North Qaidam 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.

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

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.

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.

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.

Melting of subducted continental crust during collision and exhumation: Insights from granitic rocks from the North Qaidam UHP metamorphic belt, NW China

The formation and reworking of continental crust have close connections with orogeny. Syn- and post-collisional magmatism is key to understanding crustal growth and reworking during continental collision and post-collisional extension. Here we report a series of syn-collisional felsic magmatic rocks intruded into migmatized gneiss and amphibolite/eclogite in the North Qaidam ultrahigh-pressure (UHP) metamorphic belt, to provide insights into melting/reworking of UHP metamorphosed continental crust during continental collision and exhumation. The syn-collisional magmas are mainly tonalite-trondhjemite and mica granite that occur as plutons and dykes, and high potassium granite veins in migmatites. They were formed by decompression melting of subducted oceanic and continental crust at different levels during exhumation, without any discernible mantle contribution. The tonalite-trondhjemites show adakitic signatures of both oceanic crust and mafic continental crust with varying Nd isotopic compositions. The mica granite plutons have high silica and alkali contents, are weakly to strongly peraluminous, and have crustal SrNd isotope compositions. These mica granites were formed by partial melting of previously subducted continental crust, and formed mostly in high-pressure conditions, in the garnet stability field. The protolith is predominantly granitic gneiss, with minor meta-sediment and metabasite. A compilation of published ages and our results show that these syn-collisional magmatic rocks formed between 437 and 397 Ma, overlapping with UHP metamorphic ages (440–420 Ma), and suggests that exhumation occurred synchronously with continental subduction. Individual zircon grains show metamorphic cores and magmatic rims with a ≤ 20 Myr time interval, interpreted as the time between peak UHP metamorphism and melting during exhumation. Such long-lived and episodic melting of UHP metamorphic continental crust should result from periodic processes, involving melt weakening and decompression by upward movement within the subduction channel, rather than coherent motion of the entire UHP belt. This process represents crustal reworking during collision and exhumation, without input of mantle melts.

Subduction and exhumation of Luliangshan eclogite in the North Qaidam, northern Tibet: Constraints from petrology, geochemistry and phase equilibrium modelling

Eclogites in the high‐pressure (HP) and ultrahigh‐pressure (UHP) belt record important information about the subduction process and evolution history of the orogenic belt. The Luliangshan eclogites surrounded by granitic gneiss or paragneiss as lenses are exposed in the western segment of the North Qaidam UHP metamorphic belt, northwestern China. Petrology, mineral chemistry, and P–T pseudosection modelling show that the eclogites have experienced a multi‐stage metamorphic process. The peak eclogite‐facies metamorphic stage, is characterized by omphacite in matrix and as inclusion in garnet, with the peak mineral assemblages of garnet + omphacite + rutile + quartz atT > 790°C andP > 25.5 kbar. The initial HP granulite‐facies retrogression is characterized by the symplectite of diopside + plagioclase around omphacite, with P–T conditions of 911°C and 16.9 kbar. The subsequent amphibolite‐facies stage is characterized by amphibole + plagioclase symplectite around the clinopyroxene, with the metamorphic conditions of 674–686°C and 6.4–6.9 kbar. Zircon U–Pb analyses yielded two metamorphic age clusters: (a) HP granulite‐facies metamorphic age of 422–425 Ma, and (b) amphibolites‐facies retrograde age of 397–420 Ma. The protolith of eclogite have geochemical characteristics similar to those of normal mid‐ocean ridge basalt (N‐MORB); and the varyingεNd(t) values (−6.3 to 2.1) indicate that the Luliangshan eclogites were derived from a mantle source with rare crustal contamination. Combining these data with previous studies, a multi‐stage tectonic model can be proposed: In the Early Neoproterozoic, the protolith of the Luliangshan eclogites were emplaced into ancient continental crust; during 460–430 Ma, following the oceanic subduction, the subduction of continental crust was dragged by the oceanic slab and continue to subduct towards the Qilian Block, and metamorphosed at the depth of at least 75 km. After a long and slow exhumation process, it returned to the shallow crust.

Analysis of electron backscattered diffraction (EBSD) mapping of geological materials: precautions for reliably collecting and interpreting data on petro-fabric and seismic anisotropy

Automated electron backscattered diffraction (EBSD) data yield abundant information on the lattice-preferred orientations (LPOs) and deformation microstructures and mechanisms of minerals in a rock, which aid in our understanding of the tectonic conditions and history of a region. Additionally, this information allows us to interpret seismic anisotropies in the crust and mantle. However, great care must be taken in the collection and production of crystallographic orientation data via automated EBSD to more precisely interpret LPOs, deformation microstructures, and seismic anisotropies because petro-fabrics can be different depending on sampling methods such as all-points-per-grain (whole) data and one-point-per-grain (one orientation per grain) data. Here, we report a detailed comparison of the crystallographic orientation data produced using both all-points-per-grain and one-point-per-grain techniques to analyze eclogites from the Yuka terrane in the North Qaidam ultrahigh-pressure metamorphic belt of northwestern China. By comparing eclogite crystallographic orientations between different sampling methods, we found that there was no marked difference in the LPOs, fabric strengths (J-index and M-index), and seismic anisotropies for relatively small mineral grains (e.g., garnet and omphacite). However, there were large differences in the LPOs, fabric strengths, and seismic anisotropies of relatively large minerals grains (e.g., amphibole). This result can be attributed to the acquisition method of crystallographic orientation data, which can influence the LPO, fabric strength, and seismic anisotropy of minerals and rocks. Therefore, the pole figures of minerals should be constructed from one point per grain data in order to avoid the oversampling of large grains for samples with highly heterogenous grain size distributions, as well as to permit the comparison of crystallographic orientations obtained using different tools and in other studies.

From magmatic generation to UHP metamorphic overprint and subsequent exhumation: A rapid cycle of plate movement recorded by the supra-subduction zone ophiolite from the North Qaidam orogen

The rapid subduction and exhumation of high-pressure (HP) and ultrahigh-pressure (UHP) rocks exposed in collisional belts play a critical role in understanding the evolution of geodynamics and plate tectonics in subduction zone environments. Herein, ophiolitic fragments, eclogites, and amphibolites, from a subduction channel exposed in the Dulan terrane, North Qaidam orogen were investigated. Garnet compositional zoning from the studied eclogites records only a single cycle of subduction and exhumation. New uranium–lead (U–Pb) zircon ages of the current study combined with previous rutile U–Pb ages from eclogites indicate a rapid cycle of events, starting with the generation of the magmatic protolith at ca. 446 Ma, its subsequent subduction reaching a depth of ca. 100–110 km at ca. 435 Ma, and its exhumation into shallow portions of the crust which occurred at ca. 416 Ma. Thus a rapid 30 million years (Myr) protolith generation–subduction–exhumation cycle could be recorded. According to an ophiolitic rock association with island arc basalt (IAB)-like and mid-ocean ridge basalt (MORB)- to ocean island basalt (OIB)-like trace element compositions, the slab-derived components determined by Sr–Nd–Hf isotopes and the short time span of only about 11 Myr between photolith generation and UHP metamorphism in eclogites, the protolith of eclogite is best explained as a supra-subduction zone ophiolite. This finding likely records the closure of the South Qilian ocean and the initiation of a continent-arc collision at ca. 446 Ma. Furthermore, starting with subduction erosion during continent-arc collision in the early Palaeozoic, ophiolitic and arc-related materials become subducted in the North Qaidam. It was inferred that such a rapid succession of events from protolith formation to UHP overprint and final exhumation, could be explained by movements of rock material in a subduction channel. Thus, this study provides a new view in understanding the mechanics of different episodes of subduction systems in North Qaidam UHP metamorphic belt.

Subduction channel fluid-rock interaction: Indications from rutile-quartz veins within eclogite from the Yuka terrane, North Qaidam orogen

High-pressure (HP) or ultrahigh-pressure (UHP) rutile-quartz veins that form at mantle depths due to fluid-rock interaction can be used to trace the properties and behavior of natural fluids in subduction zones. To explore the fluid flow and the associated element mobility during deep subduction and exhumation of the continental crust, we investigated the major and trace elements of Ti-rich minerals. Additionally, U–Pb dating, trace element contents, and Lu–Hf isotopic composition of zircon grains in the UHP eclogite and associated rutile-quartz veins were examined in the North Qaidam UHP metamorphic belt, Yuka terrane. The zircon grains in the rutile-quartz veins have unzoned or weak oscillatory zonings, and show low Th/U ratios, steep chondrite-normalized patterns of heavy rare earth elements (HREEs), and insignificant negative Eu anomalies, indicating their growth in metamorphic fluids. These zircon grains formed in 431 ​± ​3 ​Ma, which is consistent with the 432 ​± ​2 ​Ma age of the host eclogite. As for the zircons in the rutile-quartz veins, they showed steep HREE patterns on one hand, and were different from the zircons present in the host eclogite on the other. This demonstrates that their formation might have been related to the breakdown of the early stage of garnet, which corresponds to the abundance of fluids during the early exhumation stage. The core-rim profile analyses of rutile recorded a two-stage rutile growth across a large rutile grain; the rutile core has higher Nb, Ta, W, and Zr contents and lower Nb/Ta ratios than the rim, indicating that the rutile domains grew in different metamorphic fluids from the core towards the rim. The significant enrichment of high field strength elements (HFSEs) in the rutile core suggests that the peak fluids have high solubility and transportation capacity of these HFSEs. Furthermore, variations in the Nb vs. Cr trends in rutile indicate a connection of rutile to mafic protolith. The zircon grains from both the rutile-quartz veins and the host eclogite have similar Hf isotopic compositions, indicating that the vein-forming fluids are internally derived from the host eclogite. These fluids accumulated in the subduction channel and were triggered by local dehydration of the deeply subducted eclogite during the early exhumation conditions.

Tracking the timing and nature of protolith, metamorphism, and partial melting of tourmaline‐bearing migmatites by zircon U–Pb and Hf isotopic compositions in the Yuka terrane, North Qaidam UHP metamorphic belt

An integrated study using cathodoluminescence (CL) images, U–Pb geochronology, and Hf isotopic characterization of zircons was performed on migmatites from the Yuka terrane to provide insights into the timing and nature of the protoliths, metamorphism, and partial melting of these rocks. Zircon grains separated from the migmatites have three distinct domains. Residual zircon cores in samples YKT01 and YKT02 exhibit oscillatory zoning, high Th/U ratios (0.10–0.79), steep heavy rare earth element (HREE) patterns, and mean ages of 936 ± 5 Ma and 927 ± 11 Ma, respectively, indicating that they are igneous in origin and record Neoproterozoic magmatism during the Grenvillian Orogeny. CL‐grey cores record an ultrahigh‐pressure (UHP) peak metamorphism age of 435 ± 5 Ma and have low Th/U and 176Lu/177Hf ratios, weak negative Eu anomalies, and relatively flat HREE patterns. The CL‐bright rims have steep HREE patterns with lower middle rare earth element (MREE) concentrations but higher HREE concentrations compared with the metamorphic cores and a weighted average age of 422 ± 4 Ma, which records the timing of melt crystallization. The migmatites incorporate garnet relicts coexisting with fluid inclusions and hydrous minerals like peritectic phengite and peritectic large anhedral poikilitic tourmaline in the leucosomes. Peak temperatures are less than 750°C, suggesting that the anatectic melts may be derived from melting of felsic gneiss in the presence of water. The timing of melt crystallization is ca. 13 Ma later than UHP metamorphism, indicating that the partial melting took place during the exhumation stage. The metamorphic zircon cores have higher 176Hf/177Hf(t) ratios than the residual magmatic cores, which provides powerful evidence for the generation of the metamorphic zircons through the incorporation of internal high Hf minerals other than zircon in a closed system. Anatectic rims with lower 176Hf/177Hf(t) ratios but higher 176Lu/177Hf ratios than metamorphic cores suggest that the main formation mechanism is dissolution‐reprecipitation of pre‐existing zircons. In conclusion, the elevated 176Hf/177Hf(t), TDM2 age and decreasing 176Lu/177Hf(t) ratios in the metamorphic and anatectic domains imply that the new zircons in some migmatites and granites may not reflect the Hf isotope compositions of their source rocks, accounting for the variable Hf isotope compositions in S‐type granites.

Eclogite-facies metamorphism in impure marble from north Qaidam orogenic belt: Geodynamic implications for early Paleozoic continental-arc collision

In the North Qaidam ultrahigh-pressure (UHP) metamorphic belt, impure marble and interbedded eclogite represent a particular sedimentary provenance and tectonic setting, which have important implications for a controversial problem - the dynamic evolution of early Paleozoic subduction-collision complexes. In this contribution, detailed field work, mineral chemistry, and whole-rock geochemistry are presented for impure marble to provide the first direct evidence for the recycling of carbonate sediments under ultrahigh-pressures during subduction and collision in the Yuka terrane, in the North Qaidam UHP metamorphic belt. According to conventional geothermobarometry, pre-peak subduction to 0.8–1.3GPa/485–569°C was followed by peak UHP metamorphism at 2.5–3.3GPa/567–754°C and cooling to amphibolite facies conditions at 0.6–0.7GPa/571–589°C. U-Pb dating of zircons from impure marble reveals a large group with ages ranging from 441 to 458Ma (peak at ~450Ma), a smaller group ranging from 770 to 1000Ma (peak at 780Ma), and minor >1.8Ga zircon aged ca. 430Ma UHP metamorphism. The youngest detrital zircons suggest a maximum depositional age of ca. 442Ma and a burial rate of ca. 1.0–1.1cm/yr when combined with P–T conditions and UHP metamorphic age. The REE and trace element patterns of impure marble with positive Sr and U anomalies, negative high field strength elements (Nb, Ta, Zr, Hf, and Ti), and Ce anomalies imply that the marble had a marine limestone precursor. Impure marble intercalated with micaschist and eclogite was similar to limestone and siltstone protoliths deposited in continental fore-arc or arc setting with basic volcanic activity. Therefore, the Yuka terrane most likely evolved in a continental island arc setting during the Paleozoic. These data suggest that metasediments were derived from a mixture of Proterozoic continental crust and juvenile early Paleozoic oceanic and/or island arc crust. In addition, their protoliths were likely deposited in a terrigenous-dominated forearc marine basin rather than an intracontinental basin environment, further evidence that some continental arc volcanic rock may have been the source of eclogite in the North Qaidam. These sediments, formed in a forearc basin close to the Qaidam Block to the north, were transported in the subduction zone to 100–110km depth with UHP metamorphism prior to exhumation. Meanwhile, the new results suggest that subduction erosion occurred along the active continental margin during the Qaidam Block with north-dipping subduction, indicating that the North Qaidam UHP metamorphic belt may have formed during continental-arc collision.

Two epochs of eclogite metamorphism link ‘cold’ oceanic subduction and ‘hot’ continental subduction, the North Qaidam UHP belt, NW China

AbstractEclogites in the high-pressure (HP) and ultrahigh-pressure (UHP) belts record subduction-zone processes; exhumed eclogites of seafloor protoliths record low-temperature (mostly <600°C), high-pressure and ‘wet’ environments: that is, relatively ‘cold’ subduction with highly hydrous minerals such as lawsonite. In contrast, eclogites formed by the continental subduction record relatively ‘hot’ (T > 650°C) and ‘dry’ ultrahigh-pressure metamorphic (UHPM) conditions with syncollisional magmatism. Here, we investigate some eclogites from two ophiolite sequences that intercalated in the North Qaidam UHPM belt, which is genetically associated with continental subduction/collision. The observations of lawsonite pseudomorphs in garnets, garnet compositional zoning, mineral and fluid inclusions in zircons, and zircons with distinct trace-element patterns and U–Pb ages all suggest that these eclogites represent two exhumation episodes of subduction-zone metamorphic rocks: the early ‘cold’ and ‘wet’ lawsonite eclogite and the late ‘hot’ and ‘dry’ UHP kyanite eclogite. The early lawsonite-bearing eclogite gives metamorphic ages of 470–445 Ma and the later kysnite-bearing eclogite gives metamorphic ages of 438–420 Ma, with a time gap of c. 7–10 myr. This gap may represent the timescale for transition from oceanic subduction and continental subduction to depths greater than 100 km. We conclude that evolution from oceanic subduction to continental collision and subduction was a continuous process.In addition, we find that titanium contents in zircons have a positive correlation with U contents. Ti-in-zircon thermometry is likely to be invalid or limited for low-temperature eclogites.

Tectonic erosion related to continental subduction: An example from the eastern North Qaidam Mountains, NW China

AbstractTectonic erosion has been commonly recognized in oceanic subduction zones, but evidence of tectonic erosion related to continental subduction is rarely reported. In this contribution, we suggest a tectonic erosion process along the North Qaidam Mountains of NW China during the Early Palaeozoic continental subduction. To the north of the North Qaidam ultra‐high pressure (UHP) metamorphic belt, the microcontinental Oulongbuluke Block (OB) is characterized by Precambrian metamorphic basement. New and existing zircon U–Pb data show that the OB underwent Early Palaeoproterozoic (2.4–2.3 Ga) magmatism and Late Palaeoproterozoic (c. 1.9 Ga) high‐grade metamorphic and anatectic events. The North Qaidam UHP metamorphic belt has been considered to result from the northward subduction of the Qaidam continental block consisting mainly of Meso‐Neoproterozoic orthogneiss and paragneiss. However, our new data show some gneisses enclosing eclogites in the eastern North Qaidam UHP metamorphic belt (Dulan UHP metamorphic unit) experienced 2.4–2.3 Ga magmatism and 1.9–1.8 Ga metamorphism, similar to those of the OB, prior to Early Palaeozoic UHP metamorphism. These data suggest that tectonic erosion occurred during north‐dipping subduction: the Precambrian basement from the upper plate (OB) was transported in the subduction zone to more than 80 km depth, resulting in UHP metamorphism related to Early Palaeozoic continental subduction prior to their exhumation.

The Early Paleozoic Xitieshan syn-collisional granite in the North Qaidam ultrahigh-pressure metamorphic belt, NW China: Petrogenesis and implications for continental crust growth

Syn-collisional magmatism produced by partial melting of subducted oceanic and continental crust during the continental collision plays an important role in understanding the orogenic evolution and crustal growth. This contribution reports zircon LA-ICPMS U–Pb ages and Hf isotopic compositions, whole rock major and trace elements and Sr-Nd isotopic compositions of syn-collisional Xitieshan intrusion within the North Qaidam ultrahigh-pressure metamorphic (UHPM) belt, to study its petrogenesis and contribution to continental crust growth. LA-ICPMS zircon U–Pb dating of the Xitieshan granite yields magmatic crystallization ages of 441±2Ma and 442±2Ma, which are consistent with the peak age of ultrahigh-pressure eclogite-facies metamorphism in the Xiteishan terrane. The temporal correlation between them confirms that the Xitieshan granite was products of syn-collisional magmatism during the continental collision between Qaidam and Qilian Blocks. These granites show high-K calc-alkaline and slightly peraluminous signature, low zirconium saturation temperatures and high contents of K2O with the S-type characteristics. They have rare earth element and trace element patterns resembling those of bulk continental crust, with strong fractionation of light and heavy rare earth elements ((La/Yb)N=19–26), moderately Eu negative anomalies (δEu=0.65–0.71) and the obviously Nb, Ta, P and Ti negative anomalies. The Xitieshan granite also exhibits remarkable Sr-Nd isotopic differences (initial 87Sr/86Sr=0.70920–0.71080 and εNd(t)=−4.54–4.11) from the contemporaneous granites within North Qaidam UHPM belt. Combined with the positive εHf (t)(0.5–5.3) and ages of inherited zircons (475–518Ma), the magmatism is best explained as resulting from melting of subducted oceanic and continental crust during continental collision. Isotopic mixing calculations suggest that ca. 28–35% ocean crust and ca. 65–72% continental materials contribute to the origin of the Xitieshan granite. Thus the syn-collisional felsic magmatism represents juvenile crust with input of oceanic crustal materials derived from the depleted mantle and the hypothesis “continental collision zones are primary sites for net continental crust growth” is applicable in the North Qaidam.

Zircon geochemistry of two contrasting types of eclogite: Implications for the tectonic evolution of the North Qaidam UHPM belt, northern Tibet

Compared to the extensively documented ultrahigh-pressure metamorphism at North Qaidam, the pre-metamorphic history for both continental crust and oceanic crust is poorly constrained. Trace element compositions, U–Pb ages, O and Lu–Hf isotopes obtained for distinct zircon domains from eclogites metamorphosed from both continental and oceanic mafic rocks are linked to unravel the origin and multi-stage magmatic/metamorphic evolution of eclogites from the North Qaidam ultrahigh-pressure metamorphic (UHPM) belt, northern Tibet.For continental crust-derived eclogite, magmatic zircon cores from two samples with U–Pb ages of 875–856Ma have both very high δ18O (10.6±0.5‰) and mantle-like δ18O (averaging at 5.2±0.7‰), high Th/U and 176Lu/177Hf ratios, and steep MREE-HREE distribution patterns (chondrite-normalized) with negative Eu anomalies. Combined with positive εHf (t) of 3.9–14.3 and TDM (1.2–0.8Ga and 1.3–1.0Ga, respectively), they are interpreted as being crystallized from either subduction-related mantle wedge or recycled material in the mantle. While the metamorphic rims from the eclogites have U–Pb ages of 436–431Ma, varying (inherited, lower, and elevated) oxygen isotopes compared with cores, low Th/U and 176Lu/177Hf ratios, and flat HREE distribution patterns with no Eu anomalies. These reflect both solid-state recrystallization from the inherited zircon and precipitation from external fluids at metamorphic temperatures of 595–622°C (TTi-in-zircon).For oceanic crust-derived eclogite, the magmatic cores (510±19Ma) and metamorphic rims (442.0±3.7Ma) also show distinction for Th/U and 176Lu/177Hf ratios, and the REE patterns and Eu anomalies. Combined with the mantle-like δ18O signature of 5.1±0.3 ‰ and two groups of model age (younger TDM close to the apparent ages and older >700Ma), two possible pools, juvenile and inherited, were involved in mixing of mantle-derived magma with crustal components. The relatively high δ18O of 6.6±0.3‰ for metamorphic zircon rims suggests either the protolith underwent hydrothermal alteration prior to the ~440Ma oceanic crust subduction, or external higher δ18O fluid activities during UHP metamorphism at ~440Ma.Therefore, the North Qaidam UHPM belt witnesses multiple tectonic evolution from Late Mesoproterozoic–Neoproterozoic assembly/breakup of the Rodinia supercontinent with related magmatic emplacement, then Paleozoic oceanic subduction, and finally transition of continental subduction/collision related to UHP metamorphism.

Derivation of P–T paths from high-pressure metagranites — Examples from the Gran Paradiso Massif, western Alps

Metamorphosed granites (SiO2≥70wt.%) are, in fact, a common rock type in high pressure (HP) and ultrahigh pressure (UHP) terrains, but these rocks were rarely used to derive metamorphic P–T paths. To test the suitability of HP metagranites for such derivations, two metagranites from the Gran Paradiso Massif were studied applying elemental mapping of phengite and garnet and calculated P–T pseudosections contoured by various chemical and modal parameters. Both rocks contain phengite with maximum Si contents of about 3.42 Si per formula unit (pfu) and 3.55Sipfu in cores, and accessory garnet which is compositionally zoned. Garnet core compositions are rich in grossular component (XCa up to 0.72). Only a rough P–T path could be derived with peak pressures below 2GPa because, for instance, Si contents in phengite become geobarometrically insensitive at HP conditions, when biotite is not anymore stable. A test of the pseudosection approach to a metagranite from the North Qaidam UHP metamorphic belt resulted in an ambiguous finding. In fact, compositions of garnet and phengite in this rock are indicative of both UHP and specific HP conditions (~1.3GPa, 530°C), but the latter conditions fit the entire mineralogical observations better.

Grenville-age orogenesis in the Qaidam-Qilian block: The link between South China and Tarim

The link between the South China and Tarim blocks is a key issue in defining the tectonic framework of China. We report here that rocks from the Paleozoic (Caledonian) ultrahigh-pressure metamorphic (UHPM) belt also record a Grenville-age orogenic event in the Qaidam-Qilian block that lies between the South China and Tarim blocks. This event is marked by the presence of a juvenile Grenville-age continental margin that had been subducted to and exhumed from mantle depths of 100–200km at the present-day location of northern Tibetan Plateau. The magmatic and metamorphic ages of 900–1000Ma recorded in the gneisses provide direct evidence for this Grenville-age orogeny that extends northwestward from the Yangtze block, to the Qaidam-Qilian block, and to the Tarim block. This Grenville-age orogen along the North Qaidam UHPM belt (1) represents the link for the once South China–Qaidam-Qilian–Tarim continent, which, we name here as the “South-West China United Continent (SWCUC)”, had existed before the India–Asia collision, (2) sets a framework for precise reconstruction of Supercontinent Rodinia, and (3) presents an example of multi-epoch tectonic recycles, represented by the Neoproterozoic Grenvillian orogenesis and the Early Paleozoic Caledonian orogenesis.

Tracing the 850-Ma continental flood basalts from a piece of subducted continental crust in the North Qaidam UHPM belt, NW China

A Paleozoic ultrahigh-pressure metamorphic (UHPM) belt extends along the northern margin of the Qaidam Basin, North Tibetan Plateau. Eclogites in the Yuka eclogite terrane, northwest part of this UHPM belt, occur as blocks or layers of varying size intercalated with granitic and pelitic gneisses. These eclogites have protoliths geochemically similar to enriched-type mid-ocean ridge basalts (E-MORB) and oceanic island basalts (OIB). On the basis of Ti/Y ratios, they can be divided into low-Ti and high-Ti groups. The low-Ti group (LTG) eclogites exhibit relatively low TiO 2 (most <2.5 wt%) and Ti/Y (<500) but comparatively high Mg# (48–55), whereas the high-Ti group (HTG) eclogites have high TiO 2 (most >2.5 wt%) and Ti/Y (>500) but lower Mg# (46–52). Zircons from two eclogite samples gave a magmatic crystallization (protolith) age of ∼850 Ma and a UHPM age of ∼433 Ma. The occurrence, geochemical features and age data of the Yuka eclogites suggest that their protoliths are segments of continental flood basalts (CFBs) with a mantle plume origin, similar to most typical CFBs. Our observation, together with the tectonic history and regional geologic context, lend support for the large scale onset of mantle plume within the Rodinia supercontinent at ∼850 Ma. The Qaidam block is probably one of the fragments of the Rodinia supercontinent with a volcanic-rifted passive margin. The latter may have been dragged to mantle depths by its subducting leading edge of the oceanic lithosphere in the Early Paleozoic.

UHP metamorphic evolution and SHRIMP geochronology of a coesite-bearing meta-ophiolitic gabbro in the North Qaidam, NW China

We have for the first-time observed the well-preserved coesite inclusions in a kyanite-eclogite from the Shaliuhe terrane of the North Qaidam ultra-high pressure (UHP) metamorphic belt, NW China. This provides convincing evidence for UHP metamorphism for the North Qaidam eclogite. Detailed petrography indicates that the kyanite-eclogite records complex metamorphic histories that can be broadly described in terms of three stages: (1) a pre-peak metamorphic stage characterized by the mineral assemblage of Grt + Amp + Pl + Qtz at P = 0.49–0.67 GPa and T = 410–490 °C; (2) a peak eclogite-facies stage with a mineral assemblage of Grt + Omp + Ky + Phn + Rt ± Qtz/Coe at P = 2.7–3.4 GPa and T = 610–700 °C; (3) a retrograde “stage” represented by reaction texture which includes the garnet reaction rims, the breakdown of omphacite, the symplectitic breakdown of kyanite, the retrograded garnet amphibolite assemblage and the latest greenschist-facies overprint. Zircon U–Pb SHRIMP dating shows the peak UHP metamorphic age of the kyanite-eclogite to be ∼440 Ma. These data indicate that the kyanite-eclogite had ever been subducted to mantle depth of ∼100 km in the end of the Ordovician. All these observations allow the construction of a P-T path for the kyanite-eclogite, suggesting a cold geothermal gradient and rapid subduction, ultimately followed by rapid exhumation in response to continental collision in Shaliuhe terrane and throughout the North Qaidam UHP metamorphic belt.