Ultrahigh-pressure Metamorphic Belt Research Articles

Basal erosion during the initiation of continental deep subduction in the North Qaidam ultrahigh-pressure metamorphic belt (NW China): Constraints from geochemistry and geochronology on eclogites and gneisses in the Chachahe unit

Subduction erosion is thought to be a common process in active continental margins that removes upper-plate material and transfers it to the subduction channel. The North Qaidam ultrahigh-pressure metamorphic belt of NW China was formed by subduction of the Qaidam Block beneath the Quanji Block in the early Paleozoic. In this study, we found gneisses and eclogites in the Chachahe unit of the North Qaidam ultrahigh-pressure metamorphic belt that recorded 2.39−2.28 Ga magmatism and 1.93−1.87 Ga amphibolite-facies metamorphism prior to the early Paleozoic (452−439 Ma) eclogite-facies metamorphism. The Paleoproterozoic tectono-thermal history recorded by these gneisses and eclogites is distinct from that of the Qaidam Block but similar to that of the Quanji Block. The rock assemblages, field occurrences, geochemical characteristics, and zircon Lu-Hf isotopic compositions of these rocks closely resemble those of gneisses and enclosed mafic enclaves in the Delingha Complex in the basement of the Quanji Block and the mafic dikes intruded within it. This evidence clearly illustrates that the protoliths of gneisses and eclogites in the Chachahe unit were from the basement of the upper Quanji Block rather than the subducted Qaidam Block. Further considering the spatial location of the Chachahe unit, as well as similarities in early Paleozoic metamorphic ages, peak metamorphic conditions, and clockwise P-T paths between rocks in the Chachahe unit and those that originated from the Qaidam Block, we propose that the bottom basement of the Quanji Block was scraped off by basal erosion during the initiation of continental subduction, transported to mantle depth, and then exhumed with other slices from the subducted slab.

Titanite geochemistry traces extreme differentiation of granitic magma in the collisional orogen

The genesis of granitic magma is important for the growth and evolution of continental crust. However, whether and how granitic magma can undergo efficient crystal fractionation from partial melting at the source to magma emplacement in the shallow crust level are still controversial. To address this problem, a combined study of whole-rock and titanite geochemistry was carried out for syn-exhumation granites from the Sulu ultrahigh-pressure metamorphic belt in eastern China. These high-Si granites were produced by partial melting of the deeply subducted continental crust. Significant geochemical differentiation has been revealed in these granites. Titanite in the granites can be divided into anatectic and magmatic origins, according to the Fe/Al ratios and trace element compositions. Both types of titanite yield similar UPb ages ranging from 209 ± 23 Ma to 233 ± 26 Ma, corresponding to the late collisional stage of the Sulu orogen. These titanite domains generally have consistent εNd(t) values, suggesting that they formed in a nearly closed system without significant magma mixing of different sources. The highly variable Nb/Ta ratios of titanite show good correlations with FeO, Al2O3, Ta, Sr, LREE and (Lu/Gd)N. This interesting feature is ascribed to the extensive crystal fractionation of titanite, plagioclase and allanite/epidote during magma evolution. The results suggest that the highly silicic granitic melts can experience extensive crystal separation from the liquid during granitic magma differentiation. This study shows that accessory minerals such as titanite play a key role in constraining the geochemical differentiation of granitic melts. Furthermore, titanite geochemistry has the potential to decode the complex processes from partial melting at the source to magma differentiation at shallow crust level in collisional orogens.

Trace Elements of Multi‐stage Minerals and Titanite U‐Pb Dating for the Gneisses from Liansan Island, Sulu UHPM Belt

AbstractGneisses with anatectic characteristics from the Liansan island in the Sulu UHPM (ultra‐high pressure metamorphic) belt were studied for petrography, titanite U‐Pb dating and mineral geochemistry. Three origins of garnets are distinguished: metamorphic garnet, peritectic garnet and anatectic garnet, which are formed in the stages of peak metamorphism, retrograde anatexis and melt crystallization, respectively. The euhedral titanite has a high content of REE and high Th/U ratios, which is interpreted as indicating that it was newly‐formed from an anatectic melt. The LA‐ICP‐MS titanite U‐Pb dating yields 214–217 Ma ages for the titanite (melt) crystallization. The distribution of trace elements varies in response to the different host minerals at different stages. At the peak metamorphic stage, Y and HREE are mainly hosted by garnet, Ba and Rb by phengite, Sr, Nb, Ta, Pb, Th, U and LREE by allanite and Y, U and HREE by zircon. During partial melting, Y, Pb, Th, U and REE are released into the melt, which causes a dramatic decline of these element contents in the retrograde minerals. Finally, titanite absorbs most of the Nb, U, LREE and HREE from the melt. Therefore, the different stages of metamorphism have different mineral assemblages, which host different trace elements.

The fate of kyanite in ultrahigh-pressure metasedimentary rocks, southwestern Tianshan, NW China

Kyanite, an aluminous phase common in high- to medium-pressure metapelitic rocks, is critical in revealing complex tectonometamorphic histories of orogenic belts. So far it has not been documented in the well-established low-temperature – ultrahigh-pressure (LT-UHP) metamorphic belt of Chinese southwestern Tianshan. By detailed petrographic investigations, kyanite is recognized as minute inclusions or porphyroblasts in two different types of metasedimentary rocks. In omphacite-garnet phengite schists of the Habutengsu area, kyanite only forms needles associated with acicular phengite and lawsonite (pseudomorphed by paragonite and clinozoisite), and coesite (or its pseudomorph) in the extremely low-Ca core domain of well-zoned garnet. The acicular inclusion assemblage in garnet cores probably witnessed large overstepping of peak-stage reactions in metapelitic rocks in a cold subduction zone. The absence of matrix kyanite in this rock type may be ascribed to elevated Na and Ca activities during the growth of high-Ca garnet rim. This is supported by the common presence of sodic and calcic phases (e.g., paragonite and omphacite) in the matrix or the garnet rim domain, with absence in garnet core. In a suite of calcschists of the Muzhaerte area, kyanite occurs as a matrix and/or inclusion phase. In relatively fresh, less foliated chloritoid-ankerite calcschists, kyanite is a major matrix phase with coesite inclusions and locally surrounded by prophyroblasts or fine-grained aggregates of both paragonite and phengite. In altered chlorite-white mica calcschists, kyanite is rarely embedded in white mica aggregates but commonly enclosed in Fe-rich chloritoid porphyroblasts. The mantling of ankerite by acicular to prismatic Fe-rich chloritoid suggests locally increased Fe2+ activity and Al transport by fluids along grain boundaries. The common white mica corona around kyanite in the calcschists suggests elevated Na and/or K activities postdating kyanite formation. Outcrop-scale variations of kyanite textures and whole-rock alkali element contents support the infiltration of Na- and/or K-bearing fluids, probably a dominant mechanism responsible for the lack of kyanite in the Tianshan UHP metasedimentary rocks. P-T estimates suggest the kyanite corrosion took place at 15–21 kbar and 530–610 °C, corresponding to the early exhumation stage. This study suggests that kyanite may be a major peak phase of the Tianshan UHP metasedimentary rocks, and the complete understanding of its prograde evolution is a prerequisite to unravel the fate of subducted sediments and their chemical geodynamics in cold subduction zones.

Various fluids and complex geochemical processes in the subduction channel: Constraints from the ultrahigh pressure metamorphic belt of Southwestern Tianshan, China

Various fluids and complex geochemical processes in the subduction channel: Constraints from the ultrahigh pressure metamorphic belt of Southwestern Tianshan, China

Abiotic Methane Reservoirs in the Western Tianshan HP–UHP Metamorphic Belt, China

AbstractNatural gas, consisting primarily of methane (CH4), has become a major source of clean energy in modern society in many parts of the globe. Recent experimental observations and discoveries of deep‐sourced abiotic CH4 in cold subduction zones indicate the important ability of cold subducted slabs to generate natural gas reservoirs. However, most CH4 flux and reservoirs remain unknown and their potential is overlooked in global carbon flux estimations. Massive abiotic CH4‐rich fluid inclusions (FIs) in garnet and omphacite from ultrahigh‐pressure (UHP) eclogites have been found in the Western Tianshan (WT) UHP metamorphic belt, which provides one ideal case for quantification of abiotic CH4 stored in the cold subducted crust. By two methods, we assess the abiotic CH4 content stored in the Chinese WT HP–UHP metamorphic belt. Our calculations show that at least 113 Mt CH4 is stored in the WT eclogites. We also discuss the implications for CH4 reservoirs in subduction zones worldwide and speculate that the cold subduction zones may represent one of the largest, yet overlooked, sources of abiotic CH4 on Earth, which should not be ignored in the global natural resource and carbon flux estimations.

The transition from oceanic to continental subduction and collision: A case study of the North Qaidam ultrahigh-pressure metamorphic belt, northern Tibetan Plateau

The transition from oceanic to continental subduction and collision: A case study of the North Qaidam ultrahigh-pressure metamorphic belt, northern Tibetan Plateau

Geochemistry and zircon U-Pb-Hf-O isotopic study of granite and mafic xenoliths in the Danfeng area: Recording two stages of tectonic uplift events in the North Qinling ultrahigh-pressure metamorphic belt

北秦岭造山带中广泛出露的早古生代花岗岩及其中的基性捕虏体，对于讨论该造山带早古生代地壳及构造演化过程具有重要研究意义。本文报道了丹凤地区枣园二长花岗岩及其中的斜长辉石岩捕虏体与黄柏岔岩体中的斜长角闪岩捕虏体的全岩地球化学和锆石U-Pb-Hf-O同位素组成。斜长角闪岩和斜长辉石岩锆石U-Pb定年获得了493±8Ma原岩年龄和468±4Ma~460±5Ma的角闪岩相变质年龄。锆石U-Pb定年表明枣园花岗岩侵位于晚志留世（419±4Ma）。斜长角闪岩和斜长辉石岩SiO₂含量介于49.69%~50.73%之间，呈现镁质、准铝质和钙碱性特征；富集大离子亲石元素（LILEs）和轻稀土元素（LREE），亏损高场强元素（HFSEs）；并且大部分锆石具有较高的<i>ε</i>_Hf（<i>t</i>）值（-0.3~7.5）和锆石<i>δ</i>¹⁸O值（5.61‰~8.20‰）。这些特征指示其原岩可能源于俯冲板片交代的地幔楔部分熔融。枣园花岗岩SiO₂含量为70.11%，呈现高硅碱，贫铁镁，弱过铝，富集LILEs和LREE以及亏损HFSEs的特点；与I型花岗岩的元素地球化学特征一致；花岗岩具有较高的<i>ε</i>_Hf（<i>t</i>）值（0.1~3.9）和<i>δ</i>¹⁸O值（8.72‰~9.66‰）。这些特征指示花岗岩是由蚀变的年轻地壳经黑云母脱水熔融形成。结合北秦岭高压-超高压（HP-UHP）岩石、早古生代花岗岩和基性岩研究现状，表明该区在早古生代经历两期构造抬升事件并伴随退变质和深熔作用，分别发生在中-晚奥陶世和晚志留世。

Protolith origin of eclogites from the North Qaidam UHP metamorphic Belt, NW China: Implications for the breakup of the Rodinia supercontinent

The North Qaidam ultrahigh-pressure metamorphic belt (NQUB) is distinguished by the widespread distribution of eclogites. However, the protolith nature of these eclogites remains controversial. This study reports new zircon LA-ICP-MS U–Pb ages and whole-rock geochemistry as well as Sr–Nd isotopic data on the Yuka and Dulan eclogites in the NQUB. Zircon U–Pb dating results of these eclogites indicate that the protolith and metamorphic ages are 831–809 Ma and 439–433 Ma. Based on distinct geochemical features, the samples can be classified into high-Ti/Y type (Group 1) and low-Ti/Y type (Group 2 and Group 3). The Group 1 eclogite protoliths are geochemically similar to oceanic island basalts (OIB) and likely originated from garnet-spinel peridotite mantle sources. The protoliths of Group 2 eclogites have a similar mantle source to those of the Group 1 protoliths, but experienced a higher degree of fractional crystallization and crustal contamination. The Group 3 eclogites have N-MORB-like affinity, and may be derived from shallower mantle sources in the spinel stability field. The differences in mantle sources of enriched (Group 1 and Group 2) and depleted (Group 3) eclogite protoliths can be ascribed to varying extents of mixing between enriched plume and depleted MORB-type mantle components at various melting depths. Our results suggest that the Neoproterozoic protoliths of eclogites in the NQUB belonged to a part of widespread Neoproterozoic rift-related mafic magmatism in the Altun-Qilian-Kunlun Orogenic Belt. These rocks could be the products of mantle plume activities that caused the breakup of Rodinia. Most Neoproterozoic mafic rocks in the NQUB experienced early Paleozoic HP/UHP metamorphism.

Ultrahigh-pressure metamorphism and P-T-t path of the Yangkou eclogites and granitic gneisses from the central Sulu orogenic belt, eastern China, based on phase equilibria modelling

ABSTRACT Coesite-bearing eclogites and garnet-bearing granitic gneisses from the Yangkou area in the middle portion of Sulu ultrahigh pressure (UHP) metamorphic belt records three stages of metamorphism: I) Prograde to peak stage, II) Early retrograde stage, and III) Late retrograde stage. The prograde stage is recorded in the core to mantle in garnet and Si (3.38–3.43) in phengite in eclogite that has the assemblage of Grt-Omp-Amp-Phg-Lws (+ Qtz + Rt + H2O), and the P-T conditions are constrained at 25–27 kbar and 585–595°C. Stage II is constrained by Xgr (0.48–0.55) in garnet from mantle to rim, XAn (0.009–0.011) from mantle to rim in plagioclase of granitic gneiss, and the P–T conditions are estimated to be 29–35 kbar and 865–990°C with the mineral assemblage of Grt-Bt-Pl-Phg-Liq (+ Qtz/Coe). Stage III is constrained by two P-T ranges: mineral assemblage of Grt-Ep-Bt-Pl-Phg-Liq (+ Qtz) and Xgr (0.45–0.72) from mantle to rim in garnet in granitic gneiss constrained an earlier P-T range of 22–23 kbar and 800–820°C, while Xgr (0.34–0.39) and Xpy (0.16–0.18) from mantle to rim in garnet in eclogite in the assemblage of Grt-Omp-Amp-Phg-LL (+ Qtz + Rt + H2O) constrained a later P-T range of 17–18 kbar and 675–690°C. The formation and evolution of these assemblages have important implications for the generation and differentiation of continental crust through the operation of plate tectonics. The data suggest that both mafic and granitic rocks in the Yangkou area subducted to a depth of more than 115 km and underwent UHP metamorphism. The three stages of metamorphism in the study area are consistent with the regional well-established metamorphic evolution history, from UHP eclogite facies, through HP/HT eclogite facies, to amphibole eclogite facies with the timing of 230, 225, and 195 Ma, respectively.

Tracking the multi-stage metamorphism and exhumation history of felsic gneisses in the South Altyn ultra-high pressure metamorphic belt, Western China

Felsic gneisses are usually the dominant rock types in most high-pressure (HP) and ultrahigh-pressure (UHP) terranes, although their peak metamorphic conditions and complete P–T paths are rarely known. This lack of knowledge is attributed to the high variance of mineral assemblages at HP/UHP conditions of the felsic gneisses, or to the effects of retrograde-related overprinting. Thus, it is difficult to distinguish whether felsic gneisses underwent HP/UHP metamorphism. In this paper, we focus on the felsic gneiss that has been previously documented as having formed under HP granulite-facies metamorphism with peak P–T of > 1.15 GPa and > 860 °C from the Danshuiquan locality in the South Altyn HP–UHP belt. The Danshuiquan felsic gneiss is strongly deformed and is spatially associated with weakly deformed garnet-rich felsic gneiss. On the basis of detailed microstructure observations on mineral assemblages and phase equilibria modelling, three stages of metamorphic evolution for the weakly deformed garnet-rich felsic gneiss are identified. The first stage is represented by garnet cores, revealing a prograde P–T path from 1.5 GPa/750 °C to 2.1 GPa/900 °C. The second stage is characterized by garnet mantles with multiphase inclusions, defining HP eclogite-facies metamorphic conditions of 2.2–2.6 GPa/950–1100 °C. The third stage is represented by the composition of garnet rims, biotite, and plagioclase, suggesting an HP granulite to amphibolite facies decompression cooling process to 1.1–1.3 GPa/710–770 °C. In contrast, the strongly deformed felsic gneiss just records a deformation-related decompression process from 1.1 to 1.2 GPa/750–760 °C to 0.87–0.98 GPa/760–770 °C, and finally to 0.83 GPa/740 °C. Zircon U–Pb dating yields c. 920 Ma protolith ages, c. 500 Ma eclogite-facies ages, and c. 480 Ma granulite to amphibolite facies retrograde ages for the Danshuiquan felsic gneisses. Our results show that the Danshuiquan felsic gneisses underwent a similar metamorphic evolution process to that of Yinggelisayi HP–UHP rocks in the eastern segment of the South Altyn Tagh. Most HP mineralogical records tend to be eliminated by the strong deformation at crustal depths (∼30 km), meaning that the peak metamorphic conditions of the studied felsic rocks in UHP metamorphic terranes are previously underestimated.

Origin and metamorphic evolution of Chachahe eclogites, North Qaidam UHP metamorphic Belt, NW China: Implications for fate of overriding plate material in subduction channel

• A clockwise P–T path was established for Chachahe eclogites in North Qaidam UHP belt, NW China. • The Chachahe eclogites experienced a 1.92 Ga metamorphic event prior to eclogitization at ∼ 450 Ma. • Overriding plate material can enter subduction channel via tectonic erosion and be carried to mantle depths of ≥ 85 km. Although numerical modelling and marine seismic data show that overriding plate material can enter subduction channel and even recycle to mantle, and finally contribute to mantle heterogeneity and arc magmas, protoliths of the majority of high pressure (HP) and ultrahigh pressure (UHP) metamorphic rocks on the earth’s surface originate from the subducting plate. Whether overriding plate material can enter subduction channel and their fate in subduction channel remain enigmas. The North Qaidam UHP Metamorphic Belt (NQUB), located in northwestern China, was formed by north-dipping subduction of the Qaidam Block beneath the Quanji block in the Early Paleozoic. In this paper, we report that some eclogites in Chachahe area, NQUB experienced a 1.92 Ga low-grade metamorphism prior to eclogitization at ∼ 450 Ma. This Paleoproterozoic metamorphic event is not reported in the subducted Qaidam Block but widespread in the overriding Quanji Block, indicating that protoliths of Chachahe eclogites originated from the overriding plate. A clockwise P–T path from 12 kbar/520 °C to near UHP conditions of 25.5 kbar/690 °C, followed by isothermal decompression, was estimated for the Early Paleozoic metamorphism using a combination of garnet major and trace element zoning, phase equilibrium modelling and thermobarometry. This P–T path pattern is similar to other eclogites in this area, protoliths of which originated from the subducted plate. These results clearly show that overriding plate material can enter subduction channel and be carried to mantle depths of at least 85 km and exhume fast along subduction channel with other slices from subducted slab.

Whewellite-Bearing Fluid Inclusions in Zircons From a Stromatic Migmatite in the Chinese Sulu Ultrahigh-Pressure Metamorphic Belt

Abstract Abundant primary fluid and melt inclusions occur in zircons from a stromatic migmatite sample in the Chinese Sulu ultrahigh-pressure (UHP) metamorphic belt, reflecting an immiscible melt-fluid coexistence during the partial melting process. Raman analysis shows that the fluid inclusions include whewellite, highly disordered carbonaceous matter (CM), diamond, calcite, dawsonite, quartz, cristobalite, H2O-bearing silica glass, muscovite, chalcopyrite, molybdenite, pyrite/pyrrhotite, magnetite, hematite, rutile, CO2 and H2O. The inclusions were entrapped during the exhumation of the rock from UHP to granulite-facies metamorphic conditions. The presence of whewellite suggests that the primary fluids during the entrapment of the inclusions contained some kind of dicarboxylic compound. The highly disordered CM still includes aliphatic hydrocarbon species revealed by the C-H stretching bands at ~2886 and ~2941 cm−1, and a subsidiary band at 1250 cm−1 on the low-frequency side of the D band at 1350 cm−1, indicating that it was derived from incomplete carbonization of an organic compound. The identification of whewellite as well as highly disordered CM in the fluid inclusions provides evidence that abiotic synthesis of organic compounds can happen at elevated pressures and temperatures where both melt and fluids are stable, and in which magnetite and sulphides may play a catalyzing role.

The effect of crystal fractionation on the geochemical composition of syn-exhumation magmas: Implication for the formation of high δ56Fe granites in collisional orogens

Syn-exhumation magmatism in collisional orogens is an important process for crustal differentiation and crust-mantle interaction at convergent plate boundaries. It is intriguing to elucidate which factors control the geochemical composition of such magmatic products. To answer this question, a combined study of whole-rock major-trace elements and Nd-Fe isotopes, mineral O isotopes, and zircon U–Pb ages and trace elements was carried out for syn-exhumation granites from the Sulu ultrahigh-pressure (UHP) metamorphic belt in east-central China. Several granitic plutons of Triassic age were newly found in this typical collisional orogen, with a total exposure area of ∼10 km2, suggesting large-scale partial melting of the deeply subducted continental crust in the terminal stage of continental collision. Field observations indicate that the massive granites show little deformation. The granites are high-K calc-alkaline and exhibit enrichment in LILE and LREE but depletion in HFSE relative to HREE in trace element distribution patterns. Zircons in these granites contain relict magmatic cores characterized by steep HREE patterns with strong negative Eu anomalies. These cores show middle Neoproterozoic U-Pb ages of 724–779 Ma, consistent with the protolith age of UHP metaigneous rocks in the Dabie-Sulu orogenic belt. Zircon rims exhibit oscillatory zoning, and are characterized by Triassic U-Pb ages of 210–216 Ma, low LREE contents and steep HREE patterns with significant negative Eu anomalies, suggesting their growth from granitic magmas during exhumation of the deeply subducted continental crust. The granites have low δ18O values of 2.0–6.1‰ for quartz, 1.1–5.2‰ for whole-rock, and 0.9–3.2‰ for zircon cores. The low δ18O values and Neoproterozoic U-Pb ages of zircon cores in the granites are characteristic feature of the deeply subducted continental crust in the northern margin of the South China Block. One coesite inclusion was found in the anatectic zircon rim, pointing to the deep origin of anatectic melts at a subarc depth of >80 km. Whole-rock SiO2 contents are correlated with major and trace elements, which are ascribed to crystal fractionation of mainly biotite and plagioclase during magma evolution. In particular, these granites exhibit highly variable δ56Fe values from 0.05 to 0.30‰, which are correlated with Fe3+/ΣFe, SiO2, Al2O3, Nb/Ta and Eu/Eu*. This suggests that the crystal fractionation of biotite would have controlled the Fe isotope variation in the granites. Therefore, the syn-exhumation granites experienced significant geochemical differentiation mainly through the fractional crystallization during the magma evolution. A compilation of syn-exhumation granites in the typical collisional orogens of the world shows that such granites were mainly formed through dehydration melting of hydrous minerals during decompressional exhumation. They fall into the magnesian group with relatively low REE and HFSE (like Ti, Nb and Zr) contents, distinct from the geochemical compositions of A-type granites. Therefore, syn-exhumation granites record crustal differentiation in the terminal stage of continental collision.

Zircon U-Pb Dating for Paragneiss in the Xinxian Area in the Dabie–Sulu Orogenic Belt and Its Geochemical Characteristics

In order to understand the subduction of the Yangtze block and its collision with the North China Craton, we performed the elements analysis and zircon U-Pb dating on paragneiss from Xinxian area, which located along the west edge of the Dabie-Sulu ultrahigh-pressure (UHP) metamorphic belt. The major elements analysis showed SiO2, Al2O3, and K2O were enriched and the rare-earth element (REE) analysis showed the paragneiss had a high content of REEs, especially light rare Earth elements (LREEs). The elements analysis suggest that the protolith of paragneiss may be formed by terrestrial materials. As reflected by cathode luminescence (CL), zircon in rocks was metomorphic and had a core-edge microstructure. The edge of zircon was dated by LA-ICP-MS at 233 ± 24 Ma, which suggests that the paragneiss was formed later than the Triassic. The inherited zircon had a large age range of 456–1,727 Ma, mainly in the range of 705–811 Ma. The wide range of ages suggests that the protolith of paragneiss was sedimentary rocks formed no earlier than the Paleozoic. And the paragneiss from Xinxian area were generated in the Dabie-Sulu ocean basin or abortive ocean basin in the Paleozoic era.

Hydrothermal systems with radiogenic Sr in the North Qaidam ultrahigh-pressure metamorphic belt, NE Tibetan Plateau and implications for regional dissolved Sr budget

Hydrothermal systems, including hot springs and hydrothermal calcite, are common features of orogenic belts. The hot springs and hydrothermal calcite in the Himalayas are characterized by radiogenic Sr related to metamorphic processes. Such metamorphism-driven processes have contributed significant amounts of radiogenic Sr to rivers and groundwater, which has affected the evolution of global seawater 87Sr/86Sr during the Cenozoic. However, whether such hydrothermal systems with highly radiogenic Sr occur in other continental collisional belts with intense metamorphism remains unknown, which hinders a better understanding of regional and global Sr cycles. Compiled 87Sr/86Sr ratios in surface water around the Tibetan Plateau show that the Qilian-North Qaidam region displays a higher 87Sr/86Sr ratio than other regions, except for the Himalayas. Here, we report 87Sr/86Sr ratios of a hydrothermal system on the northern margin of the early Paleozoic North Qaidam ultrahigh-pressure metamorphic belt, NE Tibetan Plateau. The high 87Sr/86Sr ratios (0.757–0.734) of the hot spring water and the acetic acid leachates of the surrounding rocks (gneiss and schist) with other isotopic tracers (e.g., oxygen, hydrogen, lithium, and boron isotopes) indicate that 1) Sr was exchanged during metamorphism between carbonate and silicate phases in crustal rocks and the high 87Sr/86Sr ratio of the hot spring water was acquired from a deep water-rock reaction process; and 2) hydrothermal systems around the Qilian-North Qaidam region operate like those in the Himalayas. Mass balance calculations indicate that the metamorphism-forced hydrothermal system plays a key role in delivering radiogenic Sr to rivers and lakes and yielding higher 87Sr/86Sr ratios in surface water around the Qilian Shan region than in the surrounding areas. The reactivated early Paleozoic Qilian orogenic belts with ultrahigh-pressure metamorphism caused by the India-Asia collision have significantly impacted the regional water 87Sr/86Sr ratio, suggesting that the reactivated old orogenic belts with intense metamorphic processes formed by continental subduction/collision can play a pivotal role in regulating past seawater 87Sr/86Sr evolution.

Multistage metamorphism of eclogite in the South Altyn HP–UHP belt, Northwest China: Deep subduction and exhumation process of continental crust

AbstractTwo types of retrograded eclogite (phengite‐bearing eclogite and garnet pyroxenite) within eclogite‐facies granitic gneisses are reported for the first time from the Younusisayi area of the South Altyn high to ultrahigh pressure (HP–UHP) metamorphic belt. These eclogites were studied using petrology, mineral chemistry, phase equilibrium modelling, and geochronology. The phengite‐bearing eclogite records five stages of metamorphism: (1) early prograde (Grt + Cpx + Amp + Rt + Qz) at conditions of P = 11.50–17.5 kbar, T = 665–740°C; (2) peak eclogite‐facies (Grt + Omp + Ph + Rt + Qz) at conditions of P &gt; 24.2 kbar, T = 710–1000°C; (3) overprinting during retrograde transition to HP granulite‐facies conditions (Grt + Cpx ± Kfs + Pl + Rt + Qz) at P = 12.1–16.9 kbar and T = 760–890°C; (4) retrogression at granulite‐facies conditions (Grt + Cpx + Amp + Pl + Ilm + Qz) of P = 9.1–11.4 kbar, T = 730–840°C; and (5) amphibolite‐facies metamorphism (Amp + Pl + Ilm + Qz) during cooling and decompression. The garnet pyroxenite records four stages of metamorphism: (1) eclogite‐facies at conditions of P &gt; 23.2 kbar, T = 775–965°C; (2) HP granulite‐facies conditions of P = 13.2–17.8 kbar and T = 836–912°C; (3) granulite‐facies conditions of P = 8.6–9.7 kbar, T = 739–783°C; and (4) amphibolite‐facies metamorphism. Zircon U–Pb dating and trace element analysis constrain eclogite‐facies metamorphism to c. 500 Ma, consistent with the age of the surrounding eclogite‐facies granitic gneiss (497.8 ± 2.7 Ma). Retrograde metamorphism ages of c. 460 Ma are interpreted as recording zircon growth after granulite‐facies metamorphism. The P–T–t path defines the integrated process of deep subduction and multistage exhumation and places constraints on the early Palaeozoic geodynamics of the South Altyn belt. Well‐preserved major and trace element zoning of garnets in the Ph‐bearing eclogites lacks evidence of significant thermal diffusion and may record garnet growth during deep subduction, immediately followed by very rapid exhumation.

Short-lived intra-oceanic arc-trench system in the North Qaidam belt (NW China) reveals complex evolution of the Proto-Tethyan Ocean

Abstract Recognition of any intra-oceanic arc-trench system (IOAS) could provide invaluable information on the tectonic framework and geodynamic evolution of the vanished ocean basin. The Tanjianshan Complex and mafic-ultramafic rocks along the North Qaidam ultra-high pressure metamorphic belt in NW China record the subduction process of the Proto-Tethyan Ocean. Four lithotectonic units, including island arc, ophiolite, forearc basin, and accretionary complex, are recognized based on detailed field investigation. They rest on the northern margin of the Qaidam block and occur as allochthons in fault contact with underlying high-grade metamorphic rocks. The ophiolite unit mainly consists of ultramafic rocks, 527–506 Ma gabbro, 515–506 Ma plagiogranite, dolerite, and massive lava. High-Cr spinels in serpentinite, dolerite with forearc basalt affinity, and boninitic lava collectively indicate a forearc setting. The accretionary complex, exposed to the south of the ophiolite complex and island arc, is highly disrupted and contains repeated slices of basalt, 495–486 Ma tuff, chert, limestone, and mélange. Tuffs with positive zircon εHf(t) values indicate derivation from a nearby juvenile island arc. These lithotectonic units, as well as the back-arc basin, are interpreted to constitute a Cambrian IOAS that formed during the northward subduction of the Proto-Tethyan Ocean. Combined with regional geology, we propose a new geodynamic model involving short-lived Mariana-type subduction and prolonged Andean-type subduction to account for the complex evolution of the Proto-Tethyan Ocean. The reconstruction of a relatively complete IOAS from the North Qaidam belt not only reveals a systematic evolution of intra-oceanic subduction but also advances our understanding of the subduction and accretion history of the Proto-Tethyan Ocean.

Multi-Stage Metamorphism of the South Altyn Ultrahigh-Pressure Metamorphic Belt, West China: Insights into Tectonic Evolution from Continental Subduction to Arc–Backarc Extension

Abstract The South Altyn ultrahigh-pressure (UHP) metamorphic belt is claimed to host the deepest subducted continental crust based on the discovery of former stishovite, and thus can provide unique insights into the tectonic evolution from deep continental subduction and exhumation to arc–backarc extension. In this article, we present detailed studies of petrography, mineral chemistry, phase equilibria modelling and zircon U–Pb dating for three representative samples involving garnet amphibolite (A1531 &amp; A1533) and associated garnet-biotite gneiss (A1534) from the UHP belt. Three phases of metamorphism are inferred for the rocks. The first phase high-pressure (HP)–UHP-type eclogite facies is represented by the mineral assemblages of garnet and phengite inclusions in zircon and garnet cores with the high grossular (XGrs = 0.33–0.34). The Si contents of 3.40–3.53 and 3.24–3.25 p.f.u. in phengite inclusions yield pressure conditions of &amp;gt;1.7–2.3 GPa for A1533 and 2.5–2.55 GPa for A1534 at a fixed temperature of 770°C. The second phase medium-pressure (MP)-type overprinting of garnet amphibolite facies shows P–T conditions of 0.8–1.2 GPa/750–785°C based on the stability fields of corresponding mineral assemblages, the measured isopleths of Ti contents in biotite and amphibole cores, and XGrs in garnet. The third phase low-pressure (LP) type overprinting includes early-stage heating to peak granulite facies followed by cooling towards a late-stage amphibolite facies. The peak granulite facies is represented by the high Ti amphibole mantle, high Zr titanite and the intergrowths of clinopyroxene + ilmenite in A1533 &amp; A1531, with P–T conditions of 800–875°C/0.80–0.95 GPa. The late-stage is defined by the solidus assemblages, giving P–T conditions of 0.5–0.7 GPa/720–805°C. U–Pb geochronology on metamorphic zircons from A1533 and A1534 gives three ages of c. 500 Ma, c. 482 Ma and c. 460 Ma. They are interpreted to represent the HP-UHP, MP and LP types of metamorphism respectively, based on cathodoluminescence (CL) images, mineral inclusions and trace element patterns. Combining the regional geology and metamorphic evolution from the Altyn Orogen, a tectonic model is inferred, including the following tectonic scenarios. The small Altyn Microcontinent (AM) was subducted to great mantle depths with dragging of the surrounding vast oceanic lithosphere to undergo the HP-UHP eclogite facies metamorphism during the early subduction stage (c. 500 Ma) of the Proto-Tethys Ocean. Then, the subducted slabs were exhumed to a thickened crust region to be overprinted by the MP-type assemblages at c. 482 Ma. Finally, an arc–backarc extension was operated within the thickened crust region due to the retreat of subduction zones. It caused evident heating and the LP-type metamorphic overprinting at c. 460 Ma, with a fairly long interval of 30–40 Myr after the HP-UHP metamorphism, distinct from the short interval of &amp;lt;5–10 Myr in the Bohemian Massif.

Formation process of Al-rich calcium amphibole in quartz-bearing eclogites from The Sulu Belt, China

AbstractAluminum-rich and Si-poor calcium amphibole [~3.9 Al atoms per formula unit (apfu) and ~5.5 Si apfu for 23 O] occur in the quartz-bearing eclogites from the Donghai area, Sulu ultrahigh-pressure metamorphic belt, eastern China. Most of the aluminous amphibole phases are retrograde products from the exhumation and hydration stage and are texturally divided into a mantle phase around a porphyroblastic garnet and a crack-filling (vein) phase of a garnet. Less aluminous amphibole occurs as symplectite phase with plagioclase after omphacite. The formation process of the aluminous amphibole in the quartz-bearing samples is discussed on the basis of the analytical data by EPMA, FIB-TEM, and EBSD.The mantle amphibole occurs between garnet and symplectite or quartz. A set of plagioclase and aegirinediopside/argirine-hedenbergite thin monomineralic bands forms at the boundary between the mantle amphibole and matrix quartz. However, these monomineralic bands do not occur at the mantle amphibole-symplectite boundary. These textural differences indicate that the recrystallization of the aluminous amphibole around garnet was controlled by significant local reactions, and the size of equilibrate domains was probably several tens of micrometers or less.The mantle amphibole is composed of inner (garnet-side) and outer (matrix-side) zones. The inner zone is compositionally homogeneous, and its atomic Al/Si value is ~0.63–0.66 and similar to that of garnet. Atomic Ca/Si value in the inner zone is also almost uniform and is generally identical to that of garnet. The outer zone exhibits a monotonic decrease in the Al/Si and Ca/Si values outward, and its composition at the outermost margin is similar to that of the symplectitic amphibole. The crack-filling amphibole has a composition similar to the inner zone of the mantle amphibole. The CPO pattern of the crack-filling amphibole is different from that of the adjacent mantle amphibole, showing that the crack-filling amphibole is cut by the mantle amphibole. The textural relationship between the mantle and crack-filling amphibole phases and their compositional characteristics imply that: (1) the mantle type is a slightly later stage product than the crack-filling type, and (2) the boundary between the inner and outer zones of the mantle aluminous amphibole probably corresponds to the initial surface of the porphyroblastic garnet. The inner zone is considered to have grown inward by simple substitution of garnet, using the tetrahedral and octahedral cations of the garnet as the basic framework. On the other hand, most of the outer zone of the mantle-type amphibole grew outward in the matrix from the initial surface of the garnet porphyroblast. The mantle amphibole shows a CPO similar to that of amphibole in the adjacent symplectite domain, suggesting that these two types of amphibole formed almost simultaneously, sharing crystallographic orientation with each other.The formation of crack-filling aluminous amphibole was probably promoted by the hydraulic microfrac-turing process at an early stage of exhumation and hydration. The mantle and symplectitic amphibole phases formation was promoted by the subsequent infiltration of metamorphic fluid. The aluminous amphibole in the SiO2 phase-bearing eclogites probably recrystallized with the formation of a localized SiO2-undersaturated reaction domain because of rapid exhumation and subsequent rapid cooling of the Sulu UHP metamorphic belt.