Metamorphic history of eclogites and country rock gneisses in the Aktyuz area,Northern Tien‐Shan,Kyrgyzstan: a record from initiation of subduction through to oceanic closure by continent–continent collision

Eclogites and related high‐P metamorphic rocks occur in the Zaili Range of the Northern Kyrgyz Tien‐Shan (Tianshan) Mountains, which are located in the south‐western segment of the Central Asian Orogenic Belt. Eclogites are preserved in the cores of garnet amphibolites and amphibolites that occur in the Aktyuz area as boudins and layers (up to 2000 m in length) within country rock gneisses. The textures and mineral chemistry of the Aktyuz eclogites, garnet amphibolites and country rock gneisses record three distinct metamorphic events (M1–M3). In the eclogites, the first MP–HT metamorphic event (M1) of amphibolite/epidote‐amphibolite facies conditions (560–650 °C, 4–10 kbar) is established from relict mineral assemblages of polyphase inclusions in the cores and mantles of garnet, i.e. Mg‐taramite + Fe‐staurolite + paragonite ± oligoclase (An_<16) ± hematite. The eclogites also record the second HP‐LT metamorphism (M2) with a prograde stage passing through epidote‐blueschist facies conditions (330–570 °C, 8–16 kbar) to peak metamorphism in the eclogite facies (550–660 °C, 21–23 kbar) and subsequent retrograde metamorphism to epidote‐amphibolite facies conditions (545–565 °C and 10–11 kbar) that defines a clockwise P–T path. thermocalc (average P–T mode) calculations and other geothermobarometers have been applied for the estimation of P–T conditions. M3 is inferred from the garnet amphibolites and country rock gneisses. Garnet amphibolites that underwent this pervasive HP–HT metamorphism after the eclogite facies equilibrium have a peak metamorphic assemblage of garnet and pargasite. The prograde and peak metamorphic conditions of the garnet amphibolites are estimated to be 600–640 °C; 11–12 kbar and 675–735 °C and 14–15 kbar, respectively. Inclusion phases in porphyroblastic plagioclase in the country rock gneisses suggest a prograde stage of the epidote‐amphibolite facies (477 °C and 10 kbar). The peak mineral assemblage of the country rock gneisses of garnet, plagioclase (An_11–16), phengite, biotite, quartz and rutile indicate 635–745 °C and 13–15 kbar. The P–T conditions estimated for the prograde, peak and retrograde stages in garnet amphibolite and country rock are similar, implying that the third metamorphic event in the garnet amphibolites was correlated with the metamorphism in the country rock gneisses. The eclogites also show evidence of the third metamorphic event with development of the prograde mineral assemblage pargasite, oligoclase and biotite after the retrograde epidote‐amphibolite facies metamorphism. The three metamorphic events occurred in distinct tectonic settings: (i) metamorphism along the hot hangingwall at the inception of subduction, (ii) subsequent subduction zone metamorphism of the oceanic plate and exhumation, and (iii) continent–continent collision and exhumation of the entire metamorphic sequences. These tectonic processes document the initial stage of closure of a palaeo‐ocean subduction to its completion by continent–continent collision.     

Metamorphic evolution of lawsonite eclogites from the southern Motagua fault zone,Guatemala: insights from phase equilibria and Raman spectroscopy

Lawsonite eclogite (metabasalt and metadolerite) and associated metasedimentary rocks in a serpentinite mélange from an area just south of the Motagua fault zone (SMFZ), Guatemala, represent excellent natural records of the forearc slab–mantle interface. Pseudosection modelling of pristine lawsonite eclogite reproduces the observed predominant mineral assemblages, and garnet compositional isopleths intersect within the phase fields, yielding a prograde P–T path that evolves from 20 kbar, 470 °C (M1) to 25 kbar, 520 °C (M2). The dominant penetrative foliation within the eclogite blocks is defined by minerals developed during the prograde evolution, and the associated deformation, therefore, took place during subduction. Thermometry using Raman spectra of carbonaceous material in metasedimentary rocks associated with the SMFZ eclogites gives estimates of peak‐T of ～520 °C. Barometry using Raman spectroscopy shows unfractured quartz inclusions in garnet rims retain overpressures of up to ～10 kbar, implying these inclusions were trapped at conditions just below the quartz/coesite transition, in agreement with the results of phase equilibrium analysis. Additional growth of Ca‐rich garnet indicates initial isothermal decompression to 20 kbar (M3) followed by hydration and substantial cooling to the lawsonite–blueschist facies (M4). Further decompression of the hydrated eclogite blocks to the pumpellyite–actinolite facies (3–5 kbar, 230–250 °C) is associated with dehydration and veining (M5). The presence of eclogite as m‐ to 10 m‐sized blocks in a serpentinite matrix, lack of widespread deformation developed during exhumation and derived prograde P–T path associated with substantial dehydration of metabasites within the antigorite stability field suggest that the SMFZ eclogites represent the uppermost part of the forearc slab crust sampled by an ascending serpentinite diapir in an active, moderate‐T subduction zone.     

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.     

Pro- and retrograde P–T evolution of granulites of the Beit Bridge Complex (Limpopo Belt, South Africa): constraints from quantitative phase diagrams and geotectonic implications

Interpretations based on quantitative phase diagrams in the system CaO–Na₂O–K₂O–TiO₂–MnO–FeO–MgO–Al₂O₃–SiO₂–H₂O indicate that mineral assemblages, zonations and microstructures observed in migmatitic rocks from the Beit Bridge Complex (Messina area, Limpopo Belt) formed along a clockwise P–T path. That path displays a prograde P–T increase from 600 °C/7.0 kbar to 780 °C/9–10 kbar (pressure peak) and 820 °C/8 kbar (thermal peak), followed by a P–T decrease to 600 °C/4 kbar. The data used to construct the P–T path were derived from three samples of migmatitic gneiss from a restricted area, each of which has a distinct bulk composition: (1) a K, Al‐rich garnet–biotite–cordierite–sillimanite–K‐feldspar–plagioclase–quartz–graphite gneiss (2) a K‐poor, Al‐rich garnet–biotite–staurolite–cordierite–kyanite–sillimanite–plagioclase–quartz–rutile gneiss, and (3) a K, Al‐poor, Fe‐rich garnet–orthopyroxene–biotite–chlorite–plagioclase–quartz–rutile–ilmenite gneiss. Preservation of continuous prograde garnet growth zonation demonstrates that the pro‐ and retrograde P–T evolution of the gneisses must have been rapid, occurring during a single orogenic cycle. These petrological findings in combination with existing geochronological and structural data show that granulite facies metamorphism of the Beit Bridge metasedimentary rocks resulted from an orogenic event during the Palaeoproterozoic (c. 2.0 Ga), caused by oblique collision between the Kaapvaal and Zimbabwe Cratons. Abbreviations follow Kretz (1983 ).     

Evolution of a crustal-scale transpressive shear zone in the Albany–Fraser Orogen, SW Australia: 1. P–T conditions of Mesoproterozoic metamorphism in the Coramup Gneiss

The Albany–Fraser Orogen in southwestern Australia preserves an important thermo‐tectonic record of Australo‐Antarctic cratonic assembly during the Mesoproterozoic. New petrologic and thermobarometric data from the Coramup Gneiss (a 10 km wide zone of high strain rocks within the NE‐trending eastern Albany–Fraser Orogen) indicate at least two high‐grade metamorphic events during 1345–1140 Ma convergence and amalgamation of the West Australian and Mawson cratons. The first event (M1) involved c. 1300 Ma granulite facies metamorphism of the Coramup Gneiss (M1a: 800–850 °C, 5–7 kbar), followed by burial and recrystallization under high‐P conditions (M1b: 800–850 °C, c. 10 kbar) prior to high‐T decompression (M1c: 700–800 °C, 7–8 kbar) and the 1290–1280 Ma emplacement of Recherche Granite sills. The second event (M2) entailed high‐T, low‐P metamorphism within dextral D2 shear zones (M2a: 750–800 °C, 5–6 kbar), followed by fluid‐present amphibolite facies M2b retrogression. Subsequent sinistral D3 mylonites and pseudotachylites are considered contemporaneous with similar structures in the adjacent Nornalup Complex that postdate the c. 1140 Ma Esperance Granite. Our petrological and thermobarometric data permit two end‐member P–T‐time relationships between M1 and M2: (1) a single post‐M1b event involving continuous M1b–M1c–M2a–M2b cooling and decompression, and (2) a two‐stage post‐M1b evolution involving M1c metamorphism during the waning stages of an event unrelated causally or temporally to subsequent M2a metamorphism and D2 deformation. In a companion paper, new structural and U–Pb SHRIMP zircon data are presented to support a two‐stage P–T evolution for the Coramup Gneiss, with M1 and M2, respectively, reflecting thermo‐tectonic activity during Stage I (1345–1260 Ma) and Stage II (1215–1140 Ma) of the Albany–Fraser Orogeny.     

Phase relationships in grunerite–garnet-bearing amphibolites in the system CFMASH, with applications to metamorphic rocks from the Central Zone of the Limpopo Belt, South Africa

A petrogenetic grid in the model system CaO–FeO–MgO–Al₂O₃–SiO₂–H₂O is presented, illustrating the phase relationships among the minerals grunerite, hornblende, garnet, clinopyroxene, chlorite, olivine, anorthite, zoisite and aluminosilicates, with quartz and H₂O in excess. The grid was calculated with the computer software thermocalc , using an upgraded version of the internally consistent thermodynamic dataset HP98 and non‐ideal mixing activity models for all solid solutions. From this grid, quantitative phase diagrams (P–T pseudosections) are derived and employed to infer a P–T path for grunerite–garnet‐bearing amphibolites from the Endora Klippe, part of the Venetia Klippen Complex within the Central Zone of the Limpopo Belt. Agreement between calculated and observed mineral assemblages and garnet zonation indicates that this part of the Central Zone underwent a prograde temperature and pressure increase from c. 540 °C/4.5 kbar to 650 °C/6.5 kbar, followed by a post‐peak metamorphic pressure decrease. The inferred P–T path supports a geotectonic model suggesting that the area surrounding the Venetia kimberlite pipes represents the amphibolite‐facies roof zone of migmatitic gneisses and granulites that occur widely within the Central Zone. In addition, the P–T path conforms to an interpretation that the Proterozoic evolution of the Central Zone was controlled by horizontal tectonics, causing stacking and differential heating at c. 2.0 Ga.     

Combined thermobarometry and geochronology of peraluminous metapelites from the Karakoram metamorphic complex,North Pakistan; New insight into the tectonothermal evolution of the Baltoro and Hunza Valley regions

Petrographic analysis of peraluminous metapelites from two separate regions of the Karakoram metamorphic complex, North Pakistan, has produced new insights into the P–T–t evolution of the deep crust along the south Asian margin before and after the India‐Asia collision. Average P–T estimates and pseudosection construction in the MnO–Na₂O–CaO–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O–TiO₂–Fe₂O₃ (MnNCKFMASHTO) system using THERMOCALC have provided prograde and peak metamorphic conditions and U–Pb geochronology of metamorphic monazite has provided age constraints. Two new events in the tectonothermal evolution of the Hunza Valley have been documented; an andalusite‐grade contact metamorphic event at 105.5 ± 0.8 Ma, at unknown P–T conditions, associated with the widespread subduction‐related granite magmatism before the India‐Asia collision, and a kyanite‐grade overprint of sillimanite‐grade rocks with peak P–T conditions of ～7.8 kbar, 645 °C at 28.2 ± 0.8 Ma associated with the ongoing India‐Asia collision. A kyanite‐grade event observed in the Baltoro region with similar peak P–T conditions (～7.4–8.0 kbar, ～640–660 °C) is interpreted to have occurred sometime after 21.8 ± 0.6 Ma, however, previous studies have suggested that this event commenced in the Baltoro as early as c. 28 Ma. A calculated prograde P–T path for this kyanite‐grade event in the Baltoro indicates that garnet first nucleated on an initially high geothermal gradient (～30 °C km^?1) and grew during a significant increase in pressure of ～2.6 kbar over a temperature increase of ～100 °C. This event is thought to represent evidence for conductive heating of the middle crust during early stages of intrusion and lateral migration of the Baltoro batholith, with thermal conditions comparable with tectonic models of magmatic over‐accretion.     

Evidence for multiple burial–partial exhumation cycles from the Onodani eclogites in the Sambagawa metamorphic belt,central Shikoku,Japan

Eclogites from the Onodani area in the Sambagawa metamorphic belt of central Shikoku occur as layers or lenticular bodies within basic schists. These eclogites experienced three different metamorphic episodes during multiple burial and exhumation cycles. The early prograde stage of the first metamorphic event is recorded by relict eclogite facies inclusions within garnet cores (X_Sps 0.80–0.24, X_Alm 0–0.47). These inclusions consist of relatively almandine‐rich garnet (X_Sps 0.13–0.24, X_Alm 0.36–0.45), aegirine‐augite/omphacite (X_Jd 0.08–0.28), epidote, amphiboles (e.g. actinolite, winchite, barroisite and taramite), albite, phengite, chlorite, calcite, titanite, hematite and quartz. The garnet cores also contain polyphase inclusions consisting of almandine‐rich garnet, omphacite (X_Jd 0.27–0.28), amphiboles (e.g. actinolite, winchite, barroisite, taramite and katophorite) and phengite. The peak P–T conditions of the first eclogite facies metamorphism are estimated to be 530–590 °C and 19–21 kbar succeeded by retrogression into greenschist facies. The second prograde metamorphism began at greenschist facies conditions. The peak metamorphic conditions are defined by schistosity‐forming omphacites (X_Jd ≤ 49) and garnet rims containing inclusions of barroisitic amphibole, phengite, rutile and quartz. The estimated peak metamorphic conditions are 630–680 °C and 20–22 kbar followed by a clockwise retrograde P–T path with nearly isothermal decompression to 8–12 kbar. In veins cross‐cutting the eclogite schistosity, resorbed barroisite/Mg‐katophorite occurs as inclusions in glaucophane which is zoned to barroisite, suggesting a prograde metamorphism of the third metamorphic event. The peak P–T conditions of this metamorphic event are estimated to be 540–600 °C and 6.5–8 kbar. These metamorphic conditions are correlated with those of the surrounding non‐eclogitic Sambagawa schists. The Onodani eclogites were formed by subduction of an oceanic plate, and metamorphism occurred beneath an accretionary prism. These high‐P/T type metamorphic events took place in a very short time span between 100 and 90 Ma. Plate reconstructions indicate highly oblique subduction of the Izanagi plate beneath the Eurasian continent at a high spreading rate. This probably resulted in multiple burial and exhumation movements of eclogite bodies, causing plural metamorphic events. The eclogite body was juxtaposed with non‐eclogitic Sambagawa schists at glaucophane stability field conditions. The amalgamated metamorphic sequence including the Onodani eclogites were exhumed to shallow crustal/surface levels in early Eocene times (c. 50 Ma).     

Contrasting metamorphic histories of lenses of high‐pressure rocks and host migmatites with a flat orogenic fabric (Bohemian Massif,Czech Republic): a result of tectonic mixing within horizontal crustal flow?

Migmatites with sub‐horizontal fabrics at the eastern margin of the Variscan orogenic root in the Bohemian Massif host lenses of eclogite, kyanite‐K‐feldspar granulite and marble within a matrix of migmatitic paragneiss and amphibolite. Petrological study and pseudosection modelling have been used to establish whether the whole area experienced terrane‐wide exhumation of lower orogenic crust, or whether smaller portions of higher‐pressure lower crust were combined with a lower‐pressure matrix. Kyanite‐K‐feldspar granulite shows peak conditions of 16.5 kbar and 850 °C with no clear indications of prograde path, whereas in the eclogite the prograde path indicates burial from 10 kbar and 700 °C to a peak of 18 kbar and 800 °C. Two contrasting prograde paths are identified within the host migmatitic paragneiss. The first path is inferred from the presence of staurolite and kyanite inclusions in garnet that contains preserved prograde zoning that indicates burial with simultaneous heating to 11 kbar and 800 °C. The second path is inferred from garnet overgrowths of a flat foliation defined by sillimanite and biotite. Garnet growth in such an assemblage is possible only if the sample is heated at 7–8 kbar to around 700–840 °C. Decompression is associated with strong structural reworking in the flat fabric that involves growth of sillimanite in paragneiss and kyanite‐K‐feldspar granulite at 7–10 kbar and 750–850 °C. The contrasting prograde metamorphic histories indicate that kilometre‐scale portions of high‐pressure lower orogenic crust were exhumed to middle crustal levels, dismembered and mixed with a middle crustal migmatite matrix, with the simultaneous development of a flat foliation. The contrasting P–T paths with different pressure peaks show that tectonic models explaining high‐pressure boudins in such a fabric cannot be the result of heterogeneous retrogression during ductile rebound of the whole orogenic root. The P–T paths are compatible with a model of heterogeneous vertical extrusion of lower crust into middle crust, followed by sub‐horizontal flow.     

Correlation between the Wonominta and the Broken Hill Blocks in western New South Wales: Evidence from Nd isotope studies of metasediments

Nd isotope studies of the oldest metasedimentary rocks from the Wonominta Block, western New South Wales reveal that these samples have a model age (T_DM) of 1780–2010 Ma, slightly younger than that of low‐grade Willyama Supergroup metasediments (1920–2160 Ma), and significantly younger than those ages previously reported from high‐grade rocks of the Broken Hill Block (2200–2300 Ma). These differences have important implications for tectonic reconstruction in this region and support a model of transitional tectonics from the Broken Hill to Wonominta Blocks, as suggested by earlier geochemical studies of mafic rocks. Those studies revealed that the mafic rocks from the basal sequence of the Wonominta Block may have formed in a back‐arc basin, developed from a propagating rifting, an environment contiguous to that in which Willyama Supergroup was deposited. These results also carry significant implications for tectonic reconstruction of eastern Australia.     

Timing and style of high-temperature metamorphism across the Western Gawler Craton during the Paleo- to Mesoproterozoic

Abstract

Combined in situ monazite dating, mineral equilibria modelling and zircon U–Pb detrital zircon analysis provide insight into the pressure–temperature–time (P–T–t) evolution of the western Gawler Craton. In the Nawa Domain, pelitic and quartzo-feldspathic gneisses were deposited after ca 1760?Ma and record high-grade metamorphic conditions of ～7.5?kbar and 850?°C at ca 1730?Ma. Post-peak microstructures, including partial plagioclase coronae and late biotite around garnet, and subtle retrograde garnet compositional zoning, suggest that these rocks cooled along a shallow down-pressure trajectory across an elevated dry solidus. In the northwest Fowler Domain (Colona Block), monazite grains from pelitic gneisses record two stages of growth/recrystallisation interpreted to represent discrete parts of the P–T path: (1) ca 1710?Ma monazite growth during prograde to peak conditions, and (2) ca 1690?Ma Y-enriched monazite growth/recrystallisation during partial garnet breakdown and cooling towards the solidus. Relict prograde growth zoning in garnet suggests rocks underwent a steep up-P path to peak conditions of ～8?kbar at 800?°C. The new P–T–t results suggest basement rocks of the southwestern Nawa and northwestern Fowler were buried to depths of 20–25?km during the Kimban Orogeny, ca 10 Myrs after the sedimentary precursors were deposited. The P–T path for the Kimban Orogeny is broadly anti-clockwise, suggesting that at least the early phase of this event was associated with extension. Exhumation of rocks from both the southwestern Nawa and northwestern Fowler domains may have occurred during the waning stages of the Kimban Orogeny (<ca 1690?Ma). The limited low-grade overprint in these rocks may be explained by a mid-to-upper crustal position for these rocks during the subsequent Kararan Orogeny. Aluminous quartz-feldspathic gneiss of the Nundroo Block in the eastern Fowler Domain records peak conditions of ～7?kbar at 800?°C. Monazite grains from the Nundroo Block are dominated by an age peak at ca 1590?Ma, although the presence of some older ages up to ca 1690?Ma, possibly reflect partial resetting of older monazite domains. The P–T–t conditions suggest these rocks were buried to 20–25?km at ca 1590?Ma during the Kararan Orogeny. This high-grade metamorphism in the Nundroo Block is a mid-crustal expression of the same thermal anomaly that caused magmatism in the central-eastern Gawler Craton. Juxtaposition of rocks affected by the Kimban and Kararan orogenic events in the western Gawler Craton was controlled by lithospheric-scale shear zones, some of which have facilitated ～20 kilometres of exhumation.     

Variations in the transient prograde geothermal gradient from chloritoid-staurolite equilibria: a case study from the Barrovian and Buchan-type domains in the Bohemian Massif

Thermodynamic modelling of metamorphic rocks increases the possibilities of deciphering prograde paths that provide important insights into early orogenic evolution. It is shown that the chloritoid–staurolite transition is not only an indicator of temperature on prograde P–T paths, but also a useful indicator of pressure. The approach is applied to the Moravo‐Silesian eastern external belt of the Bohemian Massif, where metamorphic zones range from biotite to staurolite‐sillimanite. In the staurolite zone, inclusions of chloritoid occur in garnet cores, while staurolite is included at garnet rims and is widespread in the matrix. Chloritoid X_Fe = 0.91 indicates transition to staurolite at 5 kbar and 550 °C and consequently, an early transient prograde geothermal gradient of 29 °C km^?1. The overall elevated thermal evolution is then reflected in the prograde transition of staurolite to sillimanite and in the achievement of peak temperature of 660 °C at a relatively low pressure of 6.5 kbar. To the south and to the west of the studied area, high‐grade metamorphic zones record a prograde path evolution from staurolite to kyanite and development of sillimanite on decompression. Transition of chloritoid to staurolite was reported in two places, with chloritoid X_Fe = 0.75–0.80, occurring at 8–10 kbar and 560–580 °C, and indicating a transient prograde geothermal gradient of 16–18 °C km^?1. These data show variable barric evolutions along strike and across the Moravo‐Silesian domain. Elevated prograde geothermal gradient coincides with areas of Devonian sedimentation and volcanism, and syn‐ to late Carboniferous intrusions. Therefore, we interpret it as a result of heat inherited from Devonian rifting, further fuelled by syntectonic Carboniferous intrusions.     

Two‐stage Variscan metamorphism in the Canigou massif: Evidence for crustal thickening in the Pyrenees

The Variscan metamorphism in the Pyrenees is dominantly of the low‐pressure–high‐temperature (LP‐HT) type. The relics of an earlier, Barrovian‐type metamorphism that could be related to orogenic crustal thickening are unclear and insufficiently constrained. A microstructural and petrological study of micaschists underlying an Ordovician augen orthogneiss in the core of the Canigou massif (Eastern Pyrenees, France) reveals the presence of two syntectonic metamorphic stages characterized by the crystallization of staurolite (M1) and andalusite (M2), respectively. Garnet is stable during the two metamorphic stages with a period of resorption between M1 and M2. The metamorphic assemblages M1 and M2 record similar peak temperatures of 580°C at different pressure conditions of 5.5 and 3 kbar, respectively. Using chemical zoning of garnet and calculated P–T pseudosections, a prograde P–T path is constrained with a peak pressure at ~6.5 kbar and 550°C. This P–T path, syntectonic with respect to the first foliation S1, corresponds to a cold gradient (of ~9°C/km), suggestive of crustal thickening. Resorption of garnet between M1 and M2 can be interpreted either in terms of a simple clockwise P–T path or a polymetamorphic two‐stage evolution. We argue in favour of the latter, where the medium‐pressure (Barrovian) metamorphism is followed by a period of significant erosion and crustal thinning leading to decompression and cooling. Subsequent advection of heat, probably from the mantle, leads to a new increase in temperature, coeval with the development of the main regional fabric S2. LA‐ICP‐MS U–Th–Pb dating of monazite yields a well‐defined date at c. 300 Ma. Petrological evidence indicates that monazite crystallization took place close to the M1 peak pressure conditions. However, the similarity between this age and that of the extensive magmatic event well documented in the eastern Pyrenees suggests that it probably corresponds to the age of monazite recrystallization during the M2 LP‐HT event.     

Osumilite–melt interactions in ultrahigh temperature granulites: phase equilibria modelling and implications for the P–T–t evolution of the Eastern Ghats Province,India

The exposed residual crust in the Eastern Ghats Province records ultrahigh temperature (UHT) metamorphic conditions involving extensive crustal anatexis and melt loss. However, there is disagreement about the tectonic evolution of this late Mesoproterozoic–early Neoproterozoic orogen due to conflicting petrological, structural and geochronological interpretations. One of the petrological disputes in residual high Mg–Al granulites concerns the origin of fine‐grained mineral intergrowths comprising cordierite + K‐feldspar ± quartz ± biotite ± sillimanite ± plagioclase. These intergrowths wrap around porphyroblast phases and are interpreted to have formed by the breakdown of primary osumilite in the presence of melt trapped in the equilibration volume by the melt percolation threshold. The pressure (P)–temperature (T) evolution of four samples from three localities across the central Eastern Ghats Province is constrained using phase equilibria modelling in the chemical system Na₂O–CaO–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O–TiO₂–Fe₂O₃ (NCKFMASHTO). Results of the modelling are integrated with published geochronological results for these samples to show that the central Eastern Ghats Province followed a common P–T–t history. This history is characterized by peak UHT metamorphic conditions of 945–955 °C and 7.8–8.2 kbar followed by a slight increase in pressure and close‐to‐isobaric cooling to the conditions of the elevated solidus at 940–900 °C and 8.5–8.3 kbar. In common with other localities from the Eastern Ghats Province, the early development of cordierite before osumilite and the peak to immediate post‐peak retrograde reaction between osumilite and melt to produce the intergrowth features requires that the prograde evolution was one of contemporaneous increasing pressure with increasing temperature. This counter‐clockwise (CCW) evolution is evaluated for one sample using inverse phase equilibria modelling along a schematic P–T path of 150 °C kbar^?1 starting from the low P–T end of the prograde P–T path as constrained by the phase equilibria modelling. The inverse modelling is executed by step‐wise down temperature reintegration of sufficient melt into the residual bulk chemical composition at the P–T point of the 1 mol.% melt isopleth at each step, representing the melt remaining on grain boundaries after each prograde drainage event, to reach the melt connectivity transition (MCT) of 7 mol.%. The procedure is repeated until a plausible protolith composition is recovered. The result demonstrates that clastic sedimentary rocks that followed a CCW P–T evolution could have produced the observed mineral assemblages and microstructures preserved in the central Eastern Ghats Province. This study also highlights the role of melt during UHT metamorphism, particularly its importance to both chemical and physical processes along the prograde and retrograde segments of the P–T path. These processes include: (i) an increase in diffusive length scales during the late prograde to peak evolution, creating equilibration volumes larger than a standard thin section; (ii) the development of retrograde mineral assemblages, which is facilitated if some melt is retained post‐peak; (iii) the presence of melt as a weakening mechanism and the advection of heat by melt, allowing the crust to thicken; and (iv) the effect of melt loss, which makes the deep crust both denser and stronger, and reduces heat production at depth, limiting crustal thickening and facilitating the transition to close‐to‐isobaric cooling.     

Petrogenesis of lawsonite and epidote eclogite and blueschist, Sivrihisar Massif, Turkey

The Sivrihisar Massif, Turkey, is comprised of blueschist and eclogite facies metasedimentary and metabasaltic rocks. Abundant metre‐ to centimetre‐scale eclogite pods occur in blueschist facies metabasalt, marble and quartz‐rich rocks. Sivrihisar eclogite contains omphacite + garnet + phengite + rutile ± glaucophane ± quartz + lawsonite and/or epidote. Blueschists contain sodic amphibole + garnet + phengite + lawsonite and/or epidote ± omphacite ± quartz. Sivrihisar eclogite and blueschist have similar bulk composition, equivalent to NMORB, but record different P–T conditions: ～26 kbar, 500 °C (lawsonite eclogite); 18 kbar, 600 °C (epidote eclogite); 12 kbar, 380 °C (lawsonite blueschist); and 15–16 kbar, 480–500 °C (lawsonite‐epidote blueschist). Pressures for the Sivrihisar lawsonite eclogite are among the highest reported for this rock type, which is rarely exposed at the Earth's surface. The distribution and textures of lawsonite ± epidote define P–T conditions and paths. For example, in some lawsonite‐bearing rocks, epidote inclusions in garnet and partial replacement of matrix epidote by lawsonite suggest an anticlockwise P–T path. Other rocks contain no epidote as inclusions or as a matrix phase, and were metamorphosed entirely within the lawsonite stability field. Results of the P–T study and mapping of the distribution of blueschists and eclogites in the massif suggest that rocks recording different maximum P–T conditions were tectonically juxtaposed as kilometre‐scale slices and associated high‐P pods, although all shared the same exhumation path from ～9–11 kbar, 300–400 °C. Within the tectonic slices, alternating millimetre–centimetre‐scale layers of eclogite and blueschist formed together at the same P–T conditions but represent different extents of prograde reaction controlled by strain partitioning or local variations in fO₂ or other chemical factors.     

Prograde–retrograde P–T–t–deformation path of Austroalpine micaschists during Variscan continental collision (Eastern Alps)

A complete prograde P–T path, defined by 10 calculated P–T fields in succession, is recognized from metapelites by using geothermobarometry on garnet-bearing assemblages with microstructural control. Overstacking of several tectonic units during an early Variscan continental collision explains the complex prograde P–T history. Isostatic uplift and deformation controlled the retrograde P–T path. Deformation with changing character acted continuously during all stages of the evolution of the Austroalpine basement complex. After the intrusion of Caledonian granitoids, metapelites and magmatic rocks suffered a shearing deformation D₁–D₂, which produced sheath folds as well as the main foliation S₂. Spessartine-rich first-generation garnets, situated in microlithons enclosed by S₂, record the onset of shearing under increasing high-pressure–low-temperature conditions (7 kbar/380°C). Geothermobarometry on second-generation garnets which have been rotated during growth indicates isothermal decompression from 9 kbar to 5 kbar/500°C and subsequent recompression/heating during continuing shearing. This is explained by overthrusting of a tectonic unit (unit 2) from NE to SW upon the micaschist unit (unit 1), followed by isostatic uplift and further overstocking of a third unit (unit 3). The resulting P_max of 12 kbar at 650°C and further increasing temperatures up to 680°C accompanied by decompression have been calculated using a third generation of garnets. These high-pressure–high-temperature conditions may explain the occurrence of eclogitic metabasites in adjacent regions. Staurolite and kyanite first appeared under decreasing pressures at the last stage of prograde P–T evolution. Shortening deformation D₃ and simultaneous growth of typical amphibolite facies minerals (staurolite 2, kyanite 2, sillimanite, andalusite) occurred during the retrograde path. A final step of Variscan evolution was marked by an oppositely directed shearing D₄ (at T > 300°C and P > 3 kbar), possibly indicating backthrusting or extension. Apart from acid intrusions, no signs of a previous Caledonian thermotectonic history were found in the area to the south of the Defereggen–Antholz–Vals Line.     

Anatectic record and contrasting P–T paths of aluminous gneisses from the central Grenville Province

Anatectic aluminous gneisses, some derived from sedimentary rocks of broadly pelitic composition and others from hydrothermally altered felsic volcanic rocks, are exposed in the mid‐P and high‐P segments of the hinterland in the central Grenville Province. These gneisses consist dominantly of garnet, biotite, K‐feldspar, plagioclase and quartz, with sillimanite or kyanite, and display microstructural evidence of anatexis by fluid‐absent reactions consuming muscovite and/or biotite. Melt‐related microstructures, such as inter‐granular films and/or interstitial quartz or feldspar enclosing relict phases, are most abundant in the metasedimentary samples. Despite anatexis at granulite facies conditions, the hydrothermally altered rocks preserve earlier features attributed to the circulation of hydrothermal fluids, such as sillimanite seams, dismembered quartz veins and garnet‐rich aluminous nodules in a K‐feldspar‐dominated matrix. Microstructural and mineral chemical data, integrated with P–T pseudosections calculated with thermocalc for the metasedimentary rocks, permit qualitative constraints on the P–T paths. Data from a high‐P kyanite‐bearing sample are consistent with a steep prograde P–T path up to ~14.5 kbar and 860–900 °C, followed by decompression with minor cooling to the solidus at ~11 kbar and 870 °C. This pressure‐dominated P–T path is similar to those inferred in other parts of the high‐P segment in the central Grenville Province. In contrast, the P–T path predicted from a mid‐P sillimanite‐bearing paragneiss has a strong temperature gradient with P–T of ~9.5 kbar and 850 °C at the thermal peak, and a retrograde portion down to ~8 kbar and 820 °C. In a broad sense, these two contrasting P–T patterns are consistent with predictions of thermo‐mechanical modelling of large hot orogens in which P–T paths with strong pressure gradients exhume deeper rocks in the orogenic flanks, whereas P–T paths with strong temperature gradients in the orogenic core reflect protracted lateral transport of ductile crust beneath a plateau.     

Evidence from Fe- and Al-rich metapelites for thrust loading in the Transangarian region of the Yenisey Ridge, eastern Siberia

In the Transangarian region of the Yenisey Ridge in eastern Siberia (Russia), Fe‐ and Al‐rich metapelitic schists of the Korda plate show field and petrological evidence of two superimposed metamorphic events. An early middle Proterozoic event with age of c.1100 Ma produced low‐pressure, andalusite‐bearing assemblages at c. 3.5–4 kbar and 540–560 °C. During a subsequent late Proterozoic event at c. 850 Ma, a medium‐pressure, regional metamorphic overprint produced kyanite‐bearing mineral assemblages that replaced minerals formed in the low‐pressure event. Based on the results of geothermobarometry and P–T path calculations it can be shown that pressure increased from 4.5 to 6.7 kbar at a relatively constant temperature of 540–600 °C towards a major suture zone called the Panimba thrust. In order to produce such nearly isothermal loading of 1–7 °C km ^?1, we propose a model for the tectono‐metamorphic evolution of the study area based on crustal thickening caused by south‐westward thrusting of the 5–7 km‐thick upper‐plate metacarbonates over lower‐plate metapelites with velocity of c. 350 m Myr^?1. A small temperature increase (up to 20 ± 15 °C) of the upper part of the overlapped plate is explained by specific behaviour of steady‐state geotherms calculated using lower radioactive heat production of metacarbonates as compared with metapelites. The suggested thermal‐mechanical model corresponds well with P–T paths inferred from obtained thermobarometric data and correlates satisfactorily with P–T trajectories predicted by other two‐dimensional thermal models for different crustal thickening and exhumation histories.     

On the roles of deformation and fluid during rejuvenation of a polymetamorphic terrane: inferences on the geodynamic evolution of the Ruker Province, East Antarctica

Evaluating pressure–temperature (P–T) conditions through mineral equilibria modelling within an amphibolite facies polymetamorphic terrane requires knowledge of the fluid content of the rocks. The Archean‐Palaeoproterozoic basement rocks of the Ruker Province, East Antarctica, preserve evidence of three metamorphic events (M1–M3). Of particular interest is the M3 event, which is constrained to the early Palaeozoic (c. 550–480 Ma). Evaluation of the tectonic setting during this time is important because the Ruker Province is located within a critical region with respect to models of Gondwana assembly. Structural evidence of the early Palaeozoic event is preserved as large (up to ～500 m wide) high strain zones that cut the orthogneiss‐metasedimentary basement (Tingey Complex) of the Ruker Province. Rocks within these zones have been thoroughly recrystallized and preserve a dominant shear fabric and M3 mineral assemblages that formed at P–T conditions of 4.0–5.2 kbar and 565–640 °C. Distal to these zones, rocks preserve more complex petrographic relationships with S1 and S2 foliations, being incompletely overgrown by M3 retrograde assemblages. We show that the mineral assemblages preserved during the M3 event are highly dependent on the availability of fluid H₂O, which is strongly influenced by the structural setting (i.e. proximity to the high‐strain zones). P–T structural and fluid flow constraints support a model of basin inversion during early Palaeozoic crustal rejuvenation in the Ruker Province.     

Preservation of evidence for prograde metamorphism in ultrahigh-temperature, high-pressure kyanite-bearing granulites, South Harris, Scotland

Mineralogical and mineral chemical evidence for prograde metamorphism is rarely preserved in rocks that have reached ultrahigh‐temperature (UHT) conditions (>900 °C) because high diffusion and reaction rates erase evidence for earlier assemblages. The UHT, high‐pressure (HP) metasedimentary rocks of the Leverburgh belt of South Harris, Scotland, are unusual in that evidence for the prograde history is preserved, despite having reached temperatures of ～955 °C or more. Two lithologies from the belt are investigated here and quantitatively modelled in the system NaO–CaO–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O: a garnet‐kyanite‐K‐feldspar‐quartz gneiss (X_Mg = 37, A/AFM = 0.41), and an orthopyroxene‐garnet‐kyanite‐K‐feldspar quartzite (X_Mg = 89 A/AFM = 0.68). The garnet‐kyanite gneiss contains garnet porphyroblasts that grew on the prograde path, and captured inclusion assemblages of biotite, sillimanite, plagioclase and quartz (<790 °C, <9.5 kbar). These porphyroblasts preserve spectacular calcium zonation features with an early growth pattern overgrown by high‐Ca rims formed during high‐P metamorphism in the kyanite stability field. In contrast, Fe‐Mg zonation in the same garnet porphyroblasts reflects retrograde re‐equilibration, as a result of the relatively faster diffusivity of these ions. Peak P–T are constrained by the occurrence of coexisting orthopyroxene and aluminosilicate in the quartzite. Orthopyroxene porphyroblasts [y(opx) = 0.17–0.22] contain sillimanite inclusions, indicative of maximum conditions of 955 ± 45 °C at 10.0 ± 1.5 kbar. Subsequently, orthopyroxene, kyanite, K‐feldspar and quartz developed in equilibrated textures, constraining the maximum pressure conditions to 12.5 ± 0.8 kbar at 905 ± 25 °C. P–T–X modelling reveals that the mineral assemblage orthopyroxene‐kyanite‐quartz is compositionally restricted to rocks of X_Mg > 84, consistent with its very rare occurrence in nature. The preservation of unusual high P–T mineral assemblages and chemical disequilibrium features in these UHT HP rocks is attributed to a rapid tectonometamorphic cycle involving arc subduction and terminating in exhumation.