Alpine metamorphism in the south-east Tauern Window, Austria: 1. P–T variations in space and time

Abstract The Pennine rocks exposed in the south-east Tauern Window, Austria, contain mineral assemblages which crystallized in the mid-Tertiary ‘late Alpine’regional metamorphism. The pressure and temperature conditions at the thermal peak of this event have been estimated for rocks at four different structural levels using a variety of published and thermochemically derived geobarometers and geothermometers. The results are: (a) In the garnet+chlorite zone, 2–5 km structurally above the staurolite+biotite isograd: T= 490.50°C, P= 7° 1 kbar; (b) Within 0.5 km of the staurolite+biotite isograd: T= 560±300C, P=7.1 kbar; (c) In the staurolite+biotite zone, c. 2.5 km structurally below the staurolite+biotite isograd: T= 610±30°C, P=7.6±1.2 kbar; (d) In the staurolite+biotite zone, 3–4 km structurally below the staurolite+biotite isograd: T= 630±40°C, P= 6.6±1.2 kbar. The pressure estimates imply that the total thickness of overburden above the basement-cover interface in the mid-Tertiary was c. 26.4 km. This overburden can only be accounted for by the Austro-Alpine units currently exposed in the vicinity of the Tauern Window, if the Altkristallin (the ‘Middle Austro-Alpine’nappe) was itself buried beneath an ‘Upper Austro-Alpine’nappe or nappe-pile which was 7.4 km thick at that time. The occurrence of epidote + margarite + quartz pseudomorphs after lawsonite in garnet, indicates that part of the Mesozoic Pennine cover sequence in the south-east Tauern experienced blueschist-facies conditions (T<450°C, P<12 kbar) in early Alpine times. Evidence from the central Tauern is used to argue that the blueschist-facies imprint post-dated the main phase of tectonic thickening (D¹_A) and was thus a direct consequence of continental collision. Combined oxygen-isotope and fluid-inclusion studies on late-stage veins, thought to have been at lithostatic pressure and in thermal equilibrium with their host rocks during formation, suggest that they crystallized from aqueous fluids at 1.1±0.4 kbar and 420.20°C. Early Alpine, late Alpine and vein-formation P–T constraints have been used to construct a P–T path for the base of the Mesozoic cover sequence in the south-east Tauern Window. The prograde part of the P–T path, between early and late Alpine metamorphic imprints, is unlikely to have been a smooth curve and may well have had a low dP/dT overall; the decompression (presumably due to erosion) which occurred immediately before the thermal peak and possibly also earlier in the Tertiary, was probably partly or completely cancelled by the effects of early- to mid-Tertiary (D²_A) tectonic thickening. The thermal peak of metamorphism was followed by a phase of almost isothermal decompression, which implies a period of rapid uplift in the middle Tertiary. The peak metamorphic P–T estimates are compared with the solutions of England's (1978) one-dimensional conductive thermal model of the Eastern Alps, and are shown to be consistent with the idea that the late Alpine metamorphism was caused by tectonic burial of the Pennine Zone beneath the Austro-Alpine nappes in the absence of extraneous heat sources, such as large intrusions, at depth.     

Metamorphic P-T paths from calcic pelitic schists from the Strafford Dome, Vermont, USA

Metamorphism of the Gile Mountain Formation and Waits River Formation in the Strafford Dome and Townshend-Brownington Syncline in east-central Vermont records two nappe-style events, D1 and D2, followed by doming. D1 formed a muscovite + biotite ± ilmenite schistosity subparallel to compositional layering, SO, and was followed by heating to garnet grade. The temperature and pressure at the end of D1 are estimated to be c . 450 C and 6-8 kbar. D2 variably crenulated and folded S1 during a nearly isothermal pressure increase of 1-2 kbar, calculated from compositions of garnet, which have inclusions trails with progressive crenulation and rotation of the S1 fabric. Similar P-T paths are computed for most of the area, suggesting that the later schistosity developed during emplacement of a regional nappe 3-6 km thick. There is a general lack of D3 (dome-stage) microstructures.
Near the Strafford-Willoughby Arch, staurolite and kyanite overgrew S2 in pelites, and plagioclase with increasing X _An overgrew S2 in calcic pelites, reflecting post-D2 heating to a maximum of 550-600 C. Metamorphic pressures at the end of D2 are fairly constant on the west side of the dome, indicating minor dome-stage uplift. In contrast, pressures at the thermal peak of metamorphism decrease by more than 4 kbar east of the dome. The observed pattern of isotherms and isobars is mainly the result of post-metamorphic, differential uplift and unroofing.
Finally, a minor, retrograde metamorphism produced the assemblage albite + epidote + K-feldspar + muscovite + chlorite, with grade increasing east toward the Connecticut River.     

Chloritoid–glaucophane schist in the north Qilian orogen, NW China: phase equilibria and P–T path from garnet zonation

Chloritoid–glaucophane‐bearing rocks are widespread in the high‐pressure belt of the north Qilian orogen, NW China. They are interbedded and cofacial with felsic schists originated from greywackes, mafic garnet blueschists and low‐T eclogites. Two representative chloritoid–glaucophane‐bearing assemblages are chloritoid + glaucophane + garnet + talc + quartz (sample Q5‐49) and chloritoid + glaucophane + garnet + phengite + epidote + quartz (sample Q5‐12). Garnet in sample Q5‐49 is coarse‐, medium‐ and fine‐grained and shows two types of zonation patterns. In pattern I, X_grs is constant as X_py rises, and in pattern II X_grs decreases as X_py rises. Phase equilibrium modelling in the NC(K)MnFMASH system with Thermocalc 3.25 indicates that pattern I can be formed during progressive metamorphism in lawsonite‐stable assemblages, while pattern II zonation can be formed with further heating after lawsonite has been consumed. Garnet growth in Q5‐49 is consistent with a continuous progressive metamorphic process from ～14.5 kbar at 470 °C to ～22.5 kbar at 560 °C. Garnet in sample Q5‐12 develops with pattern I zonation, which is consistent with a progressive metamorphic process from ～21 kbar at 540 °C to ～23.5 kbar at 580 °C with lawsonite present in the whole garnet growth. The latter sample shows the highest P–T conditions of the reported chloritoid–glaucophane‐bearing assemblages. Phase equilibrium calculation in the NCKFMASH system with a recent mixing model of amphibole indicates that chloritoid + glaucophane paragenesis does not have a low‐pressure limit of 18–19 kbar as previously suggested, but has a much larger pressure range from 7–8 to 27–30 kbar, with the low‐pressure part being within the stability field of albite.     

Understanding the pre-Variscan and Variscan basement components of the central Tauern Window, Eastern Alps (Austria): constraints from single zircon U-Pb geochronology

New single-grain and within-grain U-Pb zircon ages from the central Tauern Window help sorting out the time dimension among the various Variscan and pre-Variscan basement components that were strongly overprinted by Alpine orogeny. Single-grain isotope dilution (ID-TIMS) U-Pb zircon geochronology of three Basisamphibolit samples yield protolith formation ages of 351±2, 349±1 and 343±1 Ma. Laser ablation ICP-MS and ID-TIMS U-Pb detrital zircon dating of the Biotitporphyroblastenschiefer constrained the maximum time of sedimentation to between 362±6 Ma and 368±17 Ma. Paragneisses from the Zwölferzug yield maximum sedimentation ages from 345±5 Ma (ion microprobe data) to 358±10 Ma. Zircons from gabbroic clasts and detrital zircons from a meta-agglomerate from the Habach Phyllite give an upper intercept age of 536±8 Ma and a near-concordant age of 506±9 Ma, respectively. Hence, apart from the Habach Phyllite, the maximum sedimentation ages of the metasediments investigated range from Upper Devonian to Lower Carboniferous. Consequently, the Basisamphibolit, the Biotitporphyroblastenschiefer, and the paragneisses of the Zwölferzug form parts of the Variscan basement series. The Basisamphibolit (351-343 Ma) is distinct both in space and time of formation from the Zwölferzug garnet amphibolite (c. 486 Ma), which forms part of the pre-Variscan basement.     

Using white mica 40Ar/39Ar data as a tracer for fluid flow and permeability under high‐P conditions: Tauern Window,Eastern Alps

New single‐grain‐fusion muscovite and paragonite ⁴⁰Ar/³⁹Ar data from eclogite and blueschist units exposed in the Tauern Window, Eastern Alps yield a range of apparent ages from 90 to 23 Ma. These apparent ages are generally older than expected for ⁴⁰Ar/³⁹Ar cooling ages, given constraints from other geochronological systems such as Rb–Sr and U–Pb. Numerical Ar‐in‐muscovite diffusion models for Tauern Window nappe P–T paths in an open system suggest that ⁴⁰Ar/³⁹Ar ages should lie between 29 and 24 Ma, and that they should constrain cooling and decompression following the post‐high pressure Barrovian overprint. The measured ranges of apparent ⁴⁰Ar/³⁹Ar dates suggest that the assumption of open system behaviour is not valid for this region. The local and/or regional generation of fluid during exhumation promoted pervasive recrystallization of high pressure lithologies throughout the Tauern Window to greenschist and amphibolite facies assemblages. The old apparent ⁴⁰Ar/³⁹Ar white mica dates in all lithologies are therefore interpreted as being due to inefficient removal of grain boundary Ar by the grain boundary fluids during the Barrovian overprint, due to high Ar concentrations or limited connectivity or both. This caused spatially (mm‐scale) and temporally variable fluxes of Ar out of, and probably into, white mica in both metasedimentary and metabasic lithologies.     

Glaucophane and pyroxene breakdown reactions in the Pennine units of the Eastern Alps

Abstract New occurrences of crossite and jadeitic pyroxene are described from a thick metabasite unit within the upper levels of the Peripheral Schieferhülle in the Tauern Window, Austria. Unusual textures are preserved which provide evidence for the reactions and mechanisms involved in the breakdown of crossite and jadeitic pyroxene. Zones of albite and chlorite, produced by reaction between crossite and paragonite, have been preserved due to sluggish reaction kinetics during decompression from the blueschist to the greenschist facies. The zonal sequence is interpreted in terms of chemical potential gradients in Na, Mg and Al, which have been established by overstepping the equilibrium boundary. Breakdown textures of jadeite-acmite pyroxene to a symplectite of albite + hematite + actinolite, and of crossite to talc and actinolite are also described.
The occurrence of crossite and jadeitic pyroxene at high levels within the Peripheral Schieferhülle implies that even upper levels of the structural sequence have undergone blueschist facies metamorphism with pressures in excess of 8 kbar during the Alpine collisional event.     

P–T conditions for the Arendal granulites, southern Norway: implications for the roles of P, T and CO2 in deep crustal LILE-depletion

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

P–T paths in the Handöl area, central Scandinavia: record of Caledonian accretion of outboard rocks to the Baltoscandian margin

The Seve–Köli Nappe Complex is widespread in the Scandinavian Caledonides and is composed of units representing parts of the Baltoscandian margin (Seve Nappes) now overlain by magmatic–sedimentary rocks (Köli Nappes) derived from west of this margin. The metamorphic evolution of Köli and Seve units has been studied in the Handöl area, central Scandinavian Caledonides, where a fragmented ophiolite with cover sequence in the lower Köli units is thrust over the higher grade Seve units. Thermobarometry constrains metamorphic conditions to 490–570° C/950–600 MPa, with a slight downwards increase in grade, for the lower Köli (Bunnerviken lens), 520–620° C/1000–600 MPa for the upper Seve (Täljstensvalen Complex), 630–740° C/750–650 MPa for the middle Seve (Snasahögarna Nappe) and 480–600° C/1150–1000 MPa for the lower Seve (Blåhammarfjället Nappe).
P–T paths during garnet growth have been constructed for all units, except the highest grade middle Seve. These paths record heating at the base of the Köli and cooling in the underlying Seve units. Pressure increase during garnet growth is indicated for all units leading to anticlockwise P–T paths in the Seve. The results imply thermal convergence with time for all units and spatial convergence in metamorphic grade in the Köli. It is suggested that the contrasting metamorphic histories on either side of the Seve–Köli boundary resulted from the emplacement of relatively colder Köli rocks on top of relatively hotter Seve rocks and that emplacement of structurally higher units contributed to the increase in pressure.     

Inverted metamorphic sequence in the Sikkim Himalayas: crystallization history, P–T gradient and implications

The metapelitic rocks of the Sikkim Himalayas show an inverted metamorphic sequence (IMS) of the complete Barrovian zones from chlorite to sillimanite + K‐feldspar, with the higher grade rocks appearing at progressively higher structural levels. Within the IMS, four groups of major planar structures, S₁, S₂ and S₃ were recognised. The S₂ structures are pervasive throughout the Barrovian sequence, and are sub‐parallel to the metamorphic isograds. The mineral growth in all zones is dominantly syn‐S₂. The disposition of the metamorphic zones and structural features show that the zones were folded as a northerly plunging antiform. Significant bulk compositional variation, with consequent changes of mineralogy, occurs even at the scale of a thin section in some garnet zone rocks. The results of detailed petrographic and thermobarometric studies of the metapelites along a roughly E–W transect show progressive increase of both pressure and temperature with increasing structural levels in the entire IMS. This is contrary to all models that call for thermal inversion as a possible reason for the origin of the IMS. Also, the observation of the temporal relation between crystallization and S₂ structures is problematic for models of post‐/late‐metamorphic tectonic inversion by recumbent folding or thrusting. A successful model of the IMS should explain the petrological coherence of the Barrovian zones and the close relationship of crystallization in each zone with S₂ planar structures along with the observed trend(s) of P–T variation in Sikkim and in other sections. A discussion is presented of some of the available models that, with some modifications, seem to be capable of explaining these observations.     

Oxidized eclogites and garnet-blueschists from Oman: P–T path modelling in the NCFMASHO system

Eclogites and garnet‐blueschists exposed at the deepest structural levels of the Oman Mountains in north‐eastern Saih Hatat, Oman, indicate that the Arabian continental margin was subducted and subsequently exhumed. The peak metamorphic pressure has been a matter of debate for over a decade, with initial thermobarometric estimates, based on garnet–clinopyroxene–phengite barometry and the presence of radial cracks around quartz inclusions in garnet, yielding values in excess of 20 kbar; these estimates have been questioned by some researchers. The high‐pressure minerals (glaucophane, omphacite and epidote) contain significant amounts of ferric iron, previously postulated to displace the stability fields of the eclogite and blueschist assemblages to less extreme conditions. In the present study, we have calculated phase diagrams and pseudosections in the model system NCFMASHO, using the program thermocalc and the thermodynamic database of Holland and Powell, which incorporates data for Fe³⁺‐bearing end‐members. It is found that the phase compositions and modal abundances for typical bulk compositions are matched successfully at 520 ± 15 °C and 20 ± 1.6 kbar for the eclogites and 510–530 °C and 17–20 kbar for the garnet blueschists. These results support the original high‐pressure estimates for the eclogites, and indicate that crossitic amphibole and aegirine‐rich pyroxene do not necessarily reflect lower pressure conditions. The data set and activity models are applicable to other oxidized high (and ultra‐high) pressure mineral assemblages.     

Origin and mechanical significance of honeycomb garnet in high-pressure metasedimentary rocks from the Tauern Window, Eastern Alps

High-pressure schists (2–2.5 GPa) from the Eclogite Zone in the Tauern Window contain honeycomb garnet in which fine webs of garnet surround strain-free quartz ± carbonate grains. High-resolution X-ray computed tomography shows that the garnet webs form a cellular structure that coats all surfaces of the inclusions. Electron backscatter diffraction analysis shows that the garnet cells are crystallographically continuous with more massive garnet regions, and that the quartz ± carbonate inclusions have random orientations; in contrast, matrix quartz exhibits a prominent crystallographic preferred orientation (CPO). High-resolution transmission electron microscopy shows few dislocations in either the garnet or the inclusion quartz. Most honeycomb garnet is chemically homogeneous, but some displays asymmetric core–rim zoning. Taken together, these observations are most consistent with formation of the garnet sheets via precipitation from a wetting fluid along quartz–quartz grain boundaries, or possibly via wholesale precipitation of garnet + quartz ± carbonate from a fluid. In either case, a silicate-rich aqueous fluid must have been present. The likelihood that a fully wetting fluid existed at high pressure has important implications for rheology during subduction of metasedimentary rocks: strain may be accommodated by grain rotation and sliding in an aqueous silicate slurry, rather than via dislocation creep mechanisms at high pressures. The absence of a CPO in early quartz may thus point to involvement of a pervasive grain-boundary fluid rather than requiring low differential stresses during subduction.     

The retrograde P–T–t path for low-pressure granulites from the Reynolds Range, central Australia: petrological constraints and implications for low-P/high-T metamorphism

Several aspects of the petrogenesis of low-pressure granulite facies rocks from the Reynolds Range (central Australia) are contentious, including: (a) the shape of the retrograde P–T –time path, and whether it is an artefact of repeated thermal events at different P–T conditions; (b) the type of regional metamorphism; and (c) the causes of metamorphism. Granulite facies rocks from the Reynolds Range Group experienced three major periods of mineralogical equilibration. Metapelitic rocks underwent dehydration-melting reactions to form migmatites under peak M2 P–T conditions of c. 5.0–5.3 kbar and c. 750–800 °C. Metapsammitic rocks that did not melt during M2 show spectacular garnet–orthopyroxene intergrowths that developed at c. 3.5–3.7 kbar and c. 700–750 °C after penetrative regional deformation, but prior to amphibolite facies rehydration in discrete strike-parallel zones. Rehydration occurred within the sillimanite stability field at P–T conditions close to the granite solidus (c. 3.2–3.4 kbar and 650–700 °C). Subsequently the terrane cooled into the andalusite stability field. Geochronological constraints suggest that: (a) peak-M2 conditions were reached at c. 1594 Ma; (b) the garnet–orthopyroxene intergrowths in unmelted metapsammites probably developed between c. 1594 Ma and c. 1586 Ma; and (c) upper amphibolite facies rehydration occurred between c. 1586 Ma and 1568 Ma. The lack of petrological evidence for multiple dehydration and rehydration of the rocks suggests that the three episodes of mineralogical recrystallization can be linked to yield a single continuous retrograde P–T–t path of minor initial decompression (c. 1.5 kbar) from the M2 peak, followed by cooling (c. 100 °C) to the granite solidus over a period of c. 26 Ma. Late kyanite-bearing shear zones that dissect the terrane are unrelated to this event and formed during the c. 300–400 Ma Alice Springs Orogeny. The shape of the P–T–t path and the duration of M2 metamorphism suggests that advective heating was not the major cause of high-grade metamorphism, and that some other, longer lived heat source, such as the burial of anomalously radiogenic, pre-tectonic granites, is required.     

Metamorphic textures and P–T evolution of high-P granulites from the Lelukuau terrane, NE Grenville Province

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

Dehydration melting, fluid buffering and decompressional P–T path in a granulite complex from the Eastern Ghats, India

A suite of metapelites, charnockites, calc-silicate rocks, quartzo-feldspathic gneisses and mafic granulites is exposed at Garbham, a part of the Eastern Ghats granulite belt of India. Reaction textures and mineral compositional data have been used to determine the P–T–X evolutionary history of the granulites. In metapelites and charnockites, dehydration melting reactions involving biotite produced quartzofeldspathic segregations during peak metamorphism. However, migration of melt from the site of generation was limited. Subsequent to peak metamorphism at c . 860° C and 8 kbar, the complex evolved through nearly isothermal decompression to 530–650° C and 4–5 kbar. During this phase, coronal garnet grew in the calc-silicates, while garnet in the presence of quartz broke down in charnockite and mafic granulite. Fluid activities during metamorphism were internally buffered in different lithologies in the presence of a melt phase. The P–T path of the granulites at Garbham contrasts sharply with the other parts of the Eastern Ghats granulite belt where the rocks show dominantly near-isobaric cooling subsequent to peak metamorphism.     

Prograde, peak, and retrograde P–T paths from aluminium in orthopyroxene: High-temperature contact metamorphism in the aureole of the Makhavinekh Lake Pluton, Nain Plutonic Suite, Labrador

Static heating during intrusion of the Makhavinekh Lake Pluton (MLP) caused replacement of garnet in the adjacent country rocks (Tasiuyak Gneiss) by coronal assemblages of orthopyroxene + cordierite. Thermometry based on Al solubility in orthopyroxene, applied to relict garnet and neighbouring orthopyroxene, preserves a temperature gradient from 700 to 900 °C at distances between 5750 and 20 m from the intrusion, reaffirming the robustness of this thermometry technique. Intracrystalline and intergranular variations of Al zoning in orthopyroxene are well‐preserved, suggesting that little diffusional modification of Al growth zoning occurred. Maximum Al₂O₃ in orthopyroxene ranges from c. 2.0 wt% at 5750 m from the intrusion to a maximum of 4.3 wt% at the contact. Individual orthopyroxene grains show decreasing Al from core to rim in samples < 500 m from the intrusion, while those at greater distances show an increase from core to rim. These features are interpreted with the aid of numerical models for conductive heat flow in the aureole. Coronas in samples close to the intrusion grew at high temperatures and along T‐t paths dominated by cooling, so maximum Al content in orthopyroxene in these samples occurs in the cores of grains that grew during the earliest stages of garnet consumption. In contrast, the corona‐forming reactions in rocks further from the contact proceeded along prograde heating paths, so maximum Al content in orthopyroxene occurs in the rims of grains that grew during the final stages of garnet consumption. These results document the ability of Al‐in‐orthopyroxene thermometry to preserve a detailed record of thermal histories in contact‐metamorphic granulites; they suggest that similar intracrystalline and intergranular variations of Al zoning in orthopyroxene in regional granulites may also preserve portions of both the prograde and peak‐T evolution.     

Age and early metamorphic history of the Sanbagawa belt: Lu–Hf and P–T constraints from the Western Iratsu eclogite

Two distinct age estimates for eclogite-facies metamorphism in the Sanbagawa belt have been proposed: (i) c.  120–110 Ma based on a zircon SHRIMP age for the Western Iratsu unit and (ii) c.  88–89 Ma based on a garnet–omphacite Lu–Hf isochron age from the Seba and Kotsu eclogite units. Despite the contrasting estimates of formation ages, petrological studies suggest the formation conditions of the Western Iratsu unit are indistinguishable from those of the other two units—all ∼20 kbar and 600–650 °C. Studies of the associated geological structures suggest the Seba and Western Iratsu units are parts of a larger semi-continuous eclogite unit. A combination of geochronological and petrological studies for the Western Iratsu eclogite offers a resolution to this discrepancy in age estimates. New Lu–Hf dating for the Western Iratsu eclogite yields an age of 115.9 ± 0.5 Ma that is compatible with the zircon SHRIMP age. However, petrological studies show that there was significant garnet growth in the Western Iratsu eclogite before eclogite facies metamorphism, and the early core growth is associated with a strong concentration of Lu. Pre-eclogite facies garnet (Grt1) includes epidote–amphibolite facies parageneses equilibrated at 550–650 °C and ∼10 kbar, and this is overgrown by prograde eclogite facies garnet (Grt2). The Lu–Hf age of c.  116 Ma is strongly skewed to the isotopic composition of Grt1 and is interpreted to reflect the age of the pre-eclogite phase. The considerable time gap ( c.  27 Myr) between the two Lu–Hf ages suggests they may be related to separate tectonic events or distinct phases in the evolution of the Sanbagawa subduction zone.     

Inference of a detailed P–T path from P–T pseudosections using metapelitic rocks of variable composition from a single outcrop, Shackleton Range, Antarctica

Generally, P–T pseudosections for reduced compositional systems, such as K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O, Na₂O–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O and MnO–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O, are well suited for inferring detailed P–T paths, comparing mineral assemblages observed in natural rocks with those calculated. Examples are provided by P–T paths inferred for four metapelitic samples from a 1 m² wide outcrop of the Herbert Mountains in the Shackleton Range, Antarctica. The method works well if the bulk composition used is reconstituted from average mineral modes and mineral compositions (AMC) or when X‐ray fluorescence (XRF) data are corrected for Al₂O₃ and FeO. A plagioclase correction is suitable for Al₂O₃. Correction for FeO is dependent on additional microscopic observations, e.g. the kind and amount of opaque minerals. In some cases, all iron can be treated as FeO^tot, whereas in others a magnetite or hematite correction yields much better results. Comparison between calculated and observed mineral modes and mineral compositions shows that the AMC bulk composition is best suited to the interpretation of rock textures using P–T pseudosections, whereas corrected XRF data yield good results only when the investigated sample has few opaque minerals. The results indicate that metapelitic rocks from the Herbert Mountains of the Northern Shackleton Range underwent a prograde P–T evolution from about 600 °C/5.5 kbar to 660 °C/7 kbar, followed by nearly adiabatic cooling to about 600 °C at 4.5 kbar.     

Syn‐metamorphic folding in the Tauern Window,Austria dated by Th–Pb ages from individual allanite porphyroblasts

High‐precision ²³²Th–²⁰⁸Pb dates have been obtained from allanite porphyroblasts that show unambiguous microstructural relationships to fabrics in a major syn‐metamorphic fold in the SE Tauern Window, Austria. Three porphyroblasts were analysed from a single garnet mica schist from the Peripheral Schieferhülle in the core of the Ankogel Synform, one of a series of folds which developed shortly before the thermal peak of Alpine epidote–amphibolite facies metamorphism: allanite grain 1 provided two analyses with a combined age of 27.7 ± 0.7 Ma; grain 2, which was slightly bent and fractured during crenulation, provided two analyses with a combined age of 27.7 ± 0.4 Ma; a single analysis from grain 3, which overgrew an already crenulated fabric, gave an age of 28.0 ± 1.4 Ma. The five ²³²Th–²⁰⁸Pb ages agree within error and define an isochron with an age of 27.71 ± 0.36 Ma (95% confidence level; MSWD = 0.46). The results imply that the crenulation event was in progress in a short interval (<1 Ma) c. 28 Ma, and that the Ankogel Synform was forming at this time. The thermal peak of regional metamorphism in the SE Tauern Window was probably attained shortly after 28 Ma, only c. 5 Ma after eclogite facies metamorphism in the central Tauern Window. Metasediment may contain allanite porphyroblasts with clear‐cut microstructural relationships to fabric development and metamorphic crystallization; for such rocks, ²³²Th–²⁰⁸Pb dating on microsamples offers a powerful geochronological tool.     

High-pressure granulites in the Sanggan area, North China craton: metamorphic evolution, P–T paths and geotectonic significance

High‐pressure basic granulites are widely distributed as enclaves and sheet‐like blocks in the Huaian TTG gneiss terrane in the Sanggan area of the Central Zone of the North China craton. Four stages of the metamorphic history have been recognised in mineral assemblages based on inclusion, exsolution and reaction textures integrated with garnet zonation patterns as revealed by compositional maps and compositional profiles. The P–T conditions for each metamorphic stage were obtained using thermodynamically and experimentally calibrated geothermobarometers. The low‐Ca core of growth‐zoned garnet, along with inclusion minerals, defines a prograde assemblage (M1) of garnet + clinopyroxene + plagioclase + quartz, yielding 700 °C and 10 kbar. The peak of metamorphism at about 750–870 °C and 11–14.5 kbar (M2) is defined by high‐Ca domains in garnet interiors and inclusion minerals of clinopyroxene, plagioclase and quartz. Kelyphites or coronas of orthopyroxene + plagioclase ± magnetite around garnet porphyroblasts indicate garnet breakdown reactions (M3) at conditions around 770–830 °C and 8.5–10.5 kbar. Garnet exsolution lamellae in clinopyroxene and kelyphites of amphibole + plagioclase around garnet formed during the cooling process at about 500–650 °C and 5.5–8 kbar (M4). These results help define a sequential P–T path containing prograde, near‐isothermal decompression (ITD) and near‐isobaric cooling (IBC) stages. The clockwise hybrid ITD and IBC P–T paths of the HP granulites in the Sanggan area imply a model of thickening followed by extension in a collisional environment. Furthermore, the relatively high‐pressures (6–14.5 kbar) of the four metamorphic stages and the geometry of the P–T paths suggest that the HP granulites, together with their host Huaian TTG gneisses, represent the lower plate in a crust thickened during collision. The corresponding upper‐plate might be the tectonically overlying Khondalite series, which was subjected to medium‐ to low‐pressure (MP/LP: 7–4 kbar) granulite facies metamorphism with a clockwise P–T path including an ITD segment. Both the HP and the MP/LP granulite facies events occurred contemporaneously at c. 1.90–1.85 Ga in a collisional environment created by the assembly process of the North China craton.     

Metasomatic albitites and related biotite-rich schists from a low-pressure polymetamorphic terrane, Snake Creek Anticline, Mount Isa Inlier, north-eastern Australia: microstructures and P–T–d paths

Rocks of the Snake Creek Anticline are mainly pelitic schists, psammitic schists and quartzites that were metamorphosed during multiple high‐T/low‐P events extending from D1 to D5, with the metamorphic peak occurring late to post‐D3. Albitites are widespread, but are concentrated in five areas. They are typically fine‐ to medium‐grained, and consist of albite, with or without combinations of quartz, biotite, staurolite, cordierite, garnet, andalusite, sillimanite, kyanite, gedrite and tourmaline. From the presence or absence of albite inclusions in porphyroblasts, the albitites are interpreted as forming early in the D3 event as a result of infiltration of external fluids. Psammitic schists and quartzites were preferentially altered, but pelitic schists were also albitized in localities where the alteration was more extreme, with the replacement of muscovite total and the replacement of quartz and biotite variable. Structural controls on albitization include fracturing and syn‐D3 shear zones in fold hinges. Biotite schists with abundant porphyroblasts (combinations of staurolite, garnet, andalusite and cordierite) occur adjacent to albitites, and it is argued that they formed by the addition of Fe and Mg sourced from the albitites. In several albitite‐rich areas, cordierite grew early in D3 and was partly or entirely replaced during or after D3 by combinations of biotite, andalusite, tourmaline, staurolite and sillimanite. A postulated P–T–d path involved an increase in pressure (with or without a decrease in temperature) subsequent to early D3 albitization, followed by an increase in temperature up to the metamorphic peak (late D3 to early D4. The metamorphism was contemporary in part with the emplacement of the Williams Batholith (c. 1550–1500 Ma), which probably supplied the Na‐rich fluids.