Two-pyroxene Granulite Research Articles

Pressure–temperature evolution, protolith characteristics and tectonic setting of granulite facies rocks from Laojinchang area in the Longgang Block, China

The Archean supracrustal rock series in the Laojinchang area of Jilin Province, China provide valuable insights into the Precambrian geological evolution of the North China Craton. This study presents new results on petrography, geochemistry, mineralogy, isotope geochronology and phase equilibrium modeling of the Laojinchang granulite facies metamorphic rocks. The protolith ages of garnet two-pyroxene granulite and amphibolite from Sidaolazihe Formation, and two-pyroxene granulite from Laoniugou Formation are 2565 ± 5.9 Ma, 2512 ± 17 Ma, and 2509.2 ± 8.6 Ma, respectively, with the metamorphic age recorded by amphibolite being 2475.6 ± 9 Ma. New geochemical data indicates that TTGs (tonalite-trondhjemite-granodiorite) weathering in an island arc environment formed the protolith of argillaceous gneiss of Sidaolazihe Formation. Basaltic andesite, formed under an island arc environment, was the protolith of garnet two-pyroxene granulite of Sidaolazihe Formation; while calc-alkaline basalt was the protolith of amphibolite from the same formation and two-pyroxene granulite from Laoniugou Formation. All these Archean supracrustal rocks exposed in the Laojinchang area have recorded nearly isobaric cooling (IBC) anticlockwise P-T paths with distinct prograde, peak and post-peak stages. The peak P-T conditions reached granulite facies, suggesting a heat source from underplated magmas. A tectonic model involving an island-arc setting is proposed for the evolution of these Neoarchean supracrustal rocks in Laojinchang area.

Magnesium isotope fractionation during magmatic differentiation in the lower continental crust

The Mg isotopic compositions display a significant disparity between the lower continental crust and the mantle. However, due to the limited availability of representative samples from the lower crust, our understanding remains incomplete regarding whether Mg isotope fractionation occurred during its formation and its potential influence on the observed isotopic differences. This study addresses this gap by presenting Mg isotopic data for juvenile lower crustal mafic xenoliths from Daoxian, Southeast China, providing new insights into this poorly understood process. The xenoliths, classified as websterites and two-pyroxene granulites, equilibrated at 818–912 °C and 10.6–15.4 kbar. They exhibit high Mg# (77–85), Cr (154–1451 ppm), and Ni (143–312 ppm), as well as trace element patterns resembling those of island arcs and evolved Sr–Nd–Pb isotopic compositions (87Sr/86Sri = 0.7046–0.7071; εNd(t) = −2.44 to +4.62; 206Pb/204Pbi = 18.26–19.03), indicating their derivation primarily from an enriched mantle source. Despite this, these xenoliths display a diverse range of Mg isotopic compositions. Websterite xenoliths exhibit mantle-like δ26Mg values (−0.27‰ to −0.29‰), indicating no influence from hybridized mantle source or pyroxene fractional crystallization/accumulation on their Mg isotopes. In contrast, granulite xenoliths show lower and variable δ26Mg values (−0.26‰ to −0.49‰), which positively correlate with Mg#, Cr, and V, but negatively correlate with Na2O + K2O, indicating that magmatic processes likely influenced Mg isotope fractionation in these samples. Petrological features, coupled with correlations between MgO and CaO/Al2O3, Cr, Co, and Ni, indicate that fractional crystallization and/or accumulation of olivine, spinel, clinopyroxene, orthopyroxene, and plagioclase played crucial roles in the formation of the granulite protoliths. Furthermore, the segregation of spinel during magmatic differentiation could have enriched residual melts in light Mg isotopes inherited by the cumulate Cpx + Opx + Pl (i.e., the granulite protoliths). Overall, the weighted average δ26Mg value of the lower crust in South China is estimated to be −0.34 ± 0.04‰. This finding, together with previous studies on granulite xenoliths in North China and Australia, strongly indicates heterogeneous Mg isotopic compositions in the lower continental crust relative to the mantle.

Metamorphism and P-T Evolution of High-Pressure Granulites from the Fuping Complex, North China Craton

Granulite facies rocks provide important keys to evaluating collisional metamorphism in orogenic belts. The mafic granulites of Baoding in the Fuping Complex of the North China Craton occur within the Trans-North China Orogen (TNCO), a major Paleoproterozoic collisional orogen. Here, we present results from detailed investigations on newly discovered garnet pyroxenite, garnet two-pyroxene granulite, and garnet-bearing-plagioclase amphibolite using petrographic, mineralogical, geochemical, and zircon U-Pb dating methods. Our results show that the Fuping Complex metamorphic evolution in this study evolved in four stages: prograde (M1), high-pressure granulite facies (M2), granulite facies (M3), and retrograde (M4) stages. The mineral assemblage of the prograde stage (M1) consists of Amp + Pl + Q within garnet cores. The mineral assemblage of high-pressure granulite facies at the peak stage (M2) consists of Gt + Cpx + Pl + Q ± Amp, forming the garnet pyroxenite. The granulite facies stage M3 is characterized by the occurrence of orthopyroxene, with a mineral assemblage of Gt + Cpx + Opx + Amp+ Pl + Q. The early retrograde stage M4-1 includes clinopyroxenes scattered inside amphiboles, following the breakdown of garnet and clinopyroxene. The mineral assemblage of this stage comprises Amp + Pl + Q + Ilm ± Cpx. Later, in the late retrograde stage M4-2, the composition of amphiboles changed to actinolite, and epidote and chlorite started to appear in the matrix. Traditional geothermobarometry yielded P-T conditions of 700~706 °C and 6.0~6.2 kbar for prograde stage M1, 854~920 °C and 13.0~13.8 kbar for high-pressure granulite facies stage M2, 912~939 °C and 8.1~9.9 kbar for M3, 661~784 °C and 3.1~4.4 kbar for M4-1, and 637~638 °C, 1.1~1.3 kbar for M4-2, along a clockwise P-T path with a nearly isothermal decompression (ITD) and slight heating. Zircon LA-ICP-MS U-Pb dating constrains the timing of the high-pressure granulite facies metamorphic event to be between 1.83 and 1.86 Ga. Geochemical features suggest that the protoliths of the high-pressure granulites may have formed in an island arc environment within a convergent margin setting. Together with results from previous studies, our data suggest that the ~1.85 Ga metamorphic age recorded in the Fuping Complex represents a regional metamorphism in the TNCO, associated with the subduction–collision and assembly of the Eastern and Western Blocks of the NCC.

Metamorphism and chronology of Paleoproterozoic mafic granulite from the Kuluketage area, northern Tarim and its tectonic applications

Mafic granulites provide key insights for understanding the composition and evolution of the lower crust but are rarely reported in northern Tarim. In the present study, a systematic petrographic, phase equilibrium modeling and zircon U-Pb geochronological study of a Paleoproterozoic mafic granulite (garnet two-pyroxene granulite) from the Kuluketage area, northern Tarim were conducted. Phase equilibrium modeling demonstrates a decompression–heating clockwise P-T path involving the prograde metamorphic stage (M1) at ∼ 950 °C / 12 kbar, the peak metamorphic stage (M2) at ∼ 1020 °C / 10 kbar, and the retrograde stage (M3) at ∼ 880 °C / 7 kbar. The decompression heating P-T path generally occurs in the post-collisional lithospheric extension setting which may contribute the UHT metamorphism of the M2 stage. Zircon U-Pb geochronological study reveals four age peaks at ∼ 2.45, ∼ 2.3, ∼ 1.91, and ∼ 1.85–1.78 Ga. We interpret the ∼ 2.45 Ga magmatic event as an older captured zircon age corresponding to a widely reported magmatic event of ∼ 2.4–2.5 Ga in Kuluketage. The slightly younger ∼ 2.3 Ga magmatic event may correspond to the crystallization age of the mafic granulite’s protolith, however, rarely reported in northern Tarim. The metamorphic age of ∼ 1.91 Ga represents a collisional-related high-pressure metamorphic age corresponding to the M1 stage and the age peaks of ∼ 1.85–1.78 Ga may represent post-peak cooling events corresponding to the M2-M3 stage.

Metamorphic evolution of mafic two-pyroxene granulites in the Dagele area, East Kunlun

Metamorphic evolution of mafic two-pyroxene granulites in the Dagele area, East Kunlun

The Multiple Metamorphism of Mafic Granulites From the East Hebei Terrane, North China Craton: Insights Into the Transition of Tectonic Regimes

The East Hebei terrane from the North China Craton preserves the dome–and–keel structures, which was transected by a later linear belt in the north margin. Mafic granulites from the linear belt and domes record two groups of metamorphic ages at Neoarchean and Paleoproterozoic, but their accurate metamorphic peak conditions and paths have not been well addressed. Three samples of mafic granulites, including two-pyroxene granulite (JD15120), garnet-bearing two-pyroxene granulite (YC8-43), and garnet clinopyroxene granulite (JD1546), were documented for detailed metamorphic studies. Two-episode metamorphism can be recognized. The first-episode recovered from JD15120 and YC8-43 is represented by peak assemblage of medium-grained clinopyroxene, orthopyroxene, amphibole, plagioclase, and ilmenite, which yields ultrahigh temperature (UHT) conditions of 940–960°C at 7.5–8.5 kbar and 950–990°C at 8 kbar, respectively, constrained by contours of the maximum anorthite (XAn) in plagioclase cores. The post-peak evolution is dominated by cooling with decompression, constrained mostly from the measured core-to-rim decreasing XAn in plagioclase. By contrast, the second-episode overprinting is recognized in all samples, but exhibits varying textures. In garnet-bearing samples (YC8-43 and JD1546), the overprinting assemblages are characterized by poikilitic garnet that occurs either as coronae around the first-episode pyroxenes, forming “red-eye socket” textures, or as grains in equilibrium with tiny-grained clinopyroxene, plagioclase, amphibole, rutile, and quartz, forming high-pressure (HP) granulite assemblages. These HP granulite assemblages show peak conditions of ∼12 kbar/860°C and ∼12.6 kbar/835°C, constrained by contours of the maximum grossular (XGrs) in garnet cores and the minimum XAn in plagioclase cores. The post-peak evolution is dominated by isothermal decompression, constrained from the outward decreasing XGrs in garnet and increasing XAn in plagioclase. LA-ICP-MS U-Pb zircon dating on JD15120 and JD1546 suggests two metamorphic ages of ∼2.49 Ga and ∼1.78 Ga, being considered to be correlated with the UHT and HP granulite metamorphism, respectively. Tectonically, the late Neoarchean UHT granulite metamorphism may correlate a vertical sagduction regime, whereas the late Paleoproterozoic HP granulite metamorphism is favored to register the continental collision in the northern margin of the North China Craton. This study may have indications for the Neoarchean–Paleoproterozoic tectonic transition of the craton.

Neoarchean–Paleoproterozoic HP-HT metamorphism in the Bhopalpatnam granulite belt, Bastar Craton (India): Insights from phase equilibria modelling and monazite geochronology

The Bhopalpatnam Granulite Belt (BGB), located within the southwestern part of the Bastar Craton (India), is composed of granite gneisses with numerous enclaves of high-grade rocks, including quartzofeldspathic gneisses, orthopyroxene-free mafic granulite, and garnetiferous two-pyroxene granulite. In addition, the marginal parts of the belt preserve patches of garnetiferous psammo-pelitic gneisses. New petrographic, metamorphic (conventional and phase equilibria modelling) and monazite geochronological investigations of these high-grade rocks constrain the conditions and timing of granulite-facies metamorphism and enable to demonstrate the tectonometamorphic history of the BGB. The garnet-bearing psammo-pelitic gneisses underwent medium pressure-high temperature (MP-HT) and retrograde metamorphism at P-T conditions of 9.4 ± 1.0 kbar/797 ± 39 °C and 5.1 ± 1.1 kbar/697 ± 69 °C, respectively. The quartzofeldspathic gneisses experienced high pressure-high temperature metamorphism (HP-HT; 12.0 ± 0.9 kbar/785 ± 48 °C), followed by retrogressive cooling at 10.6 ± 0.8 kbar/683 ± 42 °C. The orthopyroxene-free mafic granulites and two-pyroxene granulites witnessed HP-HT metamorphism at 11.0 ± 0.8 kbar/827 ± 45 °C, followed by MP-HT metamorphism at 8.4 ± 0.9 kbar/803 ± 66 °C and retrograde cooling at 5.7 ± 1.5 kbar/555 ± 80 °C. The U–Th–total Pb dating of monazites from the quartzofeldspathic gneiss yielded peak (HP-HT) metamorphic age at 2588–2461 Ma, MP-HT metamorphism at 2499–2308 Ma, and thermal events associated with granite magmatism at 2292–2111 Ma. Based on similar clockwise P–T history and Late Archean-Early Mesoproterozoic tectonothermal events, we suggest that the Bhopalpatnam and Karimnagar granulite belts evolved coherently until the Mesoproterozoic time and eventually rifted apart during the development of the Pranhita-Godavari rift basin between ∼ 1.6–1.4 Ga. The HP-HT and MP-HT granulite-facies metamorphic episodes identified in the BGB are correlated with the continent–continent collision between Bastar and eastern Dharwar cratons.

Acidic fluids in the Earth\u2019s lower crust

Fluid flux through Earth’s surface and its interior causes geochemical cycling of elements in the Earth. Quantification of such process needs accurate knowledge about the composition and properties of the fluids. Knowledge about the fluids in Earth’s interior is scarce due to limitations in both experimental methods and thermodynamic modeling in high/ultrahigh pressure–temperature conditions. In this study, we present halogen (Cl, F) measurements in apatite grains from the mafic (metagabbro), and felsic (two-pyroxene granulite, charnockite, hornblende-biotite gneiss) rocks preserved in the Nilgiri Block, southern India. Previous experiments show that it is difficult to incorporate Cl in apatite compared to F at high pressure and temperature conditions. Based on regional trends in Cl and F content in apatite (with highest Cl content 2.95 wt%), we suggest the presence of acidic C–O–H fluids in the lower crust (~20–40 km deep) during the high-grade metamorphism of these rocks. These fluids are capable of causing extreme chemical alterations of minerals, especially refractory ones. They also have significant potential for mass transfer, causing extensive geochemical variations on a regional scale and altering the chemical and isotope records of rocks formed in the early Earth. Our findings have important relevance in understanding speciation triggered by acidic fluids in the lower crust, as well as the role of fluids in deep Earth processes.

Exsolution intergrowth of cpx-opx and pseudosection modelling of two-pyroxene mafic granulite from Daltonganj of Chhotanagpur Granite Gneiss Complex, Eastern India

In this study, we reconstruct for the first time the evolution of a two-pyroxene mafic granulite from the Daltonganj of Chhotanagpur Granite Gneiss Complex (CGGC). Transmission electron microscopy (TEM) revealed that some of the clinopyroxene have exsolution texture, where orthopyroxene occurs as thin lamellae within the porphyroblast of clinopyroxene. To tightly constrain the P-T conditions at which exsolution lamellae developed after the metamorphic peak, we have applied both conventional and multi-equilibrium thermobarometry, as well as forward thermodynamic modelling. Results from multi-equilibrium thermobarometry, using the software THERMOCALC, suggest peak conditions of the mafic granulite at average pressure-temperature (PTav) conditions of 6.7 ± 1.19 kbar/814 ± 60 °C. In contrast, exsolution bearing opx-cpx minerals crystallised at relatively lower temperature (772 ± 14 °C), determined by the conventional geothermometers. The peak to retrograde evolution of these mafic granulites is constrained through phase equilibrium modelling in the NCKFMASHTO (Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O) model system using the software Perple_X. Phase equilibria results of peak conditions (i.e. 6.0–6.78 kbar and 775–808 °C) are consistent with respect to those obtained through multi-equilibrium and conventional thermobarometry, while the retrograde path is defined down to ~4.5 kbar and ~540 °C. Our results have twofold implications: (i) they show how the integrating of different geothermobarometric methods is the best proxy to tightly constrain the evolution of high-grade metamorphic rocks, and (ii) they pavement to new constraints on the Paleoproterozoic to Neoproterozoic evolution of the CGGC.

A new occurrence of two-pyroxene granulites at Chicholi from Betul supracrustal belt in Central Indian Tectonic Zone (CITZ), MP, India

We report the new occurrence of two-pyroxene granulites from Chicholi, the Betul Group of the Central Indian Tectonic Zone (CITZ). The common mineral assemblage observed within different thin sections is orthopyroxene–clinopyroxene–hornblende–plagioclase–biotite–quartz. The textural relationship of these mineral phases shows the reaction: hornblende + quartz = orthopyroxene + clinopyroxene + plagioclase. The estimated P–T condition of metamorphism of the two-pyroxene granulites is 901 ± 30°C and 8.68 ± 1.4 kbar.

Silicate, Oxide and Sulphide Trends in Neo-Archean Rocks from the Nilgiri Block, Southern India: the Role of Fluids During High-grade Metamorphism

Abstract The Nilgiri Block, southern India represents an exhumed section of lower, late Archean (2500 Ma) crust. The northern highlands of the Nilgiri Block are characterized by metagabbros with pyroxenite inlayers. A two-pyroxene granulite zone acts as a transition between the metagabbros and charnockites, which are exposed in the central and southern part of the Nilgiri highlands. Thermobarometry results indicate a SW–NE regional trend both in temperature (∼650–800°C) and in pressure (700–1100 MPa) over the Nilgiri highlands. In the charnockites, composite rutile–ilmenite grains are the dominant oxide assemblage. In the two-pyroxene granulites, hemo-ilmenite–magnetite is dominant with coexisting rutile–ilmenite composite grains in a few samples in the vicinity of the boundary with the charnockites. In the metagabbros, hemo-ilmenite–magnetite is the dominant oxide assemblage. The principal sulphide mineral in the charnockite is pyrrhotite with minor pyrite–chalcopyrite exsolution lamellae or blebs. In the two-pyroxene granulites and the metagabbros, the principal sulphide assemblage consists of discrete pyrite grains with magnetite rims and pyrite–pyrrhotite–chalcocopyrite associations. From these observations, a specific oxidation trend is seen. The northern granulite-facies metagabbros and two-pyroxene granulites of the Nilgiri highlands are highly oxidized compared with the charnockites from the central and southern regions. This higher oxidation state is proposed to be the result of highly oxidizing agents (probably as SO3) in low H2O activity grain boundary NaCl saline fluids with a dissolved CaSO4 component present during granulite-facies metamorphism of the metagabbros and two-pyroxene granulites. Eventually these agents became more reducing, owing to the inherent buffering of the original tonalite–granodiorite granitoids at the graphite–CO2 buffer, such that S took the form of H2S during the granulite-facies metamorphism of the charnockites. At the same time, these saline fluids were also responsible the solid-state conversion of biotite and amphibole to orthopyroxene and clinopyroxene in the metagabbro, two-pyroxene granulite, and charnockite.

Late Paleoproterozoic granulite-facies metamorphism in the North Altyn Tagh area, southeastern Tarim craton: Pressure-temperature paths, zircon U-Pb ages, and tectonic implications

Abstract Granulite occupies the root of orogenic belts, and understanding its formation and evolution may provide critical information on orogenic processes. Previous studies have mainly focused on garnet-bearing high-pressure and medium-pressure granulites, whereas the metamorphic evolution and pressure-temperature (P-T) paths of garnet-absent, low-pressure granulites are more difficult to constrain. Here, we present zircon U-Pb ages and mineral chemistry for a suite of newly discovered two-pyroxene granulites in the North Altyn Tagh area, southeastern Tarim craton, northwestern China. Conventional geothermobarometry and phase equilibrium modeling revealed that these rocks experienced a peak granulite-facies metamorphism at T = 790–890 °C and P = 8–11 kbar. The mineral compositions and retrograde symplectites record a clockwise cooling and exhumation path, possibly involving near-isothermal decompression followed by near-isobaric cooling. Zircon U-Pb dating yielded a ca. 1.97 Ga metamorphic age, which likely represents the initial cooling age, based on Ti-in-zircon thermometry. Combined with regional geological records, we interpret that these granulites originated from the basement rocks of a late Paleoproterozoic magmatic arc that was subsequently involved in a collisional orogen in the southern Tarim craton, presumably related to the assembly of the Columbia/Nuna supercontinent. The clockwise P-T paths of the granulites record crustal thickening and burial followed by crustal thinning and exhumation in the upper plate of the collisional orogen. Our data indicate that the initial exhumation of this orogen probably occurred no later than ca. 1.97 Ga, which is supported by widespread 1.93–1.85 Ga postorogenic magmatism in this area.

Metamorphic evolution of Archean ultrahigh-temperature mafic granulites from the western margin of Qian’an gneiss dome, eastern Hebei Province, North China Craton: Insights into the Archean tectonic regime

The supracrustal rocks of granulite facies occurs as enclaves in TTG gneisses in eastern Hebei terrane, North China Craton. Their peak P–T conditions, paths and tectonic regime remain controversial. Three samples of mafic two-pyroxene granulites from the western margin of Qian’an dome were chosen for detailed studies of petrography and P–T evolution from REE- and major element-based thermobarometers and pseudosection modelling. Their peak temperatures were constrained to be 1000–1100 °C using REE–in–clinopyroxene–orthopyroxene and REE–in–plagioclase–clinopyroxene thermometers. The major element-based clinopyroxene–orthopyroxene and amphibole–plagioclase thermobarometers yield cooling conditions of 740–830 °C/7–8 kbar. Pseudosection modelling suggests that the mafic granulites experienced a cooling evolution with decompression from UHT peak conditions at >1000 °C and 10–11 kbar in garnet-stability fields to normal granulite conditions at 820–900 °C and 7–8 kbar constrained from the stabilities of final solidus assemblages, which are well consistent with the results of REE- and major element-based thermobarometers. Both the P–T conditions and the cooling paths in P–T pseudosections are constrained by (i) the measured plagioclase compositions, especially the spectacular zoning patterns with increasing XAn [=Ca/(Ca + Na + K)] (0.46 → 0.52) from core to mantle followed by decreasing XAn (0.52 → 0.46) outwards in the rims; (ii) the measured zoning with core–rim increasing CaM2 [Ca in the M2 site] in clinopyroxene; and (iii) measured Ti in amphibole or biotite that yields temperature constraints for the final assemblages. The P–T evolution for the pre-peak stages is inferred to be a compression process based on the presence of a rounded magnetite–ilmenite grain included in clinopyroxene. Thus, an anticlockwise P–T path is inferred. Zircon U–Pb age dating yields a metamorphic age of ∼2.55 Ga, defining the protoliths of the mafic granulites were formed between 2.57 and 2.55 Ga. A summary of available age data reveals that the TTG magma activity, the formation of supracrustal rocks and their metamorphism took place simultaneously in a time span between 2.48 and 2.56 Ga. Therefore, a sagduction regime is favored to interpret the metamorphic evolution of supracrustal granulites in the east Hebei terrane.

High-pressure granulite from Jixian, Eastern Hebei, the North China Craton: implications for Neoarchean to early Paleoproterozoic collision tectonics

Abstract Precambrian high-pressure (HP) granulites provide important information for reconstructing ancient continental nuclei. Here we report granulites in Eastern Hebei, North China Craton (NCC). They experienced three metamorphic events related to Neoarchean–early Paleoproterozoic orogenesis. The garnet–clinopyroxene granulite defines the M 1 event at P–T conditions of 11–13 kbar and 780–830°C, while the two-pyroxene granulite was produced during the M 2 event at 7–9 kbar and 850–950°C. Both the garnet–clinopyroxene and two-pyroxene granulites experienced amphibolite retrogression during the M 3 event at 5–7 kbar and 710–730°C. Geochemical compositions of these granulites exhibit affinity to island-arc andesites. Zircon U–Pb dating shows that their magmatic precursors were erupted at c. 2538 Ma, and have experienced two-stage growth of zircon rims at c. 2458 and c. 2285 Ma, respectively. The c. 2458 Ma age may represent orogenic events during the amalgamation of micro-continental blocks of the eastern NCC, and the c. 2285 Ma age may be interpreted as the effect of late Paleoproterozoic magmatism. We suggest that the Neoarchean andesitic protoliths of the granulites were metamorphosed at HP granulite-facies conditions during collision of micro-continental blocks, and then exhumed to shallow levels. These early Paleoproterozoic HP granulites recorded the amalgamation of micro-continental blocks to reach the cratonization of the eastern NCC.

P–T–t evolution of the high-pressure mafic granulites from northern Hengshan, North China Craton: Insights from phase equilibria and geochronology

The Hengshan complex has drawn attention for the occurrence of typical high-pressure mafic granulites, representing the Paleoproterozoic collisional orogeny in the North China Craton. Mafic granulites occur as boudins of various scales in tonalite–trondhjemite–granodiorite (TTG) gneisses. They are composed of garnet, clinopyroxene, plagioclase, orthopyroxene, hornblende, biotite, quartz, rutile and ilmenite. Garnet porphyroblasts are usually surrounded by plagioclase coronas, and early jadeite-rich clinopyroxene is totally decomposed to the symplectite of clinopyroxene and plagioclase. Two high-pressure granulites and one two-pyroxene granulite were studied by the pseudosection approach, and clockwise P–T paths with four metamorphic stages were recognized. The pre-peak prograde stage was identified from garnet core to mantle compositions, with increasing P–T from 630 to 710 °C at 11–13 kbar. The peak pressure stage hasn’t been well preserved but its mineral assemblages inferred from petrography to be garnet + jadeite-rich clinopyroxene + quartz + rutile ± plagioclase, with P–T condition of 760–820 °C at ∼15 kbar. The post-peak decompression with slight heating is characterized by symplectite, plagioclase coronas, and the formation of orthopyroxene. Locally, two-pyroxene granulite assemblages develop, defining P–T condition of 6–8 kbar and 840–860 °C for the temperature peak (Tmax). The post-Tmax cooling is represented by the occurrence of amphibolite facies assemblages especially in boudin margins with P–T condition of 6–8 kbar and 720–760 °C, suggesting an isobaric cooling process with fluid infiltration. U–Pb dating of metamorphic zircons record ages mainly ranging from 1897 to 1830 Ma with weighted mean ages of 1862 ± 11 Ma for the high-pressure granulite and 1837 ± 7 Ma for the two-pyroxene granulite. These metamorphic ages were interpreted to record the post-peak cooling stage especially the time close to solidus, which is also supported by the result of Ti-in-zircon thermometer. A summary of P–T–t evolution for metamorphic terrains corresponding to different crust levels in Hengshan–Wutai area indicates that the main episode of crust thickening-dominated collisional orogeny is ∼1.95 Ga or earlier and finished at ∼1.92 Ga. This was followed by post-orogenic cooling-uplifting or the overprinting of another thermal-tectonic event during the period of 1.92–1.80 Ga.

Detrital zircon and igneous protolith ages of high-grade metamorphic rocks in the Highland and Wanni Complexes, Sri Lanka: Their geochronological correlation with southern India and East Antarctica

The high-grade metamorphic rocks of Sri Lanka place valuable constraints on the assembly of central parts of the Gondwana supercontinent. They are subdivided into the Wanni Complex (WC), Highland Complex (HC) and Vijayan Complex (VC), but their correlation with neighbouring Gondwana terranes is hindered by a poor understanding of the contact between the HC and WC. Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) U–Pb dating of remnant zircon cores from 45 high-grade metamorphic rocks in Sri Lanka reveals two domains with different age characteristics that correlate with the HC and WC and which help constrain the location of the boundary between them. The HC is dominated by detrital zircon ages of ca. 3500–1500 Ma from garnet–biotite gneiss, garnet–cordierite–biotite gneiss, some samples of garnet–orthopyroxene–biotite gneiss and siliceous gneiss (interpreted as paragneisses) and igneous protolith ages of ca. 2000–1800 Ma from garnet–hornblende–biotite gneiss, other samples of garnet–orthopyroxene–biotite gneiss, garnet–two-pyroxene granulite, two-pyroxene granulite and charnockite (interpreted as orthogneisses). In contrast, the WC is dominated by detrital zircon ages of ca. 1100–700 Ma from paragneisses and igneous protolith ages of ca. 1100–800 Ma from orthogneisses. This clearly suggests the HC and WC have different origins, but some of our results and previous data indicate their spatial distribution does not correspond exactly to the unit boundary proposed in earlier studies using Nd model ages. Detrital zircon and igneous protolith ages in the HC suggest that sedimentary protoliths were eroded from local 2000–1800 Ma igneous rocks and an older Paleoproterozoic to Archean craton. In contrast, the WC sedimentary protoliths were mainly eroded from local late Mesoproterozoic to Neoproterozoic igneous rocks with very minor components from an older 2500–1500 Ma craton, and in the case of the WC precursor sediments there was possibly additional detritus derived from early to middle Neoproterozoic metamorphic rocks. The relic zircon core ages in the HC are comparable with those of the Trivandrum Block and Nagercoil Block of southern India. In contrast, those ages in the WC match the Achankovil Shear Zone and Southern Madurai Block of southern India. These comparisons are also supported by Th/U ratios of detrital zircon cores from paragneisses (Th/U ratios of >0.10 for the former and not only >0.10 but also ≤0.10 for the latter). Comparisons with the Lützow-Holm Complex of East Antarctica indicate that the geochronological characteristics of the HC and WC broadly match those of the Skallen Group, and the Ongul and Okuiwa Groups, respectively.

Triassic rejuvenation of unexposed Archean-Paleoproterozoic deep crust beneath the western Cathaysia block, South China

Jurassic (ca. 150 Ma) Daoxian basalts from the western Cathaysia block (South China) entrained a suite of deep-seated crustal xenoliths, including felsic schist, gneiss and granulite, and mafic two-pyroxene granulite and metagabbro. Zircon U–Pb–Hf isotopic, whole-rock elemental and Sr–Nd–Pb isotopic compositions have been determined for these valuable xenoliths to reveal the poorly-known, unexposed deep crust beneath South China. Detrital zircons from the garnet-biotite schists show several populations of ages at 0.65–0.5 Ga, 1.1–0.75 Ga, 1.6–1.4 Ga, 1.8–1.7 Ga, 2.5–2.4 Ga, ~2.8 Ga, and ~3.5 Ga, representing a multi-sourced, meta-sedimentary origin with deposition time at the early Cambrian. One mafic granulite contains zircons with concordant U–Pb ages of Neoarchean (~2520 Ma), as well as Hf model ages of 2.8–2.6 Ga and positive εHf(t) values (up to 6.3), suggesting an accretion of juvenile crust in Neoarchean, probably as the main framework of the lower crust. Geochemical and geochronological evidence shows the mafic granulite and metagabbro were produced by underplating of magmas derived from the depleted asthenosphere and mixed with EM2-type materials during the Late Triassic (205–196 Ma). This magmatic underplating also resulted in the widespread metamorphism of the mafic lower crust and felsic middle crust (e.g., the felsic granulite and gneiss) at 202–201 Ma. We suggest the existence of a highly evolved Archean-Paleoproterozoic basement beneath the western Cathaysia block, which experienced episodic accretion and reworking and the strong rejuvenation during the Triassic. A three-layered structure of the lower crust could exist beneath the Daoxian area during the Jurassic time: its upper layer is an evolved Archean-Paleoproterozoic basement; the middle hybrid layer represents a mixture of Archean-Paleoproterozoic basement with newly accreted/reworked Proterozoic to Phanerozoic materials; and the deeper layer consists of mafic granulites derived from the intensive magmatic underplating during the Triassic.

Two phases of granulite facies metamorphism during the Neoarchean and Paleoproterozoic in the East Hebei, North China Craton: Records from mafic granulites

Archean cratons are commonly reworked during the late Paleoproterozoic orogeny, which may result in multi-phase granulite facies metamorphism in many domains. The East Hebei terrane of the North China Craton is developed with two phases of granulite facies metamorphism in the Neoarchean and Paleoproterozoic. The Neoarchean granulite facies metamorphism is recognized mainly in the rafts or slivers of supracrustal rocks from the Taipingzhai ovate-structural domain, characteristic of anticlockwise P-T paths. The mafic granulites at this domain contain two-pyroxene granulite assemblages which are commonly overprinted by a late phase of granulite facies marked by tiny-grained vermicular orthopyroxene-bearing assemblages. They only record the Neoarchean metamorphic ages at 2.53–2.52Ga and 2.51–2.48Ga as indicated by zircon U-Pb data, para-simultaneously with the emplacement of TTG rocks. In contrast, the Paleoproterozoic granulite facies metamorphism can be well recognized in the Saheqiao linear-structural belt, which is characterized by high-pressure granulite assemblages in mafic rocks with clockwise P-T paths. In this belt, the Archean two-pyroxene granulite assemblages are locally preserved, showing “red-eye socket” overprinting textures consisting of garnet coronas or fine-grained garnet and clinopyroxene bearing assemblages surrounding the early-stage pyroxenes. Zircon U-Pb data indicate that mafic granulites from the Saheqiao belt record not only the Neoarchean metamorphic ages of ∼2.50Ga but also some Paleoproterozoic ages of 1.97–1.83Ga. The first application of Lu-Hf garnet-whole rock isochron method to garnet-bearing mafic granulites from the Saheqiao belt yields ages of 1766.6±7.0 and 1787±22Ma, confirming that the overprinting of garnet-bearing assemblages occurred in the late Paleoproterozoic. Thus, the Saheqiao linear-structural belt may represent a strongly reworked domain during the Paleoproterozoic orogeny, rather than an Archean high-grade greenstone belt as previously considered.

Early Paleozoic granulite-facies metamorphism and anatexis in the northern West Qinling orogen: Monazite and zircon U-Pb geochronological constraints

In the northern West Qinling orogen (WQO), granulite-facies metamorphic rocks are recognized within the Qinling Complex. These rocks are composed of amphibole-bearing two-pyroxene granulite and garnet-sillimanite gneiss with widespread migmatitization. We investigate three granulite-facies samples and one leucosome sample from the Qinling Complex, which are suitable for U-Pb analyses of zircon and monazite. SHRIMP and LA-ICPMS U-Pb age dating of zircon and monazite from two pelitic granulites provides weighted mean ages of 430±4 Ma (MSWD=0.88) and 433±4 Ma (MSWD=0.27), respectively. Based on the petrographic characteristics and zircon CL imagery, we postulated a ca. 430 Ma metamorphic timing for the pelitic granulites. LA-ICPMS zircon U-Pb data from an amphibole two-pyroxene granulite sample reports two weighted mean age groups: 424±3 Ma (MSWD=0.45) and 402±3 Ma (MSWD=1.4), which were interpreted as granulite-facies metamorphic and retrograde ages, respectively. LA-ICPMS U-Pb dating of zircons from the leucosome sample yields a weighted mean age of 426±2 Ma (MSWD=0.3), which is interpreted as the crystallization age of the leucosome. These data indicate that the West QOB experienced early Paleozoic granulite-facies metamorphism and anatexis similar to the East QOB. However, it remains unclear whether the early Paleozoic granulite facies metamorphism resulted from an arc setting created by the northward subduction of the Shangdan ocean or from a continental collisional orogenic event.

Ultrahigh temperature (UHT) mafic granulites in the East Hebei, North China Craton: Constraints from a comparison between temperatures derived from REE-based thermometers and major element-based thermometers

The extrapolation of the newly developed REE-based thermometers of Liang et al. (2013) and Sun and Liang (2015) to the Neoarchean mafic granulites in the East Hebei of North China Craton shows that the REE-based thermometers yield temperatures that are 150–350°C for clinopyroxene–orthopyroxene pair and 100–150°C for garnet–clinopyroxene pair higher than those from the major element-based Fe-Mg thermometers, implying that the REE-based thermometers are more reliable to recover the peak metamorphic temperature of granulites. Detailed petrographic investigations suggest that three types of mafic granulites from east to west in the East Hebei can be recognized, as two-pyroxene granulites in the Taipingzhai zone, garnet two-pyroxene granulites in the Saheqiao zone and garnet clinopyroxene granulites in the Malanyu zone. P-T estimation using major element- and REE-based thermobarometers yield UHT conditions of 1025–1060°C/0.9GPa and 950–1070°C/1.0GPa for the clinopyroxene-orthopyroxene pair in granulites from the Taipingzhai and Saheqiao zones, and high-pressure granulite conditions of 800–875°C/1.0–1.4GPa for the garnet-clinopyroxene pair in granulites from the Malanyu zone. All the granulites show the P-T paths characterized by cooling from their peak conditions to late amphibolite-facies of 720–780°C. The peak P-T conditions of the two-pyroxene and garnet-bearing granulites indicate a paired type of metamorphism with contrasting geothermal gradients of ~40°C/km and ~20°C/km, being particular in the Neoarchean high-grade terranes.