High-pressure Granulite Facies Research Articles

Comment on “Neoarchean arc magmatism and Paleoproterozoic high-pressure granulite-facies metamorphism in the southern Motloutse Complex, eastern Botswana: Implications for the western extension of the Limpopo Complex”

Comment on “Neoarchean arc magmatism and Paleoproterozoic high-pressure granulite-facies metamorphism in the southern Motloutse Complex, eastern Botswana: Implications for the western extension of the Limpopo Complex”

Compositional and metamorphic controls on tectonic erosion along a continental subduction-collision zone: Implications from mafic granulites in the northern Sulu orogen

Abstract Subduction erosion has been reported recently in continental subduction-collision zones, and the response of the upper plate remains enigmatic. The Dabie-Sulu orogen is considered to have formed by deep northward subduction of the Yangtze block (lower plate) beneath the North China block (upper plate). However, within the northern Sulu ultrahigh-pressure (UHP) belt, the Haiyangsuo complex has intriguingly been accepted as Neoarchean–Paleoproterozoic metamorphic basement from the North China block, implying subduction-erosion could have occurred. Here, we examined new petrographic, mineral chemical, and published geochronological data from the Haiyangsuo mafic granulites to decipher their multiphase metamorphic evolution. Pressure-temperature (P-T) estimates from pseudosection modeling and geothermobarometry show two major metamorphic events: a medium-pressure granulite-facies event (7.3–8.3 kbar and 830–895 °C) and a later high-pressure granulite-facies event (12.2–16.6 kbar and 800–875 °C). Geochronology and mineral geochemistry link these events to Paleoproterozoic and Triassic metamorphism, respectively. Peak Triassic high-pressure granulite-facies metamorphism was followed by near-isothermal decompression and then near-isobaric cooling. These rocks derive from the upper plate and were metamorphosed at the same time as UHP eclogites, but maximum pressures were substantially lower, and P-T paths did not converge until late-stage exhumation. Evidently, metamorphic basement from the North China block was dragged to lowercrustal to upper-mantle depths (50–60 km) while that from the Yangtze block was subducted deeply, and then they were tectonically juxtaposed during exhumation. These data corroborate thermal-mechanical models that predict tectonic erosion as a major process during continental subduction and collision.

The effect of metamorphism on the aggregate properties of gabbroic rocks

Granitic rocks are durable materials sought after for the production of road and railroad aggregates. Granitic bedrock commonly, however, includes gabbroic components, which may enhance or decrease the aggregate performance. This study evaluates the variation in resistance to fragmentation (Los Angeles value, LA) and wear/abrasion (micro-Deval value, MDE) for the fraction 10/14 mm of gabbro in different metamorphic states. Samples were collected along a 150-km profile where metamorphic conditions grade from epidote–amphibolite to high-pressure granulite-facies, and the degree of metamorphic recrystallization varies with the amount of hydrous fluid. Rocks with no or incipient metamorphic recrystallization preserving their primary igneous fabric and interlocking texture meet the criteria for both asphalt base course and track ballast in railroad, with LA and MDE values below 25% and 14%, respectively. Mafic granulite and fine-grained amphibolite have LA values below 25% and can be used in unbound layers. Mafic granulites crystallize at high temperatures but commonly preserve a relict igneous texture due to limited hydration. Coarse-grained amphibolite and migmatitic amphibolite have the poorest performance. They recrystallized at hydrous conditions, leading to complete recrystallization, grain coarsening, and loss of interlocking igneous texture. This study shows that both temperature and infiltration of hydrous fluids significantly affect the technical properties. Even at high metamorphic temperatures, gabbroic rocks may yield aggregates of high technical performance. At hydrous conditions, however, recrystallization results in rock aggregates suitable for unbound layers only. The variation in metamorphic grade and hydration is easily assessed by the geologist in the field and by using standard petrographic microscopy.

Late Cretaceous Metamorphism and Anatexis of the Gangdese Magmatic Arc, South Tibet: Implications for Thickening and Differentiation of Juvenile Crust

AbstractMagmatic arcs are the primary sites of growth of post-Archean continental crust; however, the mechanisms and processes for transforming primary arc crust into mature continental crust are subject to disagreement. We conducted a detailed petrologic and geochronological study on mafic and felsic migmatites from the eastern Gangdese magmatic arc, which is typical of continental arcs worldwide. The studied mafic migmatites contain amphibole, garnet, plagioclase, epidote, white mica, quartz, rutile and ilmenite in melanosomes, and plagioclase, garnet, epidote, amphibole, white mica, and quartz in leucosomes. The leucosomes occur as diffuse patches, concordant bands, or concordant and discordant networks and veins in the melanosomes. The migmatites have protolith ages between ~157 and ~86–87 Ma, and metamorphic ages of ~83–87 Ma and underwent high-pressure granulite-facies metamorphism at peak P–T conditions of ~850–880°C and 15–17 kbar. Heating, burial, and associated partial melting preceded near-isobaric cooling with residual melt crystallization. Significant melt (>16 wt.%) generated during heating and loading had a granitic composition. Compositional comparison to low-grade meta-gabbros implies that any extracted melt had adakitic affinities (high Sr/Y and highly fractionated REE patterns). The eastern Gangdese magmatic arc experienced crustal thickening during Late Cretaceous late-stage evolution of the arc due to magma loading and tectonic shortening and thrusting of the arc crust. Crustal thickening and chemical differentiation of the Gangdese arc occurred during late subduction of the Neo-Tethys, prior to the India–Asia collision. Metamorphism nearly completely erased all prior igneous mineralogy and mineral chemistry, and consequent partial melting represents a potential source for Late Cretaceous granitoids of the upper arc crust. Although prior studies demonstrate the significance of fractional crystallization, deep-seated metamorphic processes largely drove chemical differentiation to produce mature continental crust in the Gangdese arc during the late Cretaceous.

Deformation mechanisms and characteristics of the meta-BIFs from an early Proterozoic shear system of the Southern Granulite Terrane (SGT), India

We studied three samples of Banded Iron Formation (BIF) rocks from Southern Granulite Terrane (SGT) of India, which was metamorphosed at high-pressure granulite facies and exhumed from 40 to 50 km depth. We examined the microstructure, texture, and deformation mechanisms of the primary minerals (quartz, magnetite, and hematite). Microstructures, variations in size and shape of quartz grains suggest that the three samples deformed to different intensities and temperatures. The EBSD derived CPOs for quartz and magnetite grains are strong, implying dislocation creep as the dominant mechanism. The presence of distinct subgrains in magnetite, rare in natural samples, attests to the same. Furthermore, twinning on {111}, as evident from the misorientation angle and axis distributions, indicates ductile deformation of magnetite below 400 °C. Subgrain boundary trace analysis in selected magnetite grains reveals that multiple-slip planes {111}, {110}, and {100} were operational. Topotaxial replacement, via redox reaction, of magnetite grains along their octahedral planes by hematite is evident. Well-developed CPOs but low (<1°) GOS values of both magnetite and hematite grains suggest that recovery processes in these were rapid. Post-replacement deformation was weak and did not affect the hematite CPOs that were inherited from the relatively more deformed host magnetite grains.

Comment on Volodichev et al. Archean Zircons with Omphacite Inclusions from Eclogites of the Belomorian Province, Fennoscandian Shield: The First Finding. Minerals 2021, 11, 1029

Volodichev et al. (Volodichev et al., 2021) reported on the first finding of omphacite (23%–25% Jd) inclusions in 2.68 Ga metamorphic zircons from Gridino eclogites and presented it as evidence for Archean eclogite-facies metamorphism in the Belomorian Mobile Belt. We believe that the Archean age of the garnets referred to by the above authors was estimated incorrectly. Our interpretation is that omphacite origin is related to Archean high-pressure granulite-facies metamorphism.

High‐pressure granulite facies re‐equilibration and zoisite–biotite dehydration melting during decompression of an ultrahigh‐pressure garnet clinopyroxenite from the island of Fjørtoft, Norway

AbstractA garnet clinopyroxenite from the island of Fjørtoft, Western Gneiss Region (WGR) in Norway, has experienced a complex metamorphic evolution demonstrated by studies involving petrographic observations, mineral chemistry, phase equilibria modelling and geothermobarometry. This rock, originally an eclogite, documents ultrahigh‐pressure (UHP) conditions higher than 3.5 GPa (Stage 1), witnessed by Ca‐rich garnet inclusions in kyanite. The UHP assemblage was pervasively re‐equilibrated at high‐pressure (HP) granulite facies conditions around 1.75 GPa and 870°C, during which and/or somewhat before Ca‐poor garnet, sodian diopside, zoisite and biotite formed (Stage 2a). Dehydration melting of zoisite and biotite (Stage 2b) took place at similar pressure–temperature conditions. The produced melt was poor in Si and rich in alkalis and crystallized mainly to analcime + plagioclase + K‐feldspar. Oriented composite inclusions of quartz + amphibole + phengite ± garnet, found in clinopyroxene, are interpreted to have resulted from interaction between a fluid (hydrous melt and/or aqueous fluid) and clinopyroxene at the early retrogression Stage 3. Stage 1 resulted from deep subduction of the rock. Stage 2 followed after early exhumation probably due to stagnation at a hot and thickened orogenic root. Consequently, other HP granulite facies metabasites, which are similar to the studied garnet clinopyroxenite and widespread in the northwestern WGR, could have been transformed from former eclogite at this stage, too. In addition, this study provides a direct link between the generation of peralkaline melts and crustal anatexis in collision zones and new insights into the retrograde formation of oriented mineral inclusions in HP‐UHP minerals.

Neoarchean arc magmatism and Paleoproterozoic high-pressure granulite-facies metamorphism in the southern Motloutse Complex, eastern Botswana: Implications for the western extension of the Limpopo Complex

We provide the first record of the occurrence of high-pressure garnet-bearing amphibolite and mafic granulite hosted by amphibolites from the Maope area of the southern Motloutse Complex in eastern Botswana. The mineral assemblages of the rocks are garnet + edenite + plagioclase + ilmenite + rutile + quartz (garnet-bearing amphibolite), magnesio-hornblende + orthopyroxene + clinopyroxene + plagioclase + quartz (mafic granulite), and magnesio-hornblende + plagioclase + quartz + ilmenite + epidote + titanite (host amphibolite). The poikiloblastic garnet in the garnet-bearing amphibolite contains inclusions of edenite, ilmenite, quartz, and plagioclase as prograde minerals, while it is mantled by ferro-edenite + plagioclase coronae formed during retrograde metamorphism. The geochemical signatures of the mafic granulite and amphibolite, combined with the enrichment of large-ion lithophile elements and depletion of high-field strength elements (Nb and Ta), indicate subduction-related volcanic arc affinity, whereas the garnet-bearing amphibolite indicates subduction-related volcanic arc or within-plate basalt affinity. The peak metamorphic condition of the garnet-bearing amphibolite was constrained as 850–910 °C and 10.0–10.5 kbar based on phase equilibrium modeling in the Na2O-CaO-FeO-MgO-Al2O3-SiO2-H2O-TiO2-Fe2O3 (NCFMASHTO) system and geothermobarometry. Slightly lower conditions were obtained from the mafic granulite (745–825 °C) and amphibolite (610–750 °C). Zircon grains from the garnet-bearing amphibolite with lower Th/U ratios of 0.02–0.23 yielded a weighted-mean 207Pb/206Pb age of 2020.2 ± 4.9 Ma as the timing of high-grade metamorphism. In contrast, oscillatory-zoned zircons from the mafic granulite with higher Th/U ratios of 0.44–0.86 recorded a magmatic age of 2648.8 ± 9.8 Ma. As the age is consistent with previously reported age of a tonalite gneiss (2645 Ma) from the same region, we infer bimodal arc magmatism during Neoarchean. The results of this study indicate that the southern Motloutse Complex underwent high-pressure granulite-facies metamorphism with post-peak decompression along a single clockwise P–T evolution during Paleoproterozoic Era (∼2.02 Ga). Our integrated petrological and geochronological studies suggest that the southern Motloutse Complex is nearly equivalent to the Beit Bridge Complex as a possible western extension of a collisional suture (the Central Zone of the Limpopo Complex) between the Archean Kaapvaal and Zimbabwe Cratons. The occurrence of ca. 2.0 Ga high-pressure granulite-facies rocks from the southern Motloutse Complex might suggest multiple collisions of discrete crustal units during Neoarchean to Paleoproterozoic.

Contrasting P-T-t paths of basement and cover within the Búzios Orogen, SE Brazil – Tracking Ediacaran-Cambrian subduction zones

Deeply eroded collisional orogens show complex structural and inverted stratigraphic relations with juxtaposition of rock stacks from distinct crustal levels, origins and ages, hence with contrasting P-T-t paths during convergent tectonics. This paper presents petrochronological data on Paleoproterozoic (basement) and Ediacaran (cover) gneisses tectonically interleaved during the Ediacaran-Cambrian Búzios Orogeny, in southeastern Brazil. U-Pb in zircon and EPMA U-Th-Pb in monazite data, coupled with geothermobarometric data, plus Zr-in-rutile, reveal that at a first orogenic stage (ca. 530–520 Ma), these units were at distinct crustal levels. Samples within the Paleoproterozoic basement show metamorphic near-peak conditions of ∼800 °C and 10 kbar, at a depth of c.37 km. Contrastingly, Ediacaran kyanite-orthoclase-garnet-biotite granulite with retrometamorphic sillimanite (cover) reached near-peak conditions of 15 kbar and 818 °C–785 °C at depths of c. 55 km, in high-pressure granulite facies. This deep burial of Ediacaran sediments in less than 20 m.y. would be compatible with a low angle subduction zone active from ca. 550 to 530 Ma. Intrusion of ca. 550 Ma tholeiitic dykes in the Paleoproterozoic gneiss indicates a high geothermal gradient for this subduction setting, which is consistent with a low subduction rate. In a second orogenic stage (ca. 520–500 Ma), these distinct stratigraphic units were placed tectonically side by side during a fast exhumation, preserving an inverted metamorphic stack. The cover underwent retrometamorphic conditions of 800 °C and 10 kbar on a clockwise return path due to decompression. It is proposed here that the contact between reworked units within a Paleoproterozoic continental crust and Ediacaran magmatic and sedimentary units represent the suture of an Ediacaran NW-subduction of the Angola continental paleomargin below the Oriental Terrane of the Ribeira belt. This paper reports the highest pressure recorded in Ediacaran-Cambrian metamorphic rocks from the Brasiliano belts along the actual South Atlantic continental margins. The suture we propose here is aligned along strike, with a medium to high-pressure Ediacaran metamorphic occurrence 700 km to the SW, in the Curitiba Terrane.

High-pressure granulite-facies metamorphism and anatexis of deep continental crust: New insights from the Cenozoic Ailao Shan–Red River shear zone, Southeast Asia

Studies of crustal anatexis have provided valuable insights into the evolution of metamorphism, deformation, and tectonic processes at convergent plate margins during the orogeny. The transition of metatexite to diatexite migmatites records crucial information about the tectonothermal evolution and rheology of the deep crust. Along the Ailao Shan–Red River shear zone, metatexite migmatites, diatexite migmatites, and leucogranites are widely distributed within the upper amphibolite and granulite facies zones of the Diancang Shan metamorphic complex. The high-pressure granulite-facies metamorphism, with the mineral assemblage comprising garnet + kyanite + K–feldspar + plagioclase + biotite + quartz + melt, is first recognized from the patch of metatexite migmatites in the complex. Detailed petrographic evidence and thermodynamic modeling revealed that the migmatite underwent nearly isothermal decompression metamorphism, presenting a clockwise P–T path. The peak metamorphic P–T conditions are constrained at approximately 11 kbar and 810 °C using thermodynamic modeling, and the amount of melt generated during heating is up to 18 mol%. The extraction and segregation of the melts are evidenced by the presence of leucosomes within the migmatites and leucogranite dikes, which record the melt flow network through the crust. Zircon and monazite geochronology of migmatites record the timing of the melting episode that began at approximately 36 Ma and lasted until 20 Ma. All these results are consistent with orogenic crust thickening accompanied by pervasive anatexis during the Late Eocene to the Early Oligocene in the Ailao Shan–Red River shear zone. Combined with the available data related to the other continental–exhumed shear zone, we propose that the crustal anatexis had a significant effect on the thermal state of deep–seated shear zones, thus controlling the rheological behavior of the lithosphere and playing an essential role in the initial localization of shearing in the lower crust.

Early Jurassic accretion of retrograde eclogites and granulites in the Amdo complex, Bangong–Nujiang suture zone, central Tibet

The timing of ocean closure and thermal evolution of the crust in the Bangong–Nujiang suture zone (BNSZ) in central Tibet, which separates the Qiangtang terrane to the north and the Lhasa terrane to the south, are controversial. In this study, we combine petrology, zircon U–Pb geochronology and phase equilibrium modeling to determine the P–T–t paths of newly discovered retrograde eclogites and retrograde granulites from the Amdo complex in the central part of BNSZ. Retrograde eclogites, which occur in the north as lenses in the orthogneisses with a prominent migmatitic texture, record decompression with heating P–T paths from medium-temperature eclogite-facies P–T fields at ~23–24.5 kbar and ~610–655 °C to high-pressure granulite-facies P–T fields at ~10.5–16.5 kbar at ~680–775 °C, followed by nearly isothermal decompression to the P of ~9 kbar at c. 182–181 Ma and further decompression. By contrast, the south orthogneisses without any migmatitic structure host lenses of retrograde granulites. They record “hairpin” clockwise P–T path characterized by compression and heating from ~5 kbar at ~450 °C to peak granulite-facies P–T conditions of 11.5–13 kbar at 640–725 °C, followed by an isothermal decompression to the P of ~8–10 kbar at c. 182 Ma and near isobaric cooling. The spatial relations (retrograde eclogites in the north) and higher peak P–T conditions of the retrograde eclogites, suggest the Amdo complex was a product of northward subduction beneath the Qiangtang terrane. Based on the protolith age for retrograde granulites (c. 184 Ma) and one young gabbro from the Amdo ophiolite (c. 184 Ma) that represents the intra-oceanic arc–backarc basin complex between the Qiangtang terrane to the north and the Amdo complex, we propose that both retrograde eclogites and granulites experienced fast burial to ~13–24 kbar before exhumation to ~9 kbar within 2–3 Ma.

High pressure granulite facies metamorphism at the interface of the Archean Bastar craton and the Proterozoic Eastern Ghats Belt, India

Nd model ages show that the suture between the Archean Bastar craton and Proterozoic Eastern Ghats Province (a part of the Eastern Ghats Belt), India is marked by a broad shear zone that contains a mixture of rocks from both the blocks. In this suture zone, amphibolite facies rocks of the Bastar craton were subjected to high pressure granulite facies metamorphism possibly at ~500 Ma (published isotopic data). New petrological, geothermobarometric and P-T pseudosection studies show that dehydration-melting of amphibole (in mafic protolith) and of biotite (in quartzofeldspathic protolith) along a clockwise P-T path led to peak metamorphic conditions of 9.75 ± 0.5 kbar; 875 ± 30 °C. Mafic granulites from the Eastern Ghats Province had an earlier metamorphic history of isobaric cooling from unknown peak metamorphic conditions, but currently record P-T maximum values of 9.25 ± 0.25 kbar; 825 ± 25 °C. Followed by cooling through ~100 °C from the peak metamorphic conditions, mafic granulites from the Bastar craton were exhumed to mid-crustal levels by decompression-cooling, probably as a tectonic wedge. This decompression-cooling is also shared by charnockitic rocks of the Bastar craton and mafic granulites of the Eastern Ghats Province. We interpret further burial of the Bastar craton rocks as a result of underthrusting beneath the Eastern Ghats Province, consistent with available seismic data. Total absence of any evidence of ocean opening or closure between the Bastar craton and the Eastern Ghats Province at around 500 Ma, coupled with interpretation of deduced P-T paths, suggest the shear zone resulted from far-field stress related to the Kuunga orogeny in an intraplate setting. This would support a model of Tonian age amalgamation of the Bastar craton and the Eastern Ghats Province as a part of the Greater Indian Landmass.

Differential exhumation of ultrahigh-pressure metamorphic terranes: A case study from South Altyn Tagh, western China

South Altyn Tagh contains ultrahigh-pressure (UHP) terranes that have been exhumed from ~300 km mantle depth. Previous zircon U–Pb geochronology has yielded an eclogite-facies age of ca. 500 Ma and a high-pressure (HP) granulite-facies retrograde age of ca. 450 Ma in the Jianggalesayi area in the western segment of South Altyn Tagh. However, in the eastern segment (Yinggelisayi and Danshuiquan localities), an age range of 500–480 Ma has been determined, and it remains uncertain as to whether this age range represents the timing of the peak metamorphic stage or the retrograde overprint. Our study of newly discovered retrograde eclogite in the Danshuiquan locality shows that it underwent three stages of metamorphism, under eclogite-facies, HP granulite-facies and amphibolite-facies P–T conditions of 2.5–4.0 GPa and 870–1050 °C, 2.0–1.4 GPa and 830–940 °C, and 0.7–1.3 GPa and 704–880 °C, respectively. The decompression-dominated P–T path evolved mainly after crossing the solidus, indicating marked retrograde modification under melt-bearing conditions. LA–ICP–MS and SIMS zircon U–Pb dating yielded ca. 500 Ma eclogite-facies and ca. 484 Ma granulite-facies retrograde ages and a later retrograde age of ca. 452 Ma. The clockwise P–T–t path indicates rapid exhumation from eclogite-facies to granulite-facies within around 16 Myr, which is faster than that of the UHP rocks in the western segment. Thus, the HP–UHP rocks in South Altyn Tagh suggest a differential exhumation process for the eastern and western segments. The distinct HP–UHT metamorphism and rapid exhumation of (U)HP rocks in the eastern segment likely resulted from local mantle heating. The continuing P–T evolution of the (U)HP rocks under UHT conditions during exhumation led to a pervasive granulite-facies overprint in the eastern segment of the South Altyn Tagh. The rapid exhumation recorded in the eastern segment provides valuable insights into the exhumation mechanism of ultra-deep subducted UHP terranes.

Metamorphism and geochronology of high-pressure mafic granulites (retrograded eclogites?) in East Cathaysia terrane of South China: Implications for Mesozoic tectonic evolution

AbstractHigh-pressure mafic granulites (retrograded eclogites?) were discovered as minor lenses enclosed in garnet-kyanite-cordierite gneiss from the Badu Complex of the East Cathaysia terrane in South China. These rocks consist mainly of garnet, clinopyroxene, hornblende, quartz, and rutile/ilmenite with or without omphacite pseudomorphs that are indicated by clinopyroxene + sodic plagioclase symplectic intergrowths. Mineral textures and reaction relationships suggest three metamorphic stages: (1) an eclogite-facies stage (M1) characterized by the mineral assemblage of garnet + clinopyroxene (omphacite) + hornblende + rutile + quartz; (2) a high-pressure granulite-facies (M2) stage mainly represented by garnet + clinopyroxene + plagioclase + hornblende + rutile + quartz in the matrix; and (3) an amphibolite retrograde stage (M3) defined by hornblende + plagioclase + ilmenite + quartz symplectites surrounding garnet porphyroblasts. Conventional geothermometers and geobarometers in combination with phase equilibria modeling constrain metamorphic P–T conditions of 15.8–18.2 kbar/625–690 °C (M1), 11.8–14.5 kbar/788–806 °C (M2), and 5.4–6.4 kbar/613–668 °C (M3), respectively. Two-staged decompression processes are defined after the peak pressure, which suggests a two-staged exhumation of these deeply buried rocks. Secondary ion mass spectrometry (SIMS) zircon U-Pb dating and trace element analysis show that the high-pressure metamorphism occurred at 240–244 Ma. Complete early Mesozoic orogenic processes characterized by initial subduction and/or crustal thickening and subsequent exhumation followed by rapid uplift are reconstructed for this part of the East Cathaysia terrane, South China.

On the origin of high-pressure mafic granulite in the Eastern Himalayan Syntaxis: Implications for the tectonic evolution of the Himalayan orogen

The Himalayan orogen, resulting from the Early Cenozoic collision of the Indian and Asian plates, is an ideal vehicle to study active orogenic processes and test geodynamic models of how the crust responds to collisional orogeny. This paper focused on migmatitic high-pressure (HP) mafic granulite and associated leucosome from the Greater Himalayan Crystallines (GHC) in the Eastern Himalayan Syntaxis (EHS) in order to understand the conditions and timescales over which high-grade rocks and partial melts were produced during the Himalayan orogeny. Combining with previous study results from the Western and Central Himalayas and Trans-Himalayan magmatic arc, we obtained the following conclusions: (1) The mafic granulites from the EHS underwent HP and high-temperature (HT) granulite facies metamorphism and partial melting, with peak metamorphic conditions of 15–17 kbar and 820–880 °C. The GHC, at least its western part of the EHS, underwent coherent HP granulite-facies metamorphism. (2) The HP mafic granulites experienced long-lived dehydration melting of amphibole from ~40 Ma to ~20 Ma during prograde metamorphism and generated up to ~16 vol% partial melt. The variable degrees of dehydration melting of the HP mafic, pelitic and felsic granulites in the EHS generated voluminous granitic melts with distinct compositions, and provided the source for the Himalayan granites. (3) Peak metamorphic pressure of the GHC gradually decreases, whereas the metamorphic temperature progressively increases from the Western to Eastern Himalayas. This indicates that the Indian continental crust deeply subducted into the mantle in the Western Himalaya after the Indo-Asia collision, whereas the Indian crust underthrusted or relaminated beneath the Asian continental crust, and formed the thickened lower crust in the Central and Eastern Himalayas and Gangdese arc. (4) The melts derived from the underthrusted Indian crust probably resulted in isotopic compositional enrichment of the Early Cenozoic mantle- and juvenile crust-derived magmatic rocks of the Gangdese arc.

Chicheng high-pressure granulites record the paleoproterozoic tectonic evolution in the northern North China Craton

High-pressure (HP) mafic granulites are widely distributed along the Trans-North China Orogen (TNCO), and they provide important insights into the timing and tectonic processes involved in the formation of the TNCO. We discovered three types of HP granulite, including mafic, intermediate, and felsic granulites, exposed in Chicheng area, in the northern segment of the TNCO. Their mineral assemblages of prograde, peak, and post-peak metamorphic stages were identified based on petrographic investigations. SIMS U-Pb zircon analyses yielded an upper intercept age of 2547 ± 38 Ma and a lower intercept age of 1916 ± 100 Ma for one HP mafic granulite and a discordant 207Pb/206Pb age of 2479 Ma with a weighted mean 207Pb/206Pb age of 1905 ± 9 Ma for the other HP mafic granulite. For the HP intermediate granulite, an upper intercept age of 2544 ± 20 Ma was obtained. The HP felsic granulite yielded an upper intercept age of 2524 ± 23 Ma and a concordant 207Pb/206Pb age of 1906–1907 Ma. These results reveal that the protoliths of HP granulites were formed at ~ 2.5–2.55 Ga and underwent high pressure granulite facies metamorphism at ~ 1.9 Ga. The elevated Hf isotopic compositions of the metamorphic zircons from two granulite samples indicate that metamorphic fluids that have high 176Hf/ 177Hf ratios but low 176Lu/177Hf ratios were involved in their metamorphism. The low 18O isotopic features of the zircons from the intermediate granulite suggest alteration by δ18O-depleted seawater or meteoric fluids at high temperatures. By integrating geochronological data from previous studies, we infer that the formation of the TNCO may have begun as early as ~ 1.9 Ga and lasted until ~ 1.8 Ga, supporting the hypothesis that it was formed by a long-lived tectono-metamorphic event

Late Neoarchean magmatic – metamorphic event and crustal stabilization in the North China Craton

The ca. 2.5 Ga as the time boundary between the Archean and the Proterozoic eons is a landmark, indicating the most important continental crust evolving stage of the Earth, that is, the global cratonization or the formation of supercraton(s) that was unseen before and is unrepeated in the following history of the Earth9s formation and evolution. The North China Craton (NCC) is one of the best recorders of the ca. 2.5 Ga event, and therefore studies in the thorough understanding of early Precambrian continental evolution are continuous. The period from 2.8 to 2.6 Ga is the major crustal growth period of the NCC and formed seven micro-blocks. All the micro-blocks in the NCC were surrounded by 2.6 to 2.54 Ga greenstone belts. The clear geological presentations are as follows: (1) Archaic basement rocks in North China (various micro-blocks) experienced strong partial melting and migmatization. The granitoid rocks derived from crustal partial melting include potassium, TTG and monzonitic granitoids, which come, respectively, from continental crust (sedimentary rocks with TTG gneisses), juvenile crust (mafic rocks with TTG gneisses) or mixed crust; (2) the BIF-bearing supracrustal rocks are mainly distribute in greenstone belts. The lithologic associations in the greenstone belts within the NCC are broadly similar, belonging to volcano-sedimentary sequences, with common bimodal volcanic rocks (basalt and dacite) interlayered with minor amounts of komatiites in the lower part, and calc-alkalic volcanic rocks (basalt, andesite and felsic rocks) in the upper part; (3) nearly all old rocks of >2.5 Ga underwent ∼2.52 to 2.5 Ga metamorphism of amphibolite–granulite facies. Most metamorphosed rocks show high-temperature-ultra-high-temperature (HT–UHT) characteristics and record anticlockwise P–T paths, albeit a small number of granulites seemingly underwent high-pressure granulite facies metamorphism and record clockwise P–T paths; (4) ∼2.5 Ga mafic dikes (amphibolites), granitic dikes (veins) and syenitic–ultramafic dikes developed across these archaic basements and were strongly deformed or un-deformed; (5) the extensive 2.52 to 2.48 Ga low-grade metamorphic supracrustal covers has been recognized in eastern, northern and central parts of the NCC, which are commonly composed of bi-modal volcanic rocks and sedimentary rocks. The above mentioned ∼2.5 Ga geological rocks and their characters imply that the seven micro-blocks have been united through amalgamation to form the NCC. The metamorphosed late Neoarchean greenstone belts, as syn-formed mobile belts, welded the micro-blocks at the end of the Neoarchean. However, the metamorphic thermal grades of the greenstone belts are lower than those of the high-grade terranes within the micro-blocks, suggesting that the latter might have developed under a higher geothermal gradient than the former. Besides, the greenstone belts surround the various micro-blocks in the late Neoarchean when both the old continental crust and the oceanic crust were hotter. The subduction during the amalgamation, if it happened, must have been much smaller in scale as compared to those in the Phanerozoic plate tectonic regime, and all stages occurred at crust-scale instead of lithosphere-scale or mantle-scale. This is why most rocks record HT-UHT and anti-clockwise metamorphism, while only a few samples record high-pressure granulite facies metamorphism with clockwise P–T paths. The micro-block amalgamation was accompanied by extensive crust partial melting and granitization, which finally gave rise to the stabilization of the NCC. Except for the vast granitoid intrusions, mafic-syenitic dike swarms and sedimentary covers are also landmarks of cratonization. The ca. 2.5 Ga cratonization is a global epoch-making geological event, although the accomplishment of cratonization in various cratons is somewhat different in time. Cratonization declared the formation and stabilization of global-scale supercratons or cratonic groups coupling with lithosphere, which was followed by a “silent period” with rare tectonic-thermal action lasting 150 to 200 Ma (from 2.5 Ga – 2.3 or 2.35 Ga), and then followed by the Great Oxidation Event (GOE).

Neoproterozoic and middle Paleozoic geological events in the eastern Wolhyeonri complex of the southwestern Gyeonggi Massif, South Korea, and their tectonic correlations in northeastern Asia

The eastern Wolhyeonri complex in the southwestern Gyeonggi Massif, South Korea, which has a close affinity with the northern margin of the South China Craton, primarily consists of Neoproterozoic igneous and sedimentary rocks that were metamorphosed by the middle Paleozoic thermal event. In this study, we present the results from thermobarometry, whole-rock chemistry, and zircon geochronology of rocks in the eastern Wolhyeonri complex near Gwangcheon. The main rock type, such as the migmatitic biotite gneisses, was deposited after ca. 722–680 Ma, and the associated foliated amphibolites exhibit a rift-related Neoproterozoic origin. The ca. 845–820 Ma arc-related metadolerite and mafic layer in the metavolcanic-sedimentary unit are also exposed. Two Neoproterozoic magmatic events support the idea that the southwestern Gyeonggi Massif developed along the northern margin of the South China Craton. These Neoproterozoic rocks experienced upper amphibolite-facies (7–10.5 kbar and 710–735 °C) to low-pressure granulite-facies metamorphism (10.5–12.0 kbar and 810–830 °C) during 424–412 Ma, were associated with ca. 418 Ma leucogranite formation, and then underwent retrograde amphibolite-facies metamorphism. Combined with the results from previous studies, the early-middle Paleozoic orogeny in the southern Gyeonggi Massif is characterized by 470–456 Ma subduction associated with a high P/T metamorphism, and the initiation of intermediate P/T metamorphism from ca. 445 Ma followed by low-pressure granulite-facies metamorphism at 432–403 Ma, and high-pressure granulite-facies metamorphism at 392–381 Ma, indicating that the tectonic setting transitioned from a continental collision to a post-collisional setting between 432 Ma and 392 Ma. These events and the absence of Paleozoic ultrahigh-pressure metamorphism strongly correlated with the Wuyi-Yunkai orogen in the South China Craton, rather than the Qinling belt in the North China Craton. Therefore, the early-middle Paleozoic orogeny in the southern Gyeonggi Massif was caused by the subduction and subsequent collision within the South China Craton.

Newly discovered MORB-Type HP garnet amphibolites from the Indus-Yarlung Tsangpo suture zone: Implications for the Cenozoic India–Asia collision

High pressure (HP) and ultrahigh pressure (UHP) oceanic rocks remain very scarce along the Indus-Yarlung Tsangpo suture zone in the Tibetan Plateau. Such rock types, however, could provide new and important insight into the understanding of the India-Asia collision tectonics and regional metamorphic processes. This contribution focuses on newly discovered HP garnet amphibolites that occur in the North Ayilari range along the Indus suture zone in the Western Himalaya. Petrographic and mineralogical studies as well as pressure–temperature (P–T) calculations show that the garnet amphibolites underwent at least three metamorphic stages. A prograde amphibolite-facies stage at 0.9–1.2 GPa and 605–680°C (M1) was followed by HP granulite-facies metamorphism at 680–750°C and 1.1–1.5 GPa (M2) and cooling to amphibolite-facies metamorphism at 628–650°C, 0.6–0.8 GPa (M3). Zircon and apatite U–Pb dating shows that the garnet amphibolites experienced granulite-facies conditions at 33–32 Ma and a cooling age of 12.5 ± 2.6 Ma at temperatures of 375–550 °C. The garnet amphibolites have light rare earth element (LREE)-depleted distribution patterns, low (87Sr/86Sr)i = 0.70727–0.70833, but high εNd(t) (+8.9 – +9.8) in whole-rock, and high εHf(t) (+13.9 – +16.0) values of zircon, which are similar to those of normal mid-ocean ridge basalt. This confirms that the protolith represents an oceanic slice that dominantly sourced from depleted mantle material. Geochemical, geochronological and geothermobarometrical data demonstrate that the garnet amphibolites represented a part of the Neo-Tethyan ocean, that experienced a profound amphibolite facies metamorphism before reaching HP granulite-facies conditions at ca. 32 Ma as a consequence of crustal thickening during the India–Asian collision. Thus, the studied HP oceanic-type slice represents an important puzzle piece that, together with other continental portions that experienced different P-T conditions such as HP and UHP eclogite and different metapelites and quartzo-feldspathic gneisses, helps to decipher the tectonic evolution of the Western Himalaya orogen, from oceanic subduction to continental collision and subsequent exhumation.

Metamorphic gradient modification in the Early Cretaceous Northern Andes subduction zone: A record from thermally overprinted high-pressure rocks

New field observations and petrological data from Early Cretaceous metamorphic rocks in the Central Cordillera of the Colombian Andes allowed the recognition of thermally overprinted high-pressure rocks derived from oceanic crust protoliths. The obtained metamorphic path suggests that the rocks evolved from blueschist to eclogite facies towards upper amphibolite to high-pressure granulite facies transitional conditions. Eclogite facies conditions, better recorded in mafic protoliths, are revealed by relic lawsonite and phengite, bleb- to worm-like diopside-albite symplectites, as well as garnet core composition. Upper amphibolite to high pressure granulite facies overprinting is supported by coarse-grained brown-colored Ti-rich amphibole, augite, and oligoclase recrystallization, as well as the record of partial melting leucosomes.Phase equilibria and pressure-temperature (P-T) path modeling suggest initial high-pressure metamorphic conditions M1 yielding 18.2–24.5 kbar and 465–580 °C, followed by upper amphibolite to high pressure granulite facies overprinting stage M2 yielding 6.5–14.2 kbar and 580–720 °C. Retrograde conditions M3 obtained through chlorite thermometry yield temperatures ranging around 286–400 °C at pressures below 6.5–11 kbar. The obtained clockwise P-T path, the garnet zonation pattern revealing a decrease in Xgrs/Xprp related to Mg# increment from core to rim, the presence of partial melting veins, as well as regional constraints, document the modification of the thermal structure of the active subduction zone in Northern Andes during the Early Cretaceous. Such increment of the metamorphic gradient within the subduction interface is associated with slab roll-back geodynamics where hot mantle inflow was triggered. This scenario is also argued by the reported trench-ward magmatic arc migration and multiple extensional basin formation during this period. The presented example constitutes the first report of Cretaceous roll-back-related metamorphism in the Caribbean and Andean realms, representing an additional piece of evidence for a margin-scale extensional event that modified the northwestern border of South America during the Early Cretaceous.