High-pressure Metamorphic Belt Research Articles

Bridging the Gap in Garnet Diffusion Models at Low Temperatures: Recalibration Using Western Tianshan Eclogitic Breccia

Abstract Models concerning the diffusion of divalent cations (Ca2+, Mg2+, Fe2+, and Mn2+) in garnet have been subject to extensive research and application over several decades, yet discrepancies among available models persist. Particularly the diffusion rate of Mn2+, which is the fastest in garnet, varies by more than two orders of magnitude for garnets in eclogite. In this study, we use an eclogitic breccia sample from the Western Tianshan (ultra-) high-pressure metamorphic belt for calibration. The thermobarometry indicates that the sample experienced exhumation from ~2.45 GPa, ~480°C to 1.85 GPa, ~515°C. Previous geochronological constraints estimate the exhumation duration to be a couple of million years to up to 15–20 Myr, with an average slab exhumation rate of ~3.5 mm/year. Although the estimates entail significant uncertainties, this range is still smaller than the discrepancy of Mn diffusion rate predicted among diffusion models. Thus, this natural sample provides valuable insights for calibrating the available diffusion models. Our analysis of garnet compositional profiles demonstrates that the diffusion rates (Di) at the pressure and temperature of interest are DCa:DFe:DMg:DMn = 0.2:0.4:1:2.4. By integrating garnet profiles, pressure–temperature–time information, and existing experimental data, we refine each diffusion model for Mn2+, thereby reducing the uncertainties associated with down-temperature extrapolation. Application of the newly calibrated models indicates that the oscillatory zoning of Mn2+ at the garnet rim is best described by a brief thermal excursion (~0.4 Myr, >20°C) at ~1.9 GPa. This minor thermal pulse punctuating the general exhumation path could be indicative of shear heating between nappes or momentary involvement in the mantle wedge, suggesting stacking or partial reactivation of the thrusts during exhumation.

Early Mesozoic accretionary complex scattered throughout NE China and its tectonic implications: Insights from the mylonitic meta-sedimentary rocks in Huanan, Heilongjiang Province

In NE Asia, the early stage (T-J) subduction history of the Paleo-Pacific oceanic plate was largely reconstructed by studies on the Heilongjiang complex, a blueschist-containing N-S trending high-pressure metamorphic belt, which is suggested to represent the “suture zone” between the Jiamusi Block and the rest of the Central Asian Orogenic Belt. However, recently more accretionary complexes with similar geochronological data were reported on both sides of the “suture zone”, making the tectonic affinities of the “suture zone” unclear. Here, we select the mylonitic meta-sedimentary rocks in Huanan, one of the potential accretionary complexes near the central Jiamusi Block to reveal its depositional and metamorphic history and discuss the tectonic implications. Our results show that these rocks were deposited in/after the Late Triassic (defined by the youngest detrital zircon group) and accepted clastics from both the Jiamusi and Songliao blocks. Before the Early Jurassic (c. 178 Ma by apatite UPb dating results in this study), they experienced high-P/low-T metamorphism (510–520 °C and 10.5–11 kbar), and then were heated to about 610 °C at 8.5–9.2 kbar during exhumation. The meta-sedimentary rocks in Huanan are components of the early Mesozoic accretionary complex in NE China. However, their present location is c. 85 km to the east of the previous proposed “suture zone” from Luobei to Mudanjiang. Considering the spatially scattering distribution of other similar accretionary complexes, such as the Zhangguangcai complex (c. 40 km to the west of the “suture zone”), we suggest that these complexes dispersedly exposed throughout NE China were either nappes transported by post-exhumation thrusting, or remnants of archipelagic oceanic crusts in the NW Paleo-Pacific region during the early Mesozoic.

Petrology of UHP eclogite-facies felsic schist in the Western Tianshan subduction zone, China

Although quartzo-feldspathic metasedimentary rocks are widespread in high pressure-ultrahigh pressure (HP-UHP) metamorphic belts worldwide, their petrogenesis and metamorphic evolution is poorly understood. We discovered an UHP eclogite-facies felsic schist in the Western Tianshan metamorphic belt, China. Petrological observations and phase equilibria modeling both indicate the felsic schist experienced UHP metamorphism in the coesite stability field. In particular, it experienced prograde metamorphism at 21–24 kbar, 445°C–470°C, a pressure peak at 25–28 kbar and 490°C–525°C, and eventually heating with decompression to 20 kbar and 560°C. The obtained clockwise P-T path was consistent with those of other lithologies (eclogite and pelitic schist) from the same belt, which provides new evidence for the coherent exhumation of the UHP unit of the Western Tianshan metamorphic belt. The final uplift of the Western Tianshan oceanic crust to the surface is attributed to fluid activity and late tectonic deformation.

Did the Neoproterozoic Revolution Extend to the Deep Mantle?

Did the Neoproterozoic Revolution Extend to the Deep Mantle?

Discovery of a >1,000km Cambrian eclogite-bearing high-pressure metamorphic belt in the Central Asian Orogenic Belt: Implications for the final closure of the Pan-Rodinian Ocean

Discovery of a >1,000km Cambrian eclogite-bearing high-pressure metamorphic belt in the Central Asian Orogenic Belt: Implications for the final closure of the Pan-Rodinian Ocean

Discovery of a >1,000km Cambrian eclogite-bearing high-pressure metamorphic belt in the Central Asian Orogenic Belt: Implications for the final closure of the Pan-Rodinian Ocean

Discovery of a >1,000km Cambrian eclogite-bearing high-pressure metamorphic belt in the Central Asian Orogenic Belt: Implications for the final closure of the Pan-Rodinian Ocean

Tectonic transition from the Paleo-Asian Ocean to the Paleo-Pacific Ocean in central Jilin (NE China): Constraints from the early Mesozoic high-Mg andesites

The Jilin–Yanji Suture is situated at the convergent margin between the North China Craton and Jiamusi-Khanka Massif, playing as a key hinge to link up the tectonic transition from the Paleo-Asian Ocean to the Paleo-Pacific Ocean. However, many parameters of this suture, e.g. the timing of its emplacement, tectonic setting and precise location remain controversial. For addressing these issues, we focused on the high-Mg andesites within the Seluohe Group which is a typical subduction-related volcanic association along the Jilin–Yanji Suture. Seven chlorite schist and six andesite samples were collected for petrological, geochemical and geochronological analyses. The geochemical results indicate the protoliths of chlorite schists and the andesite samples are high-Mg andesites, and their magma source was produced by the metasomatized mantle wedge by subducted slab-derived fluids in a continental island arc setting. Eight high-Mg andesites give the crystallization ages of 249 ± 3 Ma–246 ± 4 Ma, with a weighted mean age of 247 ± 1 Ma (MSWD = 0.27) yielded by 69 youngest zircons, indicative of an Early Triassic age. Based on our new results and the field investigation, we propose that the Seluohe Group is a set of Early Triassic volcanic-sedimentary association with continental island arc affinity related to the south-westward subduction of the Heilongjiang Ocean, rather than a Mesoproterozoic sequence or Late Permian accretionary complex as previously considered. Integrated with previous studies on the regional tectonic evolution, we suggest the Jilin-Yanji Suture belongs to the southern extension of the Jilin-Heilongjiang high-pressure metamorphic belt, which records the final collision between the Jiamusi-Khanka Massif and North China Craton during the Triassic caused by the subduction of the Paleo-Pacific Ocean, and the Late Permian-Late Triassic is a key period of tectonic transition from the Paleo-Asian Ocean to the Paleo-Pacific Ocean in the eastern edge of Eurasia.

Evolution of the Sumdo Paleo-Tethyan Ocean: Constraints from Permian Luobadui Formation in Lhasa terrane, South Tibet

Reconstructing the evolution of the Sumdo Paleo-Tethyan Ocean (SPTO) is important for understanding the transition from the Paleo-Tethyan system to the Neo-Tethyan system. The existence of the SPTO is inferred from the presence of an (ultra-)high pressure metamorphic belt and dismembered ophiolite bodies that separate the southern Lhasa and northern Lhasa terrane. This paper presents new geochronological and geochemical data for the Luobadui Formation in the Lhasa terrane. The geochemical difference between the Lower Luobadui volcanics and the Upper Luobadui volcanics suggests a source transition from a depleted mantle source to an enriched mantle source, which indicates continental-arc subduction. The integrated geological evidence suggests that the SPTO initially opened during the late Carboniferous (ca. 304 Ma), as verified by the earliest record of oceanic crust. Our new data for the Luobadui volcanic rocks, together with the metamorphic and sedimentary records, support northward subduction of the SPTO lithosphere beneath North Lhasa, most likely starting during the Early Permian (ca. 275 Ma) and lasting until the Middle Permian (ca. 260 Ma). The main closure of the SPTO occurred during Middle–Late Triassic (ca. 240–220 Ma), as recorded by medium-pressure metamorphism and syn-collisional magmatism. The Wilson cycle of the SPTO is hereby reconstructed, providing significant insights into the amalgamation of the Tibetan Plateau.

Stability conditions and compositional variations of deerite in high-pressure meta-ironstone during subduction–exhumation processes (SW Tianshan, China)

Index minerals in metamorphic rocks, such as coesite, glaucophane, or lawsonite, are of great importance to understand high-pressure (HP) to ultrahigh-pressure (UHP) metamorphism, the thermal gradient, and the geodynamics of subduction zones. Deerite is thought to be diagnostic for blueschist-facies metamorphism in cold subduction environments and a sensitive indicator of the oxygen fugacity (fO2) in Fe-rich rocks. However, little is known concerning its stability and compositional variation during subduction and exhumation cycles. In this study, we provide the first detailed investigation of deerite in eclogite-facies meta‑ironstones (deerite–garnet riebeckitite) from the Southwestern Tianshan (U)HP metamorphic belt. Petrological observation and textural evidence suggest that the meta‑ironstones –enclosing eclogite blocks– form part of an initially coherent volcano-sedimentary sequence that underwent a common tectono-metamorphic evolution, including HP metamorphism and subsequent exhumation. Two types of deerite, which display a different texture (inclusion vs. matrix) and composition (TiO2 content), occur in the deerite–garnet riebeckitite. Dee_1 and aegirine–augite appear exclusively as fine-grained inclusions in garnet, and together with garnet, riebeckite, quartz and magnetite form the peak metamorphic mineral assemblages at P–T conditions of ~510 °C and 2.6 GPa. Matrix deerite (Dee_2), on the other hand, mainly occurs as coarse-grained rods and acicular needle-like grains in quartz, and rarely as inclusions in riebeckite and garnet intimately in the riebeckitite, representing the blueschist-facies retrogression of the meta‑ironstones during exhumation. The garnet–clinopyroxene oxybarometer and thermodynamic calculations show that the fO2 of the deerite–garnet riebeckitite increases from ΔFMQ–0.64 at the peak eclogite-facies stage to ΔFMQ+1.5 at the blueschist-facies retrogression. This fO2 variation corresponds to the chemical variation of deerite, whereby the TiO2 content increases gradually from <0.1 wt% in the Dee_1 in garnet to about 5 wt% in the matrix Dee_2. Our study reveals the detailed metamorphic P–T–fO2 evolution of the prograde eclogite- and retrograde blueschist-facies deerite-bearing meta‑ironstones during subduction–exhumation processes. This study further emphasizes the potential of deerite to reveal the P–T–fO2 evolution of subduction-related high-pressure rocks.

New Crustal Vs Model Along an Array in South‐East China: Seismic Characters and Paleo‐Tethys Continental Amalgamation

AbstractThe assembly of East Asia was closely linked to the closure of the Tethyan oceans. In south‐east China, the closure of the Paleo‐Tethys ocean led to a continental collision between the South China and North China blocks (SCB and NCB), forming the world renowned (ultra‐) high pressure (UHP) metamorphic belt of the Dabie‐Sulu Orogen. The region was subsequently reworked by postorogenic extensional processes. These tectonic processes likely have left lithospheric scars identifiable by seismic imaging techniques. Here we characterize seismic structures across the orogen and analyze processes related to the closure of the Paleo‐Tethys. Using cutting‐edge tomographic approaches and ambient noise dispersion data, we developed a fine‐scale crustal shear‐wave velocity model beneath key crustal domains in the region. Distinct crustal scale velocity domains are identified, corresponding to the normal Precambrian crust, slow‐velocity suture zones and fault systems, and fast‐velocity orogens, suggesting a deep root of the corresponding surface geological features. By combining recent models of active‐source, gravity and magnetotellurics, characteristic lithospheric deformation patterns such as crustal thrust systems and lithospheric wedges can be inferred, which are attributed to a northward subduction of the SCB lithosphere and the eventual continental collision after the closure of the Paleo‐Tethys Ocean.

Water Content in Garnet from Eclogites: Implications for Water Cycle in Subduction Channels

Garnet from eclogites often shows very heterogenous and extremely high hydroxyl concentration. Eight eclogite samples were selected from the Sulu ultrahigh-pressure terrane and the Sumdo high-pressure metamorphic belt (Lhasa). The mean hydroxyl concentration in pyrope-rich and almandine-rich garnet varies from 54 to 427 ppm H2O and increases with the retrogression degree of eclogites. TEM observations reveal nanometer-sized anthophyllite exsolutions and clinochlore inclusions in water-rich domains in garnet, where anthophyllite is partly replaced by clinochlore. Because of overlapping of the infrared stretching absorption bands for structural OH in garnet and chlorite, it is impossible to exclude contribution of chlorite inclusions to the estimated hydroxyl concentration in garnet. The broad band near 3400 cm−1 is attributed to molecular water and nanometer-sized chlorite inclusions. Anthophyllite exsolutions may be formed by decomposition of hydrous garnet from ultrahigh-pressure eclogites during exhumation. Significant amounts of water can be stored in garnet from massif eclogites in the forms of hydroxyl in garnet and nanometer-sized inclusions of anthophyllite and clinochlore, as well as fluid inclusions. Amphibolite facies retrograde metamorphism can significantly increase both hydroxyl concentration and water heterogeneity in garnet from massif eclogites. These nano-inclusions in garnet provide a window to trace the water cycle in subduction channels.

The tectonic evolution of the Changchun-Yanji suture zone: Constraints of zircon U-Pb ages of the Yantongshan accretionary complex (NE China)

The Changchun-Yanji suture zone is composed of a sequence of high-pressure metamorphic rocks. It lies to the southern margin of the Jilin-Heilongjiang high-pressure metamorphic belt and separates the Jiamusi-Khanka Block to the north from the North China Craton. In this study, to understand the nature of the Changchun-Yanji suture zone, we studied four samples in diverse lithology (i.e., two chlorite schists, one felsic dike sample, and one piemontite schist) from the Yantongshan Complex. These rocks have been intensely deformed without preservation of original sedimentary features. Integrated evidence from high-pressure metamorphic minerals and zircon U-Pb age results suggests that most of the Paleozoic strata in this region might be part of a tectonic mélange. Phengite in the piemontite schist contains high Si (≥3.304 based on 11 oxygens), indicative of high-pressure metamorphism. The piemontite schist contains 6 zircon age populations from 2505 ± 40 to 231 ± 10 Ma. The 22 youngest zircon grains define a weighted mean age of 237 ± 5 Ma, placing an upper limit on the depositional time of the protolith of the piemontite schist. The felsic dike intruded into the metasedimentary rocks gave concordant ages with a weighted mean age of 217 ± 5 Ma, providing a minimum emplacement age for the Yantongshan Complex. We conclude from these data that the final collision between the Jiamusi-Khanka Block and North China Craton along the Changchun-Yanji suture zone occurred before the Late Triassic. Moreover, this closure time is at least 30 Myr later than closure along the Solonker-Xar Moron suture in the late Permian. As such, we consider that the Changchun-Yanji suture zone was not related to amalgamation of the Siberian and North China cratons, but was the result of the subduction of the Paleo-Pacific Plate.

Tectonic position of the Early Neoproterozoic–Early Paleozoic metamorphic belts within the Tuva–Mongolian terrane of the Central Asian Orogenic Belt

The Tuva–Mongolian terrane (TMT) of the Central Asian Orogenic Belt is a composite structure with a Vendian–Cambrian terrigenous-carbonate cover. The formation of the northern part of TMT is marked by the granitoids of the Sumsunur Complex with an age of 785 ± 11 Ma. The Sangilen and Khan-Khukhay blocks of its southern part also form a composite structure, which originated during Early Paleozoic(500–490 Ma) low-moderate pressure regional metamorphism reaching amphibolites-granulite facies. The earlier high-pressure metamorphism was established in the Moren Complex of both the blocks. In the Sangilen block, this metamorphism reached conditions of kyanite-garnet-biotite-orthoclase subfacies of amphibolites facies (temperature ~750oC, pressure 9–10 kbar). The upper age limit of this metamorphism is determined by granites with an age of 536 ± 6 Ma, which cut across migmatized biotite gneisses of the Moren Complex. The latter are intruded by the granitoids of the Ortoadir pluton, which were previously dated at 521 ± ± 12 Ma (U-Pb method, TIMS). Its emplacement predated the Early Paleozoic low-moderate pressure metamorphism, the timing of which is constrained by syn- and postmetamorphic granitoids with ages of 496 ± 4 and 489 ± 3 Ma. The age of 513 ± 4 Ma established for the granitoids of the Ortoadir Complex in the Khan-Khukhay Block more accurately constrains the lower age boundary of collision processes. This determined the amalgamation of the fragments of the high-pressure metamorphic belt with basement and carbonate-shelf cover units of the Tuva–Mongolian terrane, as well as the upper age boundary of early metamorphism. The timing of the main mappable structure of the Khan–Khukhay Block and low-moderate pressure regional metamorphism is marked by the synmetamorphic granitoids with an age of 505 ± ± 2 Ma. In general, the metamorphic rocks of the Sangilen, Khan–Khukhay, and Kaakhem blocks can be considered as fragments of the Late Ediacaran high-pressure metamorphic belt, which were amalgamated to the western margin of TMT within 515–505 Ma, after emplacement of the granitoids of the Ortoadir Complex, and were reworked by regional Early Paleozoic low-moderate pressure metamorphism.

Timing and nature of the partial melting processes during the exhumation of the garnet–bearing biotite gneiss in the southern Altyn Tagh HP/UHP belt, Western China

Partial melting of the garnet (Grt)–bearing biotite gneiss from the Jianggalesayi area in the southern Altyn Tagh high pressure (HP) – ultra high pressure (UHP) metamorphic belt has been identified and reported here. Felsic veins were distributed along the foliation plane and deformed together with the Grt–bearing biotite gneiss, which dominated the main outcrop and preserved diagnostic features of anataxis. Highly cuspate and irregular quartz (Qz)-melt and plagioclase (Pl)-melt surrounded or stretched into biotite and/or quartz grains with obvious eroded boundaries further provided the microstructural evidences for partial melting of the gneiss. CL images showed that zircons from the Grt–bearing biotite gneiss were characterized by multizone structures of core – Rim I – Rim II – Rim III. The different zircon zones from the gneiss had integrated recordings of the protolith ages 836–1613 Ma; the peak metamorphic age of 503 ± 5 Ma; the retrograded age of 456 ± 4 Ma; and the anatectic age of 418 ± 3 Ma. Zircons from felsic veins also exhibited core – rim structures on CL images. The felsic veins yielded similar protolith ages (1099–1072 Ma) and anatectic age (417 ± 2 Ma) as the host gneiss. The zircons from the host gneiss and felsic veins had plotted identical crustal evolution curves in the εHf(t) vs. U-Pb age diagram with identical TDM ages. The U-Pb ages and Lu-Hf isotope both suggested that the felsic veins were internally derived by the partial melting of the host gneiss. Moreover, the high content of Al2O3 and Rb in felsic veins indicated that the decomposition of biotite accounted for the partial melting of the gneiss. The temperatures calculated by Ti–in–zircon thermometer for the anatectic zircons from the felsic veins (582–694 °C) were identical to the conditions of the amphibolite facies stage of the Grt–bearing biotite gneiss (T = 619–646 °C), which indicated that the crystallization of the melt had taken place during the amphibolite facies stage of the exhumation. The anatectic ages (418 ± 3 Ma and 417 ± 2 Ma) of the gneiss and felsic veins were determined to be within the range of the fourth episode of the early Paleozoic magmatism (420–385 Ma), which had formed in a post-collisional extensional setting. Therefore, the combination of the magmatic heat events and stress release may have played an important role in the partial melting of the gneiss.

Two plutonic complexes of the Sanandaj-Sirjan magmatic-metamorphic belt record Jurassic to Early Cretaceous subduction of an old Neotethys beneath the Iran microplate

The Neotethyan tectonics of the Zagros orogenic belt, SW Iran remains still hotly debated in comparing with its western counterparts. One major issue concerns the timing and nature of the Sanandaj-Sirjan magmatic-metamorphic belt (SSMB), which is made predominantly of metamorphic rocks and Jurassic to Early Cretaceous large plutonic complexes. The Alvand and Qory are two largest plutonic complexes locating in north-central and southern segments, respectively, of the SSMB. Careful LA-ICP-MS U/Pb analyses of the magmatic zircons from the Alvand plutonic complex reveal a smooth spectra, along which the concordant age increase gradually from 120 to 190 Ma; while that of Qory is step-like consisting of two stages, a Jurassic and a late Early Cretaceous ones, respectively. New geochemical data, combined with zircon Lu/Hf results suggest that (1) the Alvand granitoids mostly resulted from a long-lived, successive injection of juvenile-crust-sourced magma batches without obvious fractionation crystallization (FC); but (2) the two stages granitoids of the Qory complex both generated by FC of juvenile-crust-sourced magmas; and (3) the gabbros of the Alvand complex are geochemically of E-MORB-affinity while those of the Qory complex are typical continental arc mafic rocks. Previously published petrological and 40Ar/39Ar data have identified a broken, Jurassic to Early Cretaceous high-pressure metamorphic belt to the southwest of the SSMB, which likely represents the closed, southeastern equivalent of the northern Neotethyan Ocean, north of the Taurides-Anatolia-Armenia block. Thus, the SSMB in Iran, the Kapan belt in Caucasus, and the Serbo-Macedonian belt in northern Turkey form a huge Jurassic to Early Cretaceous continental margin arc system recording northeastwards subduction of the older Northern Neotethyan Ocean beneath Eurasia. The Albian-Cenomanian ophiolites such as Koy, Kermanshah, and Neyriz in Iran represent the eastern counterpart of the younger Southern Neotethyan Ocean, south of the Taurides-Anatolia-Armenia block. During the subduction of the Old Neotethys, an intraplate transform fault likely opened and generated a slab-window beneath the Alvand region, which provided a constant energy source to steadily heat the low crust. This model satisfactorily interprets the unusual geochronological framework and geochemistry of the Alvand complex.

The zircon record of high-pressure metasedimentary rocks of New Caledonia: Implications for regional tectonics of the south-west Pacific

We report the U-Pb age, and trace element and hafnium isotope composition of zircons recovered from clastic metasedimentary rocks that span a range of metamorphic grades (prehnite-pumpellyite to eclogite facies) across the high-pressure metamorphic belt of northern New Caledonia. We use these data to evaluate the sedimentary source and environment of formation of these rocks, as well as their respective metamorphic evolution.Metasediments from the low-grade Koumac and blueschist-facies Diahot sequences contain zircon with age populations of 1800–1500Ma, 700–500Ma, 300–250Ma and 140–80Ma. These grains have highly variable Hf isotope compositions (εHf −36 to +12), and some have thin metamorphic rim zones dated to ~38Ma. Zircon grains from the high-grade metamorphic section (Pouebo Eclogite Mélange) are mostly Upper Cretaceous and lowermost Eocene in age, with ~40Ma metamorphic overgrowth zones. The Upper Cretaceous zircons from across the belt have similar isotopic compositions, whereas the lowermost Eocene zircons have relatively high εHf of +6 to +13. The zircon record of the eclogites often lacks the diverse old detrital signature that is prevalent in the blueschist-facies rocks; rather the age populations of the eclogite grade zircons reflect derivation from nearby volcanic sources.We suggest that the bulk of Koumac and Diahot sequences represent the erosional products of the Norfolk Ridge that were transported eastwards and accumulated into the East New Caledonia Basin. Paleozoic and Precambrian zircon grains were originally sourced from Paleozoic orogenic belts of the eastern margin of Australia, and possibly from now-submerged ridges of continental character. We suggest that the Mesozoic zircon grains derived from volcanic arc activity on the eastern margin of Gondwana. Extension led to a reduced continental sediment input into the East New Caledonia Basin during the Cretaceous, which explains the limited amounts of ancient detritus in the mid-Cretaceous to Eocene sedimentary protoliths of the Pouebo Eclogite Mélange. The combined Hf isotope and age data for our zircon record point to the existence of a west-dipping subduction zone active throughout the Late Triassic to the Late Cretaceous, possibly with some short-lived periods of volcanic hiatus. In the early Eocene, a new east-dipping subduction formed following a major change in plate kinematics, producing younger metavolcanic zircons and eventually leading to the metamorphism of sedimentary units in the middle Eocene.

Discovery of eclogite in the Bangong Co–Nujiang ophiolitic mélange, central Tibet, and tectonic implications

An eclogite has been recently identified within ophiolitic mélange in the western segment of the Bangong Co–Nujiang suture zone, at Shemalagou in the Gaize area of central Tibet. The eclogite consists of garnet, omphacite, phengite, rutile, quartz, diopside, and amphibole. The omphacite, which has not been recognized in the suture zone until this study, occurs as rare relics within diopside grains in the eclogite. Phase equilibria modeling shows that the eclogite formed under P–T conditions of 22–28kbar and 600–650°C with a low geothermal gradient of ca. 8°C/km, suggesting that it formed during the subduction of oceanic crust. The protoliths of the eclogite and coexisting garnet amphibolites have geochemical characteristics similar to those of normal mid-ocean ridge basalt (N-MORB), confirming that the eclogites formed from oceanic crust. The presence of high-pressure (HP) eclogite indicates that the ophiolitic mélange in the Bangong Co–Nujiang suture zone underwent oceanic subduction and was subsequently exhumed. We conclude that this ophiolitic belt represents a newly identified HP metamorphic belt in the Tibetan Plateau, adding to the previously recognized Songduo and Longmucuo–Shuanghu eclogite belts. This discovery will result in an improved understanding of the tectonic evolution of the Bangong Co–Nujiang suture zone and the Tibetan Plateau as a whole.

Detrital zircon U–Pb ages and Hf isotopic composition of the Ordovician Duguer quartz schist, central Tibetan Plateau: constraints on tectonic affinity and sedimentary source regions

AbstractMany previous studies have investigated the late Palaeozoic ophiolites, migmatites and high-pressure metamorphic belts of the Tibetan Plateau, whereas the early Palaeozoic evolution of the regions is relatively poorly understood. Lower Palaeozoic strata, including the Duguer quartz schist, occur in the Himalaya, Lhasa and South Qiangtang terranes of the Tibetan Plateau. In this study, we report the depositional age and sedimentary provenance of the Duguer quartz schist of the central South Qiangtang terrane, which enables us to interpret the tectonic affinity of the terrane. We obtained U–Pb ages, trace-element compositions and Hf isotopic data from zircons from the Duguer quartz schist. A total of 162 U–Pb analyses of detrital zircons from the schist yielded two pronounced age peaks at c. 600 Ma and c. 960 Ma. These results indicate that the provenance of the Duguer quartz schist is India Gondwana or the terranes that share an affinity with India Gondwana in the Tibetan Plateau, which include the South Qiangtang and Himalaya terranes. Detrital zircon crystals show large variations in Hf isotope compositions, with εHf(t), TDM and TDMC values of −52.5 to 13.2, 900–3300 Ma and 1010–4240 Ma, respectively. This suggests that the source area for the Duguer quartz schist included Precambrian rocks and, more specifically, Pan-African and Grenville–Jinning crustal material. During Pan-African and Grenville–Jinning events, crustal recycling and the addition of mantle material occurred in the source regions of the quartz schist, when the South Qiangtang, Lhasa and Himalaya terranes were all part of the northern margin of Gondwana.

Cambrian granitic gneiss within the central Qiangtang terrane, Tibetan Plateau: implications for the early Palaeozoic tectonic evolution of the Gondwanan margin

ABSTRACTThe Tibetan Plateau is located in the eastern Himalayan–Alpine orogen, an area where previous research has focused on ophiolites and a high-pressure metamorphic belt, whereas comparatively little research has been undertaken on the Tibetan basement. Cambrian granitic gneiss crops out in the Duguer area of the South Qiangtang terrane in northern Tibet and yields zircon laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U–Pb ages of 502–492 Ma, providing insight into the possible existence of basement rocks within the South Qiangtang terrane. The granitic gneisses are geochemically similar to high-K, calc-alkaline S-type granites, and Hf isotopic analysis of zircons within the gneisses yields negative εHf(t) values (–7.4 to – 1.1) and old zircon Hf model ages (TDMC = 1757–1406 Ma). These granitic gneisses were generated by partial melting of ancient pelitic rocks, and the resulting melts were contaminated by a small amount of mantle-derived material. Combining our new data with previous research, we conclude that these Cambrian granitic gneisses developed in a post-collisional tectonic setting after Pan-African tectonism. This suggests that the South Qiangtang terrane might have the same early Palaeozoic crystalline basement as the Lhasa, Himalaya, Baoshan, Gongshan, and Tengchong terranes.

The timing of final closure along the Changchun–Yanji suture zone: Constraints from detrital zircon U–Pb dating of the Triassic Dajianggang Formation, NE China

The Dajianggang Formation is located in the Changchun–Yanji suture zone of central Jilin Province and unconformably overlies the Changchun–Yanji Accretionary Complex (CYAC), which is a mélange resulting from subduction of the Jiamusi–Khanka Block (JKB) beneath the North China Craton (NCC). LA-MC-ICP-MS U–Pb dating of detrital zircon from four samples of the formation yields ages of 2516 to 216Ma. Zircons with U–Pb ages at 2516–2501Ma and 1897–1832Ma indicate a provenance from Precambrian basement rocks of the NCC. The 525–482Ma ages indicate a provenance from metamorphic rocks of Late Pan-African age in the JKB that have a tectonic affinity to the Central Asian Orogenic Belt (CAOB). Zircon grains with ages of 383–314Ma and 275–250Ma were likely derived from the underlying CYAC. The youngest population has a peak age of ca. 225Ma, which together with Late Triassic fossils, suggests that deposition of the Dajianggang Formation was Late Triassic or younger. This result supports the view that the final collision of the JKB and NCC along the Changchun–Yanji suture took place before the Late Triassic. Furthermore, this closure time is at least 10–20Ma later than closure along the Solonker–Xar Moron–Changchun suture in the Late Permian. We thus establish that the Changchun–Yanji suture is not related to the collision between the Siberia Craton (SC) and the NCC but was instead related to the Paleo-Pacific plate subduction. Consequently, the Changchun–Yanji suture is not the eastward extension of the Solonker–Xar Moron–Changchun suture as previously considered, but the southern margin of the Jilin–Heilongjiang high-pressure metamorphic belt (Ji–Hei HP belt), and resulted from westward subduction of the Paleo-Pacific Ocean. Thus, the Late Triassic marked the switch in subduction from the Paleo-Asian Ocean to the Paleo-Pacific Ocean in NE China.