Rapid formation of eclogites during a nearly closed ocean: Revisiting the Pianshishan eclogite in Qiangtang, central Tibetan Plateau

Geophysical Research Letters

et al. 2019

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To better constrain the origin and drift history of the North Qiangtang terrane (NQT), we report a well‐dated paleomagnetic pole from the Late Permian volcanics of the NQT that appears to average out secular variation. Our new results yield a paleolatitude of −7.6 ± 5.6°N at ~259 Ma for our sampling area, which confirms the NQT drifted northward during the Permian and Triassic periods. The equatorial paleolatitude of the NQT is similar to that of the coeval South China block, demonstrating that they were in close proximity. Combined with palaeontological and magmatic evidence, paleomagnetic constraints on the drift of the NQT in the Permian indicate that the NQT moved northward together with the South China block at this time. The paleolatitude evolution of the NQT implies that the NQT rifted from the northern margin of the Gondwana in the Devonian, which is earlier than the departure time of the South Qiangtang terrane.

Section: 1029/2018gl080964supporting

confidence: 87%

Paleomagnetic Constraints on the Origin and Drift History of the North Qiangtang Terrane in the Late Paleozoic

Geophysical Research Letters

et al. 2019

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“…All of these features demonstrate that the eastern Palaeo‐Tethys Ocean was formed at latest by Late Cambrian and evolved from the Late Cambrian to Permian in the CMOB and LCSS (Metcalfe, , , , ; Zhai et al., ; Zhong, ). Considering the protolith formation ages of 260–238 Ma for the LCSS eclogites and blueschists (Dan et al., ; Liu, Lu, et al., ), and 451 ± 3 Ma and 260 ± 3 Ma for retrograded eclogites and blueschists in CMOB (Fan et al., ; this study), both the Ordovician and Permian–Triassic Palaeo‐Tethys oceanic crust is inferred to have undergone blueschist and eclogite facies metamorphism at 246–223 Ma.…”

Section: Discussionmentioning

confidence: 74%

“…All of these features demonstrate that the eastern Palaeo-Tethys Ocean was formed at latest by Late Cambrian and evolved from the Late Cambrian to Permian in the CMOB and LCSS (Metcalfe, 2002(Metcalfe, , 2006(Metcalfe, , 2011(Metcalfe, , 2013Zhai et al, 2016;Zhong, 1998). Considering the protolith formation ages of 260-238 Ma for the LCSS eclogites and blueschists (Dan et al, 2018;Liu, Lu, et al, 2017), and 451 ± 3 Ma and 260 ± 3 Ma for retrograded eclogites and blueschists in CMOB (Fan et al, 2015; this study), both the Ordovician and Permian-Triassic Palaeo-Tethys oceanic crust is inferred to have undergone blueschist and eclogite facies metamorphism at 246-223 Ma. Lithological, geochemical and geochronological analyses have revealed that both the CMOB and LCSS represent the main convergent boundary in the Palaeo-Tethys domain between the Sibumasu Terrane derived from Gondwana and Indochina-Yangtze Block derived from "Asiatic Hunic superterrane" (Huang et al, 1984;Metcalfe, 1992Metcalfe, , 1996Metcalfe, , 2002Metcalfe, , 2006Metcalfe, , 2013Sengör et al, 1984;Wu et al, 1995;Zhong, 1998).…”

Section: Tectonic Significancementioning

confidence: 78%

“…Taking into account the distinctive clockwise P-T vectors for the eclogites in the two belts with the comparable prograde P-T path at lower geothermal gradient (5-6°C/km), and similar isothermal decompression history during retrogression, a cold oceanic subduction process at depth of ≥75 km and initial rapid exhumation were definitely confirmed in both the LCSS and CMOB. Moreover, the Middle to Late Triassic metamorphism for the LCSS eclogites as constrained from garnet Lu-Hf isochron dating (244-223 Ma;Pullen, Kapp, Gehrels, Vervoort, & Ding, 2008) and zircon U-Pb dating (243-230 Ma) from eclogites (Dan et al, 2018; also overlaps with that of CMOB retrograded eclogites (Table 3). Both the LCSS and CMOB eclogites exhibit OIB-type geochemical affinity (Table 3; this study).…”

Section: Tectonic Significancementioning

confidence: 90%

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Petrology, geochemistry and P–T–t path of lawsonite‐bearing retrograded eclogites in the Changning–Menglian orogenic belt, southeast Tibetan Plateau

Journal Metamorphic Geology

Liu

Li³

et al. 2018

The Changning–Menglian orogenic belt (CMOB) in the southeastern Tibetan Plateau, is considered as the main suture zone marking the closure of the Palaeo‐Tethys Ocean between the Indochina and Sibumasu blocks. Here, we investigate the recently discovered retrograded eclogites from this suture zone in terms of their petrological, geochemical and geochronological features, with the aim of constraining the metamorphic evolution and protolith signature. Two types of metabasites are identified: retrograded eclogites and mafic schists. The igneous precursors of the retrograded eclogites exhibit rare earth element distribution patterns and trace element abundance similar to those of ocean island basalts, and are inferred to have been derived from a basaltic seamount in an intra‐oceanic tectonic setting. In contrast, the mafic schists show geochemical affinity to arc‐related volcanics with the enrichment of Rb, Th and U, and depletion of Nb, Ta, Zr, Hf and Ti, and their protoliths possibly formed at an active continental margin tectonic setting. Retrograded eclogites are characterized by peak metamorphic mineral assemblages of garnet, omphacite, white mica, lawsonite and rutile, and underwent five‐stage metamorphic evolution, including pre‐peak prograde stage (M1) at 18–19 kbar and 400–420°C, peak lawsonite‐eclogite facies (M2) at 24–26 kbar and 520–530°C, post‐peak epidote–eclogite facies decompression stage (M3) at 13–18 kbar and 530–560°C, subsequent amphibolite facies retrogressive stage (M4) at 8–10 kbar and 530–600°C, and late greenschist facies cooling stage (M5) at 5–8 kbar and 480–490°C. Laser ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) U–Pb spot analyses of zircon show two distinct age groups. The magmatic zircon from both the retrograded eclogite and mafic schist yielded protolith ages of 451 ± 3 Ma, which is consistent with the ages of Early Palaeozoic ophiolitic complexes and ocean island sequences in the CMOB reported in previous studies. In contrast, metamorphic zircon from the retrograded eclogite samples yielded consistent Triassic metamorphic ages of 246 ± 2 and 245 ± 2 Ma, which can be interpreted as the timing of closure of the Palaeo‐Tethys Ocean. The compatible peak metamorphic mineral assemblages, P–T–t paths and metamorphic ages, as well as the similar protolith signatures for the eclogites in the CMOB and Longmu Co–Shuanghu suture (LCSS) suggest that the two belts formed part of a cold oceanic subduction system in the Triassic. The main suture zone of the Palaeo‐Tethyan domain extends at least 1,500 km in length from the CMOB to the LCSS in the Tibetan Plateau. The identification of lawsonite‐bearing retrograded eclogites in the CMOB provides important insights into the tectonic framework and complex geological evolution of the Palaeo‐Tethys.

“…The Qiangtang Terrane can be further subdivided into the NQT and the southern Qiangtang Terrane by the Longmuco-Shuanghu suture (LSS; Li et al, 1995). This suture zone is characterized by Carboniferous and Permian ophiolites Zhang et al, 2016) and Triassic blueschists and eclogites (Dan et al, 2018;Zhai et al, 2011;K. J. Zhang et al, 2006).…”

Section: Geological Settingmentioning

confidence: 99%

Late Permian Bimodal Volcanic Rocks in the Northern Qiangtang Terrane, Central Tibet: Evidence for Interaction Between the Emeishan Plume and the Paleo‐Tethyan Subduction System

Zhang

et al. 2018

JGR Solid Earth

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Many studies provide compelling evidence for a fossil mantle plume beneath the Late Permian Emeishan large igneous province located near the southeastern margin of the central Tibetan Plateau, a region associated with the Paleo‐Tethyan subduction system. However, little is known about whether direct plume‐subduction interaction occurred during the Late Permian. Here we report geochronological, mineralogical, geochemical, and Sr‐Nd‐Hf‐O isotopic data for the Late Permian (~259–256 Ma) bimodal volcanic rocks (BVRs) in the northern Qiangtang Terrane (NQT), central Tibet. These BVRs consist mainly of basalts, rhyolites, and rhyolitic tuffs. The rhyolites with A‐type affinity were generated by partial melting of newly underplated basalts. Geochemical and isotopic data suggest that two components (ocean island basalt‐type mantle and subducted sediment‐derived fluid) were involved in the generation of the basalts in an extensional back‐arc region. Considering the Permian tectonic evolution of the NQT, we propose that this enriched ocean island basalt‐type mantle was distinct from the Paleo‐Tethyan depleted upper mantle. It most likely originated from Emeishan‐plume material that flowed westward from the South China Block to the nearby NQT, driven by slab rollback‐induced counterflow. The presence of high‐temperature A‐type rhyolites in bimodal back‐arc magmatism also indicates a special subduction setting influenced by a nearby plume. Thus, we propose that the Late Permian BVRs were the result of interaction between the Emeishan plume and the Paleo‐Tethyan subduction system.