Early Mesozoic retrograded eclogite and mafic granulite from the Badu Complex of the Cathaysia Block,South China: Petrology and tectonic implications

The high-grade metamorphic terrane in the Badu region along the northeastern Cathaysia Block in South China preserves retrograded eclogites and mafic granulites. Here we present the petrology, mineral phase equilibria and P-T conditions based on pseudosection computations, as well as zircon U-Pb ages of these rocks. Mineral textures and reaction relationships suggest four metamorphic stages for the retrograded eclogite as follows: (1) eclogite facies stage (M1), (2) clinopyroxene retrograde stage (M2), (3) amphibole retrograde stage (M3), and (4) chlorite retrograde stage (M4). For the mafic granulite, three stages are identified as: (1) plagioclase-absent stage (M1), (2) granulite facies stage (M2) and (3) amphibolite facies stage (M3). Metamorphic evolution of both of the rock types follows clockwise P-T path. Conventional geothermometers and geobarometers in combination with phase equilibria modelling yield metamorphic P-T conditions for each metamorphic stage for the eclogite as 500–560 °C, 23–24 kbar (M1), 640–660 °C, 14–16 kbar (M2), 730–750 °C, and 11–13 kbar (M3). The chlorite retrograde stage (M4) is inferred to have occurred at lower amphibolite to greenschist facies conditions. Phase equilibria modelling of the mafic granulite shows P-T conditions for each metamorphic stage as 600–720 °C, > 13 kbar (M1) and 860–890 °C, 5–6 kbar (M2) and M3 at amphibolite facies conditions. LA-ICPMS zircon U-Pb dating and trace element analysis show that the high pressure metamorphism occurred at 245–251 Ma. Protolith age of the mafic granulite is 997 Ma, similar to that of the mafic to ultramafic rocks widely distributed in the Cathaysia Block and also along the Jiangnan belt. Subduction of ancient oceanic lithospheric materials (or crustal thickening) during Mesozoic and formation of eclogites suggest that the Cathaysia Block was perhaps in the Tethyan oceanic domain at this time. The granulite formation might have been aided by Mesozoic mafic magma underplating associated with lithospheric delamination, heating and retrogression of the eclogite accompanied by rapid uplift.     

Thermobaric structure of the Himalayan Metamorphic Belt in Kaghan Valley,Pakistan

The thermobaric structure of the Himalayan Metamorphic Belt (HMB) has been constructed along the Kaghan Valley transect, Pakistan. The HMB in this valley represents mainly the Lesser Himalayan Sequence (LHS) and Higher Himalayan Crystallines (HHC). Mineral parageneses of 474 samples, from an approximately, 80-km traverse from southwest to northeast, were examined. Microprobe analyses were carried out to quantify the mineral composition. To determine the pressure–temperature (P–T) conditions, 65 thin sections (7 pelites from LHS and 25 pelites, 9 mafic rocks/amphibolites and 19 eclogites from HHC) were selected. Based on field observations and mineral paragenesis, low-grade to high-grade metapelites, show Barrovian-type progressive metamorphic sequence, with chlorite, biotite, garnet and staurolite zones in LHS and staurolite, kyanite and sillimanite zones in HHC. By using well-calibrated geothermobarometers, P–T conditions for pelitic and mafic rocks are estimated. P–T estimates for pelitic rocks from the garnet zone indicate a condition of 534 ± 17 °C at 7.6 ± 1.2 kbar. P–T estimates for rocks from the staurolite and kyanite zones indicate average conditions of 526 ± 17 °C at 9.4 ± 1.2 kbar and 657 ± 54 °C at 10 ± 1.6 kbar, respectively. P–T conditions for mafic rocks (amphibolites) and eclogites from HHC are estimated as 645 ± 54 °C at 10.3 ± 2 kbar and 746 ± 59 °C at 15.5 ± 2.1 kbar, respectively. The coesite-bearing ultrahigh-pressure (UHP) eclogites record a peak P–T condition of 757–786 °C at 28.6 ± 0.4 kbar and retrograde P–T conditions of 825 ± 59 °C at 18.1 ± 1.7 kbar.These results suggest that HMB show a gradual increase in metamorphic grade from southwest to northeast. The P–T conditions from Pelitic and adjacent mafic rocks having identical peak conditions in the same metamorphic zone, while the structural middle in HHC reached the highest P–T condition upto the UHP grade.     

The youngest eclogite in central Himalaya: P–T path,U–Pb zircon age and its tectonic implication

Relict omphacite inclusions have been discovered in mafic granulite at Dinggye of China, confirming the existence of eclogite in central Himalayan orogenic belt. Detailed petrological studies show that relict omphacite occur as inclusions in both garnets and zircons, and the peak mineral assemblage of eclogite-facies should be garnet, omphacite, rutile, muscovite and quartz which was strongly overprinted by granulite-facies minerals during the exhumation. Phase equilibria modeling and associated geothermometer predict that the minimum P–T conditions for peak eclogite-facies stage are 720–760 °C and 20–21 kbar, and those of overprinted granulite-facies are 750 °C and 7–9 kbar in water-undersaturated condition. Thus, a near isothermal decompression P–T path for central Himalayan eclogite has been obtained. Zircon SHRIMP U–Pb dating of two studied eclogite samples at Dinggye yields the peak metamorphic ages of 13.9 ± 1.2 Ma and 14.9 ± 0.7 Ma, respectively, which indicates that the Dinggye eclogite should be the youngest eclogite in Himalayan orogenic belt. Geochemical characteristics and zircon analyses show that the protoliths of eclogite in Dinggye are predicted to be continental rift-related basaltic rocks. The eclogite at Dinggye in central Himalaya should be formed by the crustal thickening during the long-lasting continental overthrusting by Indian plate beneath Euro-Asian continent, and its exhumation process may be related with channel flow and orogen-parallel extension. In the middle Miocene (~ 14 Ma), Indian continental crust had reached at least ~ 65 km depth in southern Tibet.     

High-temperature,high-pressure granulites (retrogressed eclogites) in the central region of the Lewisian,NW Scotland: Crustal-scale subduction in the Neoarchaean

Eclogites and associated high-pressure (HP) rocks in collisional and accretionary orogenic belts preserve a record of subduction and exhumation, and provide a key constraint on the tectonic evolution of the continents. Most eclogites that formed at high pressures but low temperatures at > 10–11 kbar and 450–650 °C can be interpreted as a result of subduction of cold oceanic lithosphere. A new class of high-temperature (HT) eclogites that formed above 900 °C and at 14 to 30 kbar occurs in the deep continental crust, but their geodynamic significance and processes of formation are poorly understood. Here we show that Neoarchaean mafic–ultramafic complexes in the central granulite facies region of the Lewisian in NW Scotland contain HP/HT garnet-bearing granulites (retrogressed eclogites), gabbros, lherzolites, and websterites, and that the HP granulites have garnets that contain inclusions of omphacite. From thermodynamic modeling and compositional isopleths we calculate that peak eclogite-facies metamorphism took place at 24–22 kbar and 1060–1040 °C. The geochemical signature of one (G-21) of the samples shows a strong depletion of Eu indicating magma fractionation at a crustal level. The Sm–Nd isochron ages of HP phases record different cooling ages of ca. 2480 and 2330 Ma. We suggest that the layered mafic–ultramafic complexes, which may have formed in an oceanic environment, were subducted to eclogite depths, and exhumed as HP garnet-bearing orogenic peridotites. The layered complexes were engulfed by widespread orthogneisses of tonalite–trondhjemite–granodiorite (TTG) composition with granulite facies assemblages. We propose two possible tectonic models: (1) the fact that the relicts of eclogitic complexes are so widespread in the Scourian can be taken as evidence that a > 90 km × 40 km-size slab of continental crust containing mafic–ultramafic complexes was subducted to at least 70 km depth in the late Archaean. During exhumation the gneiss protoliths were retrogressed to granulite facies assemblages, but the mafic–ultramafic rocks resisted retrogression. (2) The layered complexes of mafic and ultramafic rocks were subducted to eclogite-facies depths and during exhumation under crustal conditions they were intruded by the orthogneiss protoliths (TTG) that were metamorphosed in the granulite facies. Apart from poorly defined UHP metamorphic rocks in Norway, the retrogressed eclogites in the central granulite/retrogressed eclogite facies Lewisian region, NW Scotland have the highest crustal pressures so far reported for Archaean rocks, and demonstrate that lithospheric subduction was transporting crustal rocks to HP depths in the Neoarchaean.     

Pan-African metamorphic evolution in the southern Yaounde Group (Oubanguide Complex,Cameroon) as revealed by EMP-monazite dating and thermobarometry of garnet metapelites

Garnet-bearing micaschists and paragneisses of the Yaounde Group in the Pan-African Central African Orogenic Belt in Cameroon underwent a polyphase structural evolution with the deformation stages D₁–D₂, D₃ and D₄. The garnet-bearing assemblages crystallized in course of the deformation stage D₁–D₂ which led to the formation of the regional main foliation S₂. In X_Ca–X_Mg coordinates one can distinguish several zonation trends in the garnet porphyroblasts. Zonation trends with increasing X_Mg and variably decreasing X_Ca signalize a garnet growth during prograde metamorphism. Intermineral microstructures provided criteria for local equilibria and a structurally controlled application of geothermobarometers based on cation exchange and net transfer reactions. The syndeformational P–T path sections calculated from cores and rims of garnets in individual samples partly overlap and align along clockwise P–T trends. The P–T evolution started at ～450 °C/7 kbar, passed high-pressure conditions at 11–12 kbar at variable temperatures (600–700 °C) and involved a marked decompression toward 6–7 kbar at high temperatures (700–750 °C). Th–U–Pb dating of metamorphic monazite by electron microprobe (EMP-CHIME method) in eight samples revealed a single period of crystallization between 613 ± 33 Ma and 586 ± 15 Ma. The EMP-monazite age populations between 613 ± 33 Ma enclosed in garnet and 605 ± 12 Ma in the matrix apparently bracket the high temperature–intermediate pressure stage at the end of the prograde P–T path. The younger monazites crystallized still at amphibolite-facies conditions during subsequent retrogression. The Pan-African overall clockwise P–T evolution in the Yaounde Group with its syndeformational high pressure stages and marked pressure variations is typical of the parts of orogens which underwent contractional crustal thickening by stacking of nappe units during continental collision and/or during subduction-related accretionary processes.     

Ultrahigh-temperature mafic granulites from Panrimalai,south India: Constraints from phase equilibria and thermobarometry

The Panrimalai area constitutes part of the granulite-facies rocks of the Madurai block in the Southern Granulite Terrain (SGT), India. Garnet-bearing mafic granulites in Panrimalai occur as small enclaves within charnockite. The common stable assemblage during peak metamorphism contains hornblende, garnet, orthopyroxene, clinopyroxene, quartz and plagioclase. The resorption of garnet in various reaction textures and the development of spectacular orthopyroxene–plagioclase and hornblende–plagioclase symplectites characterize the subsequent stages of metamorphism. Application of multi-equilibrium calculation procedures for mineral core compositions of the early assemblage yields near peak conditions at  ≥ 900 °C at 9 kbar. These estimates are the highest yet reported in mafic granulites from the Madurai block. The post-peak P–T path is constructed for the mafic granulites based on observed microstructural relations and thermobarometric results is characterized by a steep clockwise decompressional P–T segment from  ≥ 9 to  < 4.5 kbar. Constraints from model Nd ages provide evidence for Paleoproterozoic magmatism restricted to the Madurai block in the Southern Granulite Terrain. The early part of the crustal evolution of the Panrimalai granulites could be coeval with the Paleoproterozoic event. Subsequent development of symplectitic assemblages via near-isothermal decompression can be ascribed to a distinctly later tectonic event. Available U–Pb and Sm–Nd mineral dates suggest a widespread Pan-African tectonothermal event in the SGT. Given the general recognition of ultrahigh-temperature (UHT) and isothermal decompression (ITD) in Pan-African age metamorphism in the East-African–Antarctic Orogen (EAAO) , the Panrimalai UHT history is considered to be part of this record.     

Oligocene HP metamorphism and anatexis of the Higher Himalayan Crystalline Sequence in Yadong region,east-central Himalaya

The Higher Himalayan Crystalline Sequence (HHCS) provides an excellent natural laboratory to study continental subduction, crustal melting and tectonic evolution of orogenic belt generated through the collision of India with Eurasia. Our petrological study and phase equilibrium modeling reveal that the pelitic migmatites in the HHCS of Yadong region, east-central Himalaya, preserve an early mineral assemblage garnet, kyanite, biotite, quartz, plagioclase, K-feldspar, rutile and ilmenite, and a late sillimanite- and/or cordierite-bearing assemblage, and underwent the high pressure (HP) and high temperature (HT) granulite-facies metamorphism and associated partial melting under P–T conditions of ca. 12 kbar and 825–845 °C, followed by nearly isothermal decompression and isobaric cooling. The anatexis of the migmatites occurred dominantly through dehydration-melting of both muscovite and biotite during the prograde metamorphism. The melt produced in the peak metamorphic conditions is about 20 to 30 vol.% of the rocks, and a significant amount of melt has been extracted from the source leading to the formation of Himalayan leucogranites. The zircon U–Pb dating data shows that the migmatites probably witnessed a prolonged melting episode that began at ca. 30 Ma and lasted to ca. 20 Ma. These results show that the thickening lower crust of the Himalayan orogen experienced long-lived and continued HP and HT metamorphism and pervasive anatexis, supporting the models on channel flow.     

Partial melting and retrogression during exhumation of high-grade metapelites,the Tatra Mountains,Western Carpathians

Partial melting and retrogression have been recognized in high-grade metapelites of the Tatra Mountains, Western Carpathians (Slovakia) related to exhumation during Variscan orogeny. Reaction textures and phase equilibria define a clockwise P-T path. The prograde metamorphism from ca 600 °C and 9–10 kbar to >700 °C at 11–12 kbar resulted in muscovite dehydration-melting in the kyanite stability field. Further heating at decreasing pressure led to the dehydration-melting of biotite at >750 °C in the sillimanite stability field. This was followed by nearly isothermal decompression down to 4–5 kbar, producing cordierite and some additional melt. Later nearly isobaric cooling led to melt crystallization and sub-solidus retrogression. CO₂-N₂ fluids (5–30 mol. % N₂) were generated at pressures <6 kbar by interaction between the melt-derived water and graphite at oxidizing conditions.     

Ultrahigh-temperature metamorphism and anticlockwise P–T–t path of Paleozoic granulites from north Qinling-Tongbai orogen,Central China

Mafic and semi-pelitic granulites from the Qinling-Tongbai orogen in central China preserve petrological evidence and mineral paragenesis suggesting four distinct stages of metamorphic evolution. The prograde history (M₁) is recorded by the occurrence of cordierite, orthopyroxene and biotite inclusions in garnet porphyroblasts of the peak-metamorphic (M₂) assemblage. Peak-metamorphism was followed by cooling with minor decompression (M₃), which formed symplectites and coronitic textures. The greenschist facies retrograde metamorphic assemblage (M₄) is represented by hydrous minerals replacing minerals of the M₂ and M₃ assemblages. We present LA-ICPMS zircon U-Pb data which show ages of 432 ± 4 Ma for the peak metamorphism and 403 to 426 Ma for the retrograde stage. Microstructural analysis, P–T pseudosections, and mineral isopleths in conjunction with the zircon U-Pb ages define an anticlockwise P–T–t path. The P–T estimates for peak metamorphic conditions of 880–920 °C and 8.0–10 kbar suggest that these rocks witnessed extreme crustal metamorphism under ultrahigh-temperature conditions. The anticlockwise trajectory reported in this study is comparable with similar P–T paths recorded from subduction–collision settings, and correlate the Tongbai granulites to hot orogens developed within a Paleozoic collisional suture. We propose a ridge subduction and slab window setting to explain the formation of the Tongbai orogen, in a convergent plate setting associated with the northward subduction of the Paleo-Tethyan Qinling Ocean.     

Systematic changes in metamorphic styles along the Dabie–Hongseong and Himalayan collision belts,and their tectonic implications

Different continental collision belts show contrasting metamorphic trend along their length, including the distribution of extreme metamorphism; i.e., ultrahigh-pressure (>100 km depth) and ultrahigh-temperature (900–1150 °C) metamorphisms. However, no previous study has succeeded in explaining these trends. The present study investigates the main factors that control the metamorphic trends along collision belts, with reference to the Dabie–Hongseong collision belt between the North and South China blocks and the Himalayan collision belt between the Indian and Asian blocks. In the Dabie–Hongseong collision belt, collision began in the east before 245 Ma and propagated westward until ca. 220 Ma. In the eastern part of the belt, the amount of oceanic slab that subducted before collision was insufficient to pull down the continental crust to the depths of ultrahigh-pressure metamorphism; however, ultrahigh-pressure metamorphism occurred in the western part of the belt. Slab break-off also migrated from east to west, with a westward increase in the depth of break-off (from ca. 10 kbar in the west to ca. 35 kbar in the east). These lateral trends along the belt resulted in a westward change from ultrahigh-temperature (915–1160 °C, 9.0–10.6 kbar) to high-pressure (835–860 °C, 17.0–20.9 kbar) and finally ultrahigh-pressure metamorphism (680–880 °C, 30–40 kbar). In the Himalayan collision belt, collision started from the west at 50 Ma and propagated eastward. The amount of oceanic slab subducted prior to collision was sufficient to pull down the continental crust to the depths of ultrahigh-pressure metamorphism in the west, but not in the east. Slab break-off started in the west at ca. 46 Ma and propagated eastward, with an eastward decrease in the depth of slab break-off from 27–29 to 17–18 kbar. Consequently, the metamorphic trend along the belt changes eastward from ultrahigh-pressure (690–750 °C, 27–29 kbar) to high-pressure and finally high-pressure granulite facies metamorphism (890 °C, 17–18 kbar). The differences in metamorphic trend between the Dabie–Hongseong and Himalayan collision belts reflect the amount of oceanic crust subducted prior to collision and the depth and timing of slab break-off along each belt.     

Metamorphic P–T–t–d evolution of (U)HP metabasites from the South Tianshan accretionary complex (NW China) — Implications for rock deformation during exhumation in a subduction channel

The late Carboniferous accretionary system of the South Tianshan orogen (North-Western China) underwent complex structural and polymetamorphic evolution. Combined petrological, geochronological and microstructural analysis of (ultra)high-pressure (UHP) metabasites (eclogites and blueschists) enclosed in metapelites show a relict coarse-grained eclogitic fabric S2 surrounded by a dominant fine-grained eclogite and blueschist facies retrograde fabric S2. The S2 fabric is reworked by upright folds F3 that are responsible for a major shortening of the whole accretionary system. For both the eclogite and blueschist, peak and retrograde P–T conditions have been thermodynamically constrained at 25–26 kbar and 425–500 °C and 10–13 kbar and 500−550 °C respectively, suggesting a shared exhumation history. The garnet-whole rock-amphibole isochron in the blueschist yielded Lu–Hf and Sm–Nd ages of 326.0 ± 2.9 Ma and 318.4 ± 3.9 Ma respectively, interpreted to date the prograde to peak metamorphic assemblage. The retrograde path of the eclogite is characterized by heterogeneous omphacite recrystallization into a mylonitic fine-grained matrix and crystallization of blue amphibole. Microstructures in both pristine porphyroclastic and recrystallized fine-grained domains in the eclogite indicate a gradual evolution from constriction-dominated (L>S-type) to flattening-dominated (S>L-type) type of deformation, increase of fabric intensity reflected by gradually growing M-indexes and the development of lattice preferred orientation (LPO) typical for dislocation creep under slightly hydrated conditions. Recrystallization of the matrix in the blueschist is homogeneous, which indicates a matrix dominated channel flow during exhumation. These LPOs evolutions suggest a significant mechanical coupling with the upper plate concomitant with oroclinal bending of the Kazakh orocline. Lock up of Kazakh orocline is responsible for further stress increase resulting in horizontal shortening of South Tianshan accretionary wedge and development of D3 upright folding and steepening of the whole sequence.     

Metamorphic <Emphasis Type="Italic">P</Emphasis>–<Emphasis Type="Italic">T</Emphasis> paths of the Zanhuang amphibolites and metapelites: constraints on the tectonic evolution of the Paleoproterozoic Trans-North China Orogen

Garnet-bearing metapelites and amphibolites are exposed in the south and middle parts of the Zanhuang complex, which is located in the central segment of the nearly NS-striking Trans-North China Orogen. These rocks preserve three metamorphic mineral assemblages forming at the prograde, peak and post-peak decompression stages. The prograde metamorphic stage (M₁) is represented by mineral inclusions within garnet porphyroblasts, the peak metamorphic stage (M₂) is represented by garnet rims and matrix minerals, whereas the retrograde stage (M₃) is represented by amphibole + plagioclase symplectite rimming garnet porphyroblasts in the amphibolites and biotite + plagioclase symplectite rimming garnet porphyroblasts in the metapelites. All garnet porphyroblasts in the metapelites preserve prograde chemical zoning except for the ubiquitous, quite narrow zones from the underwent post-peak decompression. It has been determined through thermobarometric computation that the metamorphic conditions are 650–710°C at 8.2−9.2 kbar for the M₁ (inclusion) assemblages, >810°C at >12.5 kbar for the metamorphic peak M₂ (matrix) assemblages, and 660–680°C at 4.4–4.5 kbar for the retrograde M₃ (symplectite) assemblages. These rocks are thus determined to have undergone metamorphism with clockwise P–T paths involving nearly isothermal decompression (ITD) segments, which is inferred to be related to the amalgamation of the Eastern and Western Blocks to form the coherent basement of the North China Craton along the Trans-North China Orogen in the late Paleoproterozoic (1.88–1.85 Ga).     

P–T–t–d evolution of orogenic middle crust of the Roc de Frausa Massif (Eastern Pyrenees): a result of horizontal crustal flow and Carboniferous doming?

Structural, petrological and textural studies are combined with phase equilibria modelling of metapelites from different structural levels of the Roc de Frausa Massif in the Eastern Pyrenees. The pre‐Variscan lithological succession is divided into the Upper, Intermediate and Lower series by two orthogneiss sheets and intruded by Variscan igneous rocks. Structural analysis reveals two phases of Variscan deformation. D1 is marked by tight to isoclinal small‐scale folds and an associated flat‐lying foliation (S1) that affects the whole crustal section. D2 structures are characterized by tight upright folds facing to the NW with steep NE–SW axial planes. D2 heterogeneously reworks the D1 fabrics, leading to an almost complete transposition into a sub‐vertical foliation (S2) in the high‐grade metamorphic domain. All structures are affected by late open to tight, steeply inclined south‐verging NW–SE folds (F3) compatible with steep greenschist facies dextral shear zones of probable Alpine age. In the micaschists of the Upper series, andalusite and sillimanite grew during the formation of the S1 foliation indicating heating from 580 to 640 °C associated with an increase in pressure. Subsequent static growth of cordierite points to post‐D1 decompression. In the Intermediate series, a sillimanite–biotite–muscovite‐bearing assemblage that is parallel to the S1 fabric is statically overgrown by cordierite and K‐feldspar. This sequence points to ~1 kbar of post‐D1 decompression at 630–650 °C. The Intermediate series is intruded by a gabbro–diorite stock that has an aureole marked by widespread migmatization. In the aureole, the migmatitic S1 foliation is defined by the assemblage biotite–sillimanite–K‐feldspar–garnet. The microstructural relationships and garnet zoning are compatible with the D1 pressure peak at ~7.5 kbar and ~750 °C. Late‐ to post‐S2 cordierite growth implies that F2 folds and the associated S2 axial planar leucosomes developed during nearly isothermal decompression to <5 kbar. The Lower series migmatites form a composite S1–S2 fabric; the garnet‐bearing assemblage suggests peak P–T conditions of >5 kbar at suprasolidus conditions. Almost complete consumption of garnet and late cordierite growth points to post‐D2 equilibration at <4 kbar and <750 °C. The early metamorphic history associated with the S1 fabric is interpreted as a result of horizontal middle crustal flow associated with progressive heating and possible burial. The upright F2 folding and S2 foliation are associated with a pressure decrease coeval with the intrusion of mafic magma at mid‐crustal levels. The D2 tectono‐metamorphic evolution may be explained by a crustal‐scale doming associated with emplacement of mafic magmas into the core of the dome.     

The Guelb Moghrein Cu–Au deposit: Neoarchaean hydrothermal sulfide mineralization in carbonate-facies iron formation

The Guelb Moghrein copper–gold deposit in the Islamic Republic of Mauritania reopened in 2006 and has produced copper concentrate and gold since then. The deposit is hosted in Neoarchaean–Palaeoproterozoic Fe–Mg carbonate-dominated metamorphic rocks interpreted as carbonate-facies iron formation. It forms tabular orebodies controlled by shear zones in the hanging wall and footwall of this meta-iron formation. Copper and gold are hosted in a complex sulfide ore in tectonic breccia replacing Fe–Mg carbonate and magnetite. Hydrothermal monazite dates the mineralization at 2492 ± 9 Ma. Two types of aqueous fluid inclusions suggest fluid mixing at 0.75–1.80 kbar and ~ 410 °C as the mineralization and precipitation mechanism, which is temporally coincident with regional retrograde metamorphism at 410 ± 30 °C (garnet-biotite). Distal alteration zones are enriched in K, Rb and Cu, whereas orebodies are depleted in K, Rb, Sr and Ba. The copper–gold mineralization at Guelb Moghrein formed during retrograde shearing in metamorphic rocks and contemporaneous hydrothermal alteration. The stable isotope signature of alteration and ore minerals suggest an external crustal fluid source. Fluids were focused in the reactive and competent meta-iron formation. Potassium alteration, magnetite and copper–gold mineralization suggest an IOCG mineral system akin similar deposits in Australia and Brazil.     

Metamorphic P–T path of mafic granulites from Eastern Hebei: Implications for the Neoarchean tectonics of the Eastern Block,North China Craton

This study documents the metamorphic evolution of mafic granulites from the Eastern Hebei Complex in the Eastern Block of the North China Craton. Mafic granulites from Eastern Hebei occur as boudins or enclaves within Neoarchean high-grade TTG gneisses. Petrographic observations reveal three characteristic metamorphic mineral assemblages in the mafic granulites: the pre-peak hornblende + plagioclase + ilmenite + quartz + sphene assemblage (M₁) existing as mineral inclusions within coarse-grained peak assemblage (M₂) represented by garnet + clinopyroxene + orthopyroxene + plagioclase + hornblende + ilmenite + quartz, and post-peak assemblage (M₃) marked by garnet + quartz ± ilmenite symplectites surrounding the peak pyroxene and plagioclase. Based on pseudosection modeling calculated in the NCFMASHTO model system using the program THERMOCALC, P–T conditions of the pre-peak (M₁), peak (M₂) and post-peak (M₃) assemblages are constrained at 600–715 °C/6.0 kbar or below, 860–900 °C/9.6–10.3 kbar, and 790–810 °C/9.6–10.4 kbar, respectively. These P–T estimates, combined with their mineral compositions and reaction relations, define an anticlockwise P–T path incorporating isobaric cooling subsequent to the peak medium-pressure granulite-facies metamorphism for the mafic granulites from Eastern Hebei. Such an anticlockwise P–T path suggests that the end-Neoarchean metamorphism of the Eastern Hebei Complex correlated closely with underplating and intrusion of voluminous mantle-derived magmas. In conjunction with other geological considerations, a mantle-plume model is favored to interpret the Neoarchean tectonothermal evolution of the Eastern Hebei Complex and other metamorphic complexes in the Eastern Block. The prograde amphibolite-facies metamorphism (M₁) was initiated due to the upwelling of the relatively cooler mantle plume head, followed by the peak medium-pressure granulite-facies metamorphism (M₂) as triggered by the uprising hotter plume “tail”, and finally when plume activity ceased, the heated metamorphic crust experienced nearly isobaric cooling (M₃).     

Analysis of parageneses of metapelite gneisses of the Okhotsk granulite complex by minimization of Gibbs thermodynamic potential

The problem of modeling of real parageneses has been solved by minimization of Gibbs thermodynamic potential for metapelites of the Okhotsk granulite complex. Model mineral assemblages completely reproduce the composition of minerals and their modal contents in the studied rocks. This fundamental fact directly verifies the solution of the problem, proving the validity of the principle of local equilibrium in the studied assemblages and the agreement of all thermodynamic data accepted on the modeling. The pressure and temperature during the metamorphism of granulites of the Okhotsk complex, estimated by modeling, are 5.2–7.0 kbar and 620–770 °C, which corresponds to the near-boundary conditions of the amphibolitic and granulitic facies. Model mineral assemblages similar to real parageneses in the composition of minerals and their modes can be successfully obtained with the Selektor software under conditions of both inert and moving water. The composition of the external metamorphic fluid and the approximate weight ratio of fluid to rock have been determined. The oxidation potential of this fluid is similar to the potential of oxygen at the buffer C–CO–CO₂ if the fluid/rock ratio is 0.03–0.30 and the low partial pressure of water varies from 1.80 to 0.35 kbar. The Okhotsk metamorphic complex is not an analog of the granulites of the southern Aldan Shield, because considerably higher pressure and temperature are typical of the latter.     

High-pressure and ultrahigh-temperature metamorphism at Komateri,northern Madurai Block,southern India

We report for the first time the evidence for prograde high-pressure (HP) metamorphism preceding a peak ultrahigh-temperature (UHT) event in the northernmost part of the Madurai Block in southern India. Mg–Al-rich Grt–Ged rocks from Komateri in Karur district contain poikiloblastic garnet with numerous multi-phase inclusions. Although most of the inclusion assemblages are composed of gedrite, quartz, and secondary biotite, rare staurolite + sapphirine and spinel + quartz are also present. The X_Mg (=Mg/[Fe+Mg]) of staurolite (0.45–0.49) is almost consistent with that reported previously from Namakkal district in the Palghat–Cauvery Shear Zone system (X_Mg = 0.51–0.52), north of the Madurai Block. The HP event was followed by peak UHT metamorphism at T = 880–1040 °C and P = 9.8–12.5 kbar as indicated by thermobarometric computations in the Grt–Ged rock and associated mafic granulite. Symplectic intergrowth of spinel (X_Mg = 0.50–0.59, ZnO < 1.7 wt.%) and quartz, a diagnostic indicator of UHT metamorphism, probably formed by decompression at UHT conditions. The rocks subsequently underwent retrograde metamorphism at T = 720–760 °C and P = 4.2–5.1 kbar. The P–T conditions and clockwise exhumation trajectory of the Komateri rocks, comparable to similar features recorded from the Palghat–Cauvery Shear Zone system, suggest that the Madurai Block and the Palghat–Cauvery Shear Zone system underwent similar HP and UHT metamorphic history probably related to the continent–continent collision during the final stage of amalgamation of Gondwana supercontinent.     

P-T evolution of metapelites from the Bajgan complex in the Makran accretionary prism,south eastern Iran

The Bajgan Complex, one of the basement constituents of the arc massif in Iranian Makran forms a rugged, deeply incised terrain. The complex consists of pelitic schists with minor psammitic and basic schists, calc silicate rocks, amphibolites, marbles, metavolcanosediments, mafic and felsic intrusives as well as ultramafic rocks. Metapelitic rocks show an amphibolite facies regional metamorphism and contain garnet, biotite, white mica, quartz, albite ± rutile ± apatite. Thermobarometry of garnet schist yields pressure of more than 9 kbar and temperatures between 560 and 675 °C. The geothermal gradient obtained for the peak of regional metamorphism is 19 °C/km, corresponding to a depth of ca. 31 km. Replacement of garnet by chlorite and epidote suggest greenschist facies metamorphism due to a decrease in temperature and pressure through exhumation and retrograde metamorphism (370–450 °C and 3–6 kbar). The metapelitic rocks followed a ‘clockwise’ P–T path during metamorphism, consistent with thermal decline following tectonic thickening. The formation of medium-pressure metamorphic rocks is related to presence of active subduction of the Neotethys Oceanic lithosphere beneath Eurasia in the Makran.     

Syn-kinematic emplacement of the Pangong metamorphic and magmatic complex along the Karakorum Fault (N Ladakh)

This paper investigates the age, P–T conditions and kinematics of Karakorum Fault (KF) zone rocks in the NW part of the Himalaya–Karakorum belt. Granulite to greenschist facies assemblages were developed within the KF zone during strike-slip shearing. The granulites were formed at high temperature (800 °C, 5.5 kbar), were subsequently retromorphosed into the amphibolite facies (700–750 °C, 4–5 kbar) and the greenschist facies (350–400 °C, 3–4 kbar). The Tangtse granite emplaced syn-kinematically at the contact between a LT and the HT granulite facies. Intrusion occurred during the juxtaposition of the two units under amphibolite conditions. Microstructures observed within the Tangtse granite exhibit a syn-magmatic dextral S–C fabric. Compiled U–Pb and Ar–Ar data show that in the central KF segment, granulite facies metamorphism occurred at a minimum age of 32 Ma, subsequent amphibolite facies metamorphism at 20–18 Ma. Further shearing under amphibolite facies (650–500 °C) was recorded at 13.6 ± 0.9 Ma, and greenschist-facies mica growth at 11 Ma. These data give further constrains to the age of initiation and depth of the Karakorum Fault. The granulite-facies conditions suggest that the KF, accommodating the lateral extrusion of Tibet, could be at least a crustal or even a Lithosphere-scale shear zone comparable to other peri-Himalayan faults.     

High-Ti muscovite as a prograde relict in high pressure granulites with metamorphic Devonian zircon ages (Běstvina granulite body,Bohemian Massif): Consequences for the relamination model of subducted crust

High-pressure kyanite–K-feldspar granulites in the Běstvina granulite body, which belongs to the Variscan orogenic root in the Bohemian Massif, preserve muscovite, rutile and kyanite inclusions in garnet. High-Ti muscovite (Ti = 0.09–0.20 p.f.u., Si = 0.21–3.24 p.f.u.) included in garnet is associated with quartz and is in crystallographic continuity with biotite, interpreted in terms of exsolution from an original less-dioctahedral higher-Ti muscovite. The assemblage garnet–kyanite–antiperthite–perthite–quartz–rutile and the mineral compositions indicate a peak of metamorphism at about 900 °C and 17–21 kbar, based on P–T pseudosection modeling, ternary-feldspar and Zr-in-rutile thermometry. The matrix assemblage garnet–kyanite–plagioclase-K-feldspar–quartz–rutile–ilmenite and garnet rim compositions at contact with feldspars and quartz indicate the end of overall equilibration in the presence of melt at 12–14 kbar and 820–840 °C. Embayments of biotite and plagioclase locally replacing garnet, and connected with modification of garnet composition, may indicate sites of last isolated melt or diffusion of H₂O from that melt down to 10 kbar and 800 °C. Zircon with uniform cathodoluminescence (CL) pattern is present as rims around cores with faint oscillatory zoning, or as entire rounded grains. These zircons gave a cluster of ages at 359 ± 4 Ma, interpreted as the age of metamorphism. Zircon ages from the cores with common faint oscillatory zoning range from 500 to 398 Ma, and are interpreted as magmatic grains variably reset during metamorphism. Two older ages obtained on cores of 620 ± 18 Ma probably represent an inherited zircon component. Molar isopleths of zircon along the P–T path in pseudosections suggest that crystallization of metamorphic zircon occurred during decompression and cooling from 17 to 21 kbar and 900 °C to 12–14 kbar and 820–840 °C. The inferred P–T path and the age of metamorphism are discussed in the framework of a geodynamic model that considers the granulites to be a part of a subducted plate that failed to continue to subduct and was spread below the upper plate.