Problems with inferring P–T–t paths in low-P granulite facies rocks

Microstructural evidence commonly is used to infer metamorphic reactions, which are used to infer pressure–temperature–time ( P–T–t ) paths. However, this approach in low- P /high- T  (LPHT) granulite facies metamorphic terranes has two main problems. (1) Microstructural evidence may be inconclusive, so that reactions cannot be inferred with confidence. In particular, relative timing of mineral growth inferred from inclusions, moulding relationships and foliation–porphyroblast relationships is commonly ambiguous or invalid. The most reliable indicators of metamorphic reactions are partial pseudomorphs and corona structures, especially if symplectic intergrowths (indicating simultaneous growth of two or more minerals) are involved. (2) Even reactions that can be inferred with confidence do not indicate unique P–T  trends, owing to P–T  slopes of reaction curves. Where successive reactions can be shown to have occurred in the same rock, a line or curve joining reaction-curve intersections gives an apparent single-event path. However, isotopic evidence is needed to prove that polymetamorphism (involving more complex paths making fortuitous intersections with the apparent single-event path) did not occur. Although these problems are well known, their importance is not always emphasized in metamorphic investigations.
The difficulties are illustrated by published work on P–T–t paths for Proterozoic LPHT granulite facies rocks of central Australia and Antarctica. Recent work in Antarctica has shown that P–T–t paths may be episodic and more complex than the simple, single-event paths commonly inferred from microstructural evidence alone.     

Thermobarometry and P–T–t paths: the granulite to eclogite transition in lower crustal xenoliths from eastern Australia

‘Lower crustal’ suite xenoliths in basaltic and kimberlitic magmas are dominated by mafic granulites and may also include eclogites and garnet pyroxenites. Pressures of up to 25 kbar obtained from such xenoliths are well in excess of an upper value of c. 12 kbar for exposed granulite terranes. Palaeogeotherms constructed from xenoliths for the lower crust beneath the Phanerozoic fold belts of eastern Australia (SEA) and beneath the eastern margin of the Australian craton (EMAC) indicate two distinct thermal regimes. The two geotherms have similar form, with the EMAC curve displaced c. 150°C to lower temperatures. Reaction microstructures show the partial re-equilibration of primary igneous assemblages to granulite and eclogite assemblages and are interpreted to reflect the cooling from magmatic temperatures. Variations in mineral compositions and zoning are used to constrain further the history of several EMAC xenoliths to near-isobaric trajectories. Detailed graphical models are constructed to predict compositional changes for isobaric P–T paths (at 7, 14 & 21 kbar) to transform an SEA-type geotherm to a cratonic geotherm. The models show that for the assemblage grt + cpx ± ky + plag + qtz, the changes associated with falling temperature in X_gr, X_jd (increase) and X_an (decrease) will be greater at higher pressures. These results indicate that discernible zoning is more likely to be preserved in the higher pressure xenoliths. The zoning recorded in clinopyroxene from mafic granulite xenoliths over the pressure range c. 12–22 kbar suggests isobaric cooling of a large crustal thickness (30–35 km). An isobaric cooling path is consistent with magma accretion models for the transition of a crust–mantle boundary from an SEA-type geotherm to a cratonic geotherm. The coexistence of granulite and eclogite over the depth range 35–75 km beneath the EMAC indicates that the granulite to eclogite transition in the lower crust is controlled by P–T conditions, bulk chemistry and kinetic factors. At shallower crustal levels, typified by exposed granulite terranes, isobaric cooling may not result in the transition to eclogite.     

Contact metamorphic P–T–t paths from Sm–Nd garnet ages, phase equilibria modelling and thermobarometry: Garnet Ledge, south-eastern Alaska, USA

Sm–Nd garnet‐whole rock geochronology, phase equilibria, and thermobarometry results from Garnet Ledge, south‐eastern Alaska, provide the first precisely constrained P–T–t path for garnet zone contact metamorphism. Garnet cores from two crystals and associated whole rocks yield a four point isochron age for initial garnet growth of 89.9 ± 3.6 Ma. Garnet rims and matrix minerals from the same samples yield a five point isochron age for final garnet growth of 89 ± 1 Ma. Six size fractions of zircon from the adjacent pluton yield a concordant U–Pb age of 91.6 ± 0.5 Ma. The garnet core and rim, and zircon ages are compatible with single‐stage garnet growth during and/or after pluton emplacement. All garnet core–whole rock and garnet rim‐matrix data from the two samples constrain garnet growth duration to ≤5.5 my. A garnet mid‐point and the associated matrix from one of the two garnet crystals yield an age of 90.0 ± 1.0 Ma. This mid‐point result is logically younger than the 90.7 ± 5.6 Ma core–whole rock age and older than the 88.4 ± 2.5 Ma rim‐matrix age for this sample. A MnNaCaKFMASH phase diagram (P–T pseudosection) and the garnet core composition are used to predict that cores of garnet crystals grew at 610 ± 20 °C and 5 ± 1 kbar. This exceeds the temperature of the garnet‐in reaction by c. 50 °C and is compatible with overstepping of the garnet growth reaction during contact metamorphism. Intersection of three reactions involving garnet‐biotite‐sillimanite‐plagioclase‐quartz calculated by THERMOCALC in average P–T mode, and exchange thermobarometry were used to estimate peak metamorphic conditions of 678 ± 58 °C at 6.1 ± 0.9 kbar and 685 ± 50 °C at 6.3 ± 1 kbar, respectively. Integration of pressure, temperature, and age estimates yields a pressure‐temperature‐time path compatible with near isobaric garnet growth over an interval of c. 70 °C and c. 2.3 my.     

Orthopyroxene–sillimanite–quartz assemblages: distribution, petrology, quantitative P–T–X constraints and P–T paths

Granulite facies magnesian metapelites commonly preserve a wide array of mineral assemblages and reaction textures that are useful for deciphering the metamorphic evolution of a terrane. Quantitative pressure, temperature and bulk composition constraints on the development and preservation of characteristic peak granulite facies mineral assemblages such as orthopyroxene + sillimanite + quartz are assessed with reference to calculated phase diagrams. In NCKFMASH and its chemical subsystems, peak assemblages form mainly in high‐variance fields, and most mineral assemblage changes reflect multivariant equilibria. The rarity of orthopyroxene–sillimanite–quartz‐bearing assemblages in granulite facies rocks reflects the need for bulk rock X_Mg of greater than approximately 0.60–0.65, with pressures and temperatures exceeding c. 8 kbar and 850 °C, respectively. Cordierite coronas mantling peak minerals such as orthopyroxene, sillimanite and quartz have historically been used to infer isothermal decompression P–T paths in ultrahigh‐temperature granulite facies terranes. However, a potentially wide range of P–T paths from a given peak metamorphic condition facilitate retrograde cordierite growth after orthopyroxene + sillimanite + quartz, indicating that an individual mineral reaction texture is unable to uniquely define a P–T vector. Therefore, the interpretation of P–T paths in high‐grade rocks as isothermal decompression or isobaric cooling may be overly simplistic. Integration of quantitative data from different mineral reaction textures in rocks with varying bulk composition will provide the strongest constraints on a P–T path, and in turn on tectonic models derived from these paths.     

Quantitative P–T–X constraints on orthopyroxene-bearing high-pressure granulites in felsic–metapelitic rocks: evidence from the Huangtuling granulite, Dabieshan Orogen

High‐pressure granulites are generally characterized by the absence of orthopyroxene. However, orthopyroxene is reported in a few high‐pressure, felsic–metapelitic granulites, such as the Huangtuling felsic high‐pressure granulite in the North Dabie metamorphic core complex in east‐central China, which rarely preserves the high‐pressure granulite facies assemblage of garnet + orthopyroxene + biotite + plagioclase + K‐feldspar + quartz. To investigate the effects of bulk‐rock composition on the stability of orthopyroxene‐bearing, high‐pressure granulite facies assemblages in the NCKFMASHTO (Na₂O–CaO–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O–TiO₂–Fe₂O₃) system, a series of P–T–X pseudosections based on the melt‐reintegrated composition of the Huangtuling felsic high‐pressure granulite were constructed. Calculations demonstrate that the orthopyroxene‐bearing, high‐pressure granulite facies assemblages are restricted to low X_Al [Al₂O₃/(Na₂O + CaO + K₂O + FeO + MgO + Al₂O₃) < 0.35, mole proportion] or high X_Mg [MgO/(MgO + FeO) > 0.85] felsic–metapelitic rock types. This study also reveals that the X_Al values in the residual felsic–metapelitic, high‐pressure granulites could be significantly reduced by a high proportion of melt loss. We suggest that orthopyroxene‐bearing, high‐pressure granulites occur in residual overthickened crustal basement under continental subduction–collision zones and arc–continent collision belts.     

The P–T–deformation path for a mid-Proterozoic, low-pressure terrane: the Reynolds Range, central Australia

Abstract The Proterozoic low-pressure, high-temperature (LPHT) terrane of the Reynolds Range occurs in a 130-km-long, NW-trending belt in the central part of the Arunta Block, central Australia. The Reynolds Range has been affected by two mid-Proterozoic tectonic cycles, DI and DII, associated with two metamorphic events, MI and MII. DI–MI effects are restricted to the older of two sedimentary successions, the Lander Rock beds, which are separated from the younger Reynolds Range Group by an angular unconformity. The dominant structural–metamorphic features formed during DII–MII affected both sedimentary successions and the various granites that intruded them, and reworked most DI–MI effects. The DII deformation history can be subdivided into one prograde, two peak, and one retrograde stage. Average P–T calculations in the southeastern half of the range indicate a peak-metamorphic pressure of 4.1 ± 0.3 kbar. Because the calculated values are derived from the same stratigraphic level corresponding to the base of the Reynolds Range Group, which is exposed throughout the area, it is likely that pressures were similar in the entire range. In fact, however, the peak-metamorphic temperature shows a dramatic increase from greenschist facies (c. 400° C) in the northwest to granulite facies (740 ± 60° C) in the southeast, indicating that MII was associated with anomalously high heat flows. The P–T path is anticlockwise, with isobaric cooling from the metamorphic peak indicated by corona textures. However, the evidence of a prograde increase in pressure is indirect and based on the compressional nature of the structures. Peak-metamorphic mineral assemblages and retrograde mineral assemblages in amphibolite facies shear zones show the same metamorphic zonation, suggesting they formed in response to the same thermal event. If this is true, the implication is that a thermal perturbation external to the crust was maintained for a considerable period of time (110 Ma, based on zircon dating). As it is not clear whether Proterozoic, asthenosphere-active, thermal perturbations operated for this long, the alternative interpretation must be considered, namely that the peak-metamorphic events are separate from the shear zone event associated with reheating of the area.     

Metamorphism of the Central Anatolian Crystalline Complex, Turkey: influence of orogen-normal collision vs. wrench-dominated tectonics on P–T–t paths

The Central Anatolian Crystalline Complex (CACC) is a microcontinent in the Alpine–Himalayan belt. It has previously been considered as a coherent structural entity, but, although the entire CACC is comprised of similar rocks (primarily metasedimentary rocks and granitoids), it consists of at least four tectonic blocks characterized by different P–T–t paths. These blocks are the K?r?ehir (north‐west), Akda? (north‐east), Ni?de (south) and Aksaray (west) massifs. The northern massifs experienced thrusting and folding during collision and were slowly exhumed by erosion; metamorphic rocks are characterized by clockwise P–T paths at moderate P–T and local low‐P–high‐T (LP–HT) overprinting in the highest grade rocks. Apatite fission track ages are Eocene to Oligocene (47–32 Ma). The Aksaray block represents the hot, shallow mid‐crust of a Late Cretaceous–early Tertiary arc. It is dominated by intrusions; rare metapelitic rocks record low‐P (< 4 kbar) regional metamorphism overprinted by LP–HT contact metamorphism. Apatite fission track ages are 50–45 Ma. The Ni?de massif is different from the other CACC blocks because it evolved as a core complex in a wrench‐dominated setting. It is characterized by clockwise P–T paths at moderate P–T followed by widespread LP–HT metamorphism. Apatite fission track ages are Miocene (12–9 Ma), significantly younger than those in the northern massifs. Ni?de rocks resided in the mid‐crust at a time when the rest of the CACC was at or near the Earth's surface. Variations in P–T–t and tectonic histories — especially timing of exhumation — between the northern and southern CACC reflect the difference between head‐on collision vs. mid‐crustal wrenching.     

Proterozoic low-pressure/high-temperature metamorphism and an anticlockwise P–T–t path for the Hazeldene area, Mount Isa Inlier, Queensland, Australia

In the Hazeldene area, situated in the Mount Isa Inlier, Queensland, the metamorphic grade changes from chlorite zone, through biotite and cordierite zones, to sillimanite/K-feldspar zone.
Microstructural studies of rocks near the sillimanite isograd demonstrate that cordierite grew early during the development of a steep foliation (S2), was replaced by biotite, andalusite and sillimanite at the metamorphic peak late in S2, and in turn by kyanite + chlorite adjacent to localized small post-D2 shear zones. Although the anticlockwise P–T–t path is well defined, the precise P–T conditions are uncertain because of problems with experimental and thermodynamic data. The best estimate for the metamorphic peak for rocks close to the sillimanite isograd is around 600° C at 4 kbar.
The metamorphism has been dated at 1544 Ma, and was synchronous with a major crustal shortening event. Because proposed extensional events occurred more than 60 Ma earlier, their contribution to the peak metamorphic thermal perturbation would have been insignificant. The syn-metamorphic Mica Creek Pegmatites, the abundance of high heat-producing elements in the nearby pre-D2 Sybella Granite, and advective heat by fluids which caused considerable metasomatism in the Hazeldene area, may have each contributed to the thermal budget. However, the metamorphic thermal gradient may be 80°C km^-1 or higher, strongly suggesting a local magmatic control. As none are known in the area, such syn-metamorphic plutons would have to lie beneath the exposed high-grade rocks.     

High-pressure granulite facies metamorphism in the Pan-African belt of eastern Tanzania: P–T–t evidence against granulite formation by continent collision

To constrain the tectonic history of the Pan-African belt in Tanzania, we have studied the P–T evolution of granulites from northern and eastern Tanzania representative for a large part of the southern Pan-African belt of East Africa (e.g. Pare, Usambara, Ukaguru and Uluguru Mountains). Thermobarometry (conventional and multireaction equilibria) on enderbites and metapelites gives 9.5–11 kbar and 810±40 °C during peak metamorphism at 650–620 Ma. This is consistent with the occurrence of both sillimanite and kyanite in metapelites and of the high-P granulite facies assemblage garnet–clinopyroxene–quartz in mafic rocks. Peak metamorphic conditions are surprisingly similar over a very large area with N-S and E-W extents of about 700 and 200 km respectively. The prograde metamorphic evolution in the entire area started in the kyanite field but evolved mainly within the sillimanite stability field. The retrograde P–T evolution is characterized by late-stage kyanite in metapelites and garnet–clinopyroxene coronas around orthopyroxene in meta-igneous rocks. This is in agreement with thermobarometric results and isotopic dating, indicating a period of nearly isobaric and slow cooling prior to tectonic uplift. The anticlockwise P–T path could have resulted from magmatic underplating and loading of the lower continental crust which caused heating and thickening of the crust. Substantial postmetamorphic crustal thickening of yet unknown age (presumably after 550 Ma) led subsequently to the exhumation of high-P granulites over a large area. The results are consistent with formation of the Pan-African granulites at an active continental margin where tonalitic intrusions caused crustal growth and heating 70–100 Ma prior to continental collision. The P–T–t path contradicts recent geodynamic models which proposed tectonic crustal thickening due to continental collision between East and West Gondwana as the cause of granulite formation in the southern part of the Pan-African belt.     

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.     

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.     

Two types of Precambrian high-grade metamorphism, Inner Mongolia, China

Abstract Archaean and Proterozoic granulite facies complexes of Inner Mongolia differ in lithological association, tectonic style, mineral assemblage and metamorphic P–T path. A nearly isobaric cooling path for Archaean high-grade metamorphic rocks is suggested by reaction textures and geothermobarometry. Early Proterozoic metamorphic rocks show nearly isothermal decompression. Archaean metamorphism may have been caused by magmatic accretion, whereas early Proterozoic metamorphism suggests a major continental thickening event followed by exhumation.     

Prograde P–T path of medium-pressure granulite facies calc-silicate rocks, Higo metamorphic terrane, central Kyushu, Japan

This paper reports an occurrence of medium-pressure granulite facies calc-silicate rocks intercalated with pelitic gneisses in the Higo metamorphic terrane, central Kyushu, Japan, which is classified as a low- P /high- T (andalusite-sillimanite type) metamorphic belt. Three equilibrium stages are recognized in the calc-silicate rock based on reaction textures: M1 stage characterized by an assemblage of porphyroblastic garnet + coarse-grained clinopyroxene + plagioclase included in the clinopyroxene; M2 stage by two kinds of breakdown products of garnet, one is plagioclase + coronitic clinopyroxene within garnet and the other is plagioclase + vermicular clinopyroxene surrounding garnet; and M3 stage by amphibole replacing clinopyroxene. The key assemblage in the calc-silicate rock common to M1 and M2 stages is Grt + Cpx + Pl ± Qtz, which constrains the pressure and temperature ( P – T ) conditions for these stages by Fe–Mg exchange reaction and the two univariant net-transfer reactions: 2Grs + Alm + 3Qtz = 3Hd + 3An or 2Grs + Prp + 3Qtz = 3Di + 3An. The P – T conditions for M1 and M2 stages were estimated to be about 8.4 ± 1.9 kbar and 680 ± 122 °C, and 6.7 ± 1.9 to 8.9 ± 2.2 kbar and 700 ± 130 to 820 ± 160 °C, respectively. Estimates are consistent with an isobaric heating P – T path. The high peak temperature conditions at normal crustal depths and the prograde isobaric heating path probably require heat advection due to melt migration during the high- T metamorphism.     

Metamorphic history and tectonic evolution of the Qinling Complex, eastern Qinling Mountains, China

Abstract The metamorphic history and tectonic evolution of the Qinling Complex is divided into formation and modification stages. During the Proterozoic formation stage, three deformational sequences are recognized. Andalusite–muscovite, sillimanite–muscovite and sillimanite–K-feldspar zones of amphibolite facies regional metamorphism are earlier than, or synchronous with the first or second phase of folding. Ductile shear zones were formed and Caledonian granites were emplaced during the modification stage. The granites superimposed contact aureoles (garnet–K-feldspar zone) on the regional metamorphic fabric.
Metamorphic reactions, P–T conditions of metamorphism and P–T–t paths were estimated by analysis of mineral textures and standard thermobarometric techniques. The P–T–t path of the Proterozoic tectonometamorphic cycle shows prominent clockwise decompression. The P–T–t path of the Caledonian tectonometamorphic cycle is characterized by an early rise of pressure and temperature, followed by isothermal decompression (rapid uplift) and finally with isobaric cooling.
The P–T–t paths of the two tectonometamorphic cycles reflect two major stages of collision and uplift in the evolution of the Qinling orogenic belt during the Proterozoic and Caledonian–Hercynian periods, respectively.     

Proterozoic granulite facies metamorphism in the southeastern Reynolds Range, central Australia: geological context, P–T path and overprinting relationships

In the southeastern Reynolds Range, central Australia, a low- P granulite facies metamorphism affected two sedimentary sequences: the Lander Rock Beds and the Reynolds Range Group. In the context of the whole of the Reynolds Range and the adjacent Anmatjira Range, this metamorphism is M3 in a sequence M1–4 that occurred over a period of 250 Ma. In particular, M1 affected the Lander Rock Beds prior to the deposition of the Reynolds Group. M3 has an areally restricted, high-grade area in the southeastern Reynolds Range, affecting both the Reynolds Range Group and the underlying Lander Rock Beds. The effects of M3 are characterized by spinel + quartz-bearing peak metamorphic assemblages in metapelites, which imply peak conditions of ≥750°C and 4.5 ± 1 kbar, and involved isobaric cooling or compression with cooling. It is concluded that one of a series of thermal perturbations caused by thinning of mantle lithosphere contemporaneous with crustal thickening was responsible for M3. In the southeastern Reynolds Range, evidence of both the unconformity between the two rock groups and previous metamorphism/deformation has been completely erased by recrystallization during M3–D3.     

内蒙集宁群麻粒岩相中石榴石的稀土元素特征

采用现代高精度中子活化分析方法分析研究了内蒙集宁群麻粒岩相中石榴石的痕量稀土,原岩为火山质岩石和原岩为正常沉积岩中的石榴石显示出完全不同的稀土配分型式,认为变质岩中的石榴石,其稀土元素特征可用来判别深变质岩的原岩性质,区分正副变质岩。     

Contrasting P–T–t paths for Neoproterozoic metamorphism in MacRobertson and Kemp Lands, east Antarctica

Mineral equilibria modelling and electron microprobe chemical dating of monazite in granulite facies metapelitic assemblages from the MacRobertson Land coastline, Rayner Complex, east Antarctica, are consistent with an 'anticlockwise' Neoproterozoic P–T–t path. Metamorphism occurred at c. 990–970 Ma, achieving peak conditions of 850 °C and 5.6–6.2 kbar at Cape Bruce, and 900 °C and 5.4–6.2 kbar at the Forbes Glacier ∼50 km to the east. These peak metamorphic conditions preceded the emplacement of regionally extensive syntectonic charnockite. High temperature conditions are likely to have been sustained for 80 Myr by lithospheric thinning and repeated pluton emplacement; advection was accompanied by crustal thickening to maximum pressures of 6–7 kbar, followed by near-isobaric cooling. This P–T–t path is distinct from that of rocks in adjacent Kemp Land, ∼50 km to the west, where a 'clockwise' P–T–t path from higher- P conditions at c. 940 Ma may reflect the response of a cratonic margin displaced from the main magma flux. In this scenario, crustal shortening was initially accommodated in younger, fertile crust (MacRobertson Land) involving metasediments and felsic plutons with the transfer of strain to adjacent older crust (Kemp Land) subsequent to charnockite emplacement.     

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.     

Late Proterozoic High-pressure granulite facies meta-morphism in the north-east Ox inlier, north-west Ireland

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.     

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.