 kbar in cold-seal apparatus using solid oxygen buffers was completed. The data indicated breakdown of Pm17Ps17Cz66 at 645°± 10° C along the Cu2O–CuO buffer, 477°± 10° C along the Cu–Cu2O buffer, and 365°± 10° C for the hematite-magnetite buffer. Mn-free clinozoisite and epidote have been shown to be stable to much higher temperatures than piemontite at moderate fo2 (〈 Cu–Cu2O). At very high fo2, however, the presence of Mn+3 in a distorted octahedral site may permit persistence of piemontite to higher temperatures than Mn-poor epidote minerals.The compositional range of natural Fe–Mn–Al piemontites supports these results. Cation partitioning from piemontite-bearing parageneses also indicates crystallization at high fo2, but application of the cited results should be made with caution. Synthetic phases were too fine-grained for optical or microprobe analysis, and reaction was often incomplete. The assumption that all Fe + Mn was contained in piemontite and garnet, respectively, in the low- and high-temperature assemblages is supported by the cell dimensions for these synthetic minerals, which are close to predicted values. The reaction is pseudo-univariant, as solid solution in garnet and piemontite would be fo2-dependent in a more complex chemical system. Although bulk rock and fluid compositions may also have a marked effect on the stability of natural epidote–piemontite, fo2 is the major control on Fe : Mn : Al in epidote minerals.

Notes: Abstract Pelitic assemblages from all major Witwatersrand gold fields record metamorphic conditions of the greenschist facies, with minimal regional grade changes over at least 200 km strike length. Diagnostic metamorphic assemblages are less common in the volumetrically dominant quartzites, the actively-exploited auriferous conglomerates and some of the regionally persistent metapelitic horizons. Bulk rock composition has been a major control on assemblage development.Key metapelitic assemblages include pyrophyllite, chloritoid, chlorite and muscovite in each gold field, with less common metamorphic biotite. Accessory minerals are pyrite, tourmaline, rutile and zircon. The abundance of chloritoid and pyrophyllite in thin shaly units, together with their minor, but widespread, distribution in quartzites and conglomerates, indicate that metamorphic temperatures reached 350°C ± 50°C in all the gold fields. Pressures are less-well constrained, 1–2 kbar being inferred. Outside the gold fields, higher grades are indicated by andalusite and kyanite near granitoid domes and later intrusions.The temperatures during peak metamorphism and the abundance of pyrite provide ideal conditions to (re)mobilize gold and may explain its secondary textural features.

Notes: Abstract Corona textures, which developed in alternating layers in rocks in a supracrustal belt at Errabiddy, Western Australia, involved:(a) The production of staurolite, cordierite and quartz or sapphirine between Kyanite and/or sillimanite and gedrite; and(b) The production of cordierite between garnet and gedrite.These textures are inconsistent with development along the same pressure–temperature path in the system FeO–MgO–Al2O3–SiO2–H2O, but can be accounted for if CaO, mainly in garnet, is taken into account. The sapphirine-bearing kyanite–gedrite textures are explained by lower a(SiO2) during their development. The assemblages indicate a consistent pressure–temperature (P–T) trajectory involving substantial uplift with only a slight decrease in temperature. The history of these rocks includes reheating of originally high-grade rocks that had cooled to a stable conductive geotherm, followed by substantial, essentially isothermal uplift. The tectonic environment for this was presumably the one responsible for emplacement of the high-grade terrain in the upper crust.

Notes: A previous paper (Gugan, 1987) described the geometric model used to find the exterior orientation of dynamic SPOT imagery. The present paper considers the results of measurement and interpretation tests performed on a number of stereomodels. The image quality is assessed in comparison with the original digital data. Level 1A stereomodels with different base height ratios, numbers of control points and with different control point accuracies are compared. Accuracies obtained with level 1P and 1B data are also reported. The information content of the imagery is assessed by feature plotting followed by comparison with the 1:50 000 and 1:100 000 scale maps of the area. The importance of high quality photographic imagery and operator experience are noted.

Notes: Work is reported on the production of terrain elevation data from SPOT imagery. An important step in this process is determination of exterior orientation parameters for the images. Although an initial estimate is available in the SPOT header, refinements are required based on plan and height data. These are generally derived from photogrammetric measurements (or sources such as global positioning satellite systems), but accurate manual determination is time consuming.This paper reports an alternative approach by investigating the use of existing map data (located on the earth's surface and in a known projection) to find corresponding “ground control points” in both map and image. Available maps may be in digital or paper form depending on the state of development of mapping in the part of the world being studied. The last revision date and the cartographic generalisation employed will also effect the fidelity of the final result. Practical semi-automatic techniques for determining the height of ground control points from existing map data are described, including multiple profiles through digitised contour data, a commercial terrain modelling package based on Delaunay triangulation and the use of triangulation points on prominent features.Comparisons between the methods are made and results obtained are compared with Institut Géographique National datasets and photogrammetric measurements. Methods for fully automating not only the height determination but also the location of ground control features within a map database are discussed. Map and image registration is described and illustrated using Laser-Scan's Rover software. Interactive and automated methods of registration are discussed.

Notes: The Overseas Surveys Directorate (OSD) of the Ordnance Survey of Great Britain (OS) has experimented with using SPOT I HRV data for topographic line mapping at 1:100 000 scale in the Yemen Arab Republic (YAR). Results of these tests indicate that sufficient accuracy to support 1:100 000 scale mapping with a 40 m contour interval is possible but that the compilation of detail is incomplete. More emphasis on field completion and verification is required than with traditional techniques to maintain a standard 1:100 000 scale mapping specification. Costs of mapping large areas can be significantly reduced but savings are yet to be made in providing ground control. OSD has acquired expertise and hardware in a new mapping process, not to replace traditional mapping systems but to complement them under appropriate conditions. OSD has developed sufficient confidence in the use of SPOT to commit itself to mapping approximately 25 000 square kilometres of north east Yemen Arab Republic.

Notes: Bundle triangulation is an efficient tool for spatial point determination in architectural photogrammetry. All the information available in image and object space may be introduced in a combined adjustment yielding high precision and reliability. Camera calibration performed simultaneously with the object restitution process enables the application of convenient and flexible imaging systems based on professional photographic technology. Thus the survey of monuments can be carried out rapidly with a minimum of personnel and equipment. The photogrammetric recording of the basilica of San Francesco in Siena is presented as an example of the application of this survey procedure. The field work was accomplished in a few days. The equipment used consisted of a partial metric camera, an engineering theodolite and a tape. The bundle adjustment results gave spatial object co-ordinates with a r.m.s.e. of ± 15 mm for a point.

Notes: Book review in this aticle:INTRODUCTION TO THE PHYSICS AND TECHNIQUES OF REMOTE SENSING. By C. Elachi.COMPUTER PROCESSING OF REMOTELY-SENSED IMAGES. By P. M. Mather.ARCTIC AIRMEN. B E. Schofield and R. C. Nesbit.WARBURTON'S WAR. The life of Wing Commander Adrian Warburton, DSO and Bar, DFC and Two Bars, DFC (USA). By T. Spooner.MAP PROJECTIONS—A WORKING MANUAL. By. J. P. Snyder.WORLD MAPPING TODAY. By R. B. Parry and C. R. Perkins.CONTROL SURVEYS IN CIVIL ENGINEERING. By M. A. R. Cooper.INTRODUCTION TO REMOTE SENSING. By J. B. Campbell.ANALYTICAL PHOTOGRAMMETRY. By S. K. Ghosh.

Notes: The availability of data in digital formats, the rapid evolution of computer systems and the increased demands for cartographic databases are causing photogrammetrists to consider ways of integrating photogrammetric, remote sensing and database technologies. Examples are presented involving the use of aerial photographs and satellite images in combination with geographical information systems and image processing techniques to assess soil erosion, growth of aquatic vegetation and to enhance satellite images for the compilation or revision of map products.

Notes: A new lightweight vertical camera rig which can be clamped to the front legs of the front seat of a light aircraft is described. In one version, a fixed transparent fairing is attached to the door, whereas in the second version the fairing is attached to the camera rig itself which can then be extended and retracted in flight. This rig has received C.A.A. (Civil Aviation Authority) Approval.

Notes: SPOT-1, launched on 22nd February, 1986, has provided the first opportunity to produce topographic maps from space on an operational basis. The use of SPOT imagery can be justified because of reduced costs and reduced lead times for map production. However SPOT imagery cannot be handled in the same way as conventional photography and it requires new procedures to be adopted, The two main reasons for this are the large area covered by each image and the dynamic form of sensing. The procedures discussed in this paper have been implemented in a working system for map compilation.