The Early Paleozoic eclogite belt extends discontinuously for -500 km within the East Kunlun Orogen (EKO) and records the subduction history of the Proto-Tethys Ocean. This eclogite belt, therefore, is an excellent site for studying the first-order transition in convergent-margin tectonic systems, and the tectonic events associated with subduction and collision can be determined by reconstructing the highpressure (HP)-ultrahigh-pressure (UHP) P-T-time path. In this contribution, we present P-T results from phase equilibrium modeling and Zr-in-rutile/titanite thermometry for eclogites located on the east and west sides of the EKO. In the eastern EKO, MT-UHP eclogite records peak conditions of -30 kbar/650- 720 degrees C, retrograde conditions of 10 kbar at -650-700 degrees C, and then further retrograde conditions at 3-6 kbar and 500-580 degrees C. In the western EKO, LT-HP eclogite records peak conditions of -27 kbar at 570 degrees C and then retrograde conditions at 10 +/- 2 kbar/470-560 degrees C. These two types of eclogite record oceanic subduction (west) and continental collision (east), respectively. The EKO, therefore, contains two types of eclogites that indicate two end-member subduction complexes within the orogenic belt, further indicating that there are differences in the subduction-depth and collision-time from east to west. This finding is consistent with asymmetric suturing of the subducted Proto-Tethys Ocean within the EKO. Together, the P-T conditions and time-scales of the HP-UHP metamorphism can be applied to understand the process from subduction to collision between the Qaidam Block and the South Kunlun Block, as a consequence of asymmetric closure of the Proto-Tethys Ocean. (c) 2021 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

The East Kunlun Orogen (EKO) of the northwest part of the Central China Orogenic belt records long‐lived orogenic events from the Cambrian to the Triassic, involving the tectonic evolution from Proto‐Tethys to Palaeo‐Tethys. Although high‐ and ultra high pressure (HP‒UHP) metamorphism has been recognized in the EKO, other metamorphic events associated with the orogenic history are little known. The metamorphic history of high‐ and ultra high temperature granulites and their pressure–temperature – time ( P‒T–t ) path is keys to reconstructing tectonic events associated with a subduction‐to‐collision transition. In this contribution, we present P‒T–t results for the mafic and pelitic granulite facies metamorphic rocks in the Qingshuiquan region of the EKO, determined using phase equilibrium modelling and zircon geochronology. The Jingshuikou granulite terrane records a clockwise P–T path with high‐ P and ultra high temperature (HP–UHT) conditions at P max ~900 to 950°C at 13 kbar, at T max of ~950°C at 9‒12 kbar, and a final stage of 800‒895°C at 6‒10 kbar. LA‐ICP‐MS U–Pb dating of zircons provides systemic constraints on a long‐lived metamorphic age span of 530‒465 Ma, with a peak of c . 503 Ma. The HP–UHT metamorphism of the Qingshuiquan granulite terrane reveals a complete process from arc compression at the early stage of subduction to extension on the active continental margin related to rollback of the subducted slab in the Early Palaeozoic, driven by heat transfer from upwelling of the sub‐arc hot asthenosphere. The timing and P‒T conditions of the high geothermal gradient metamorphism can be used to understand the subduction and collision process of the Proto‐Tethys Ocean, including the relationship between high‐ P and high‐ to ultra‐temperature metamorphism in the East Kunlun and other orogenic belts.

AbstractThe East Kunlun Orogen (EKO) is the NW part of the Central China Orogenic Belt, which records the evolutionary history of the Proto- and Palaeo-Tethys Oceans from the Cambrian to the Triassic. An Early Palaeozoic eclogite belt has been recognized in recent years, which extends discontinuously for ∼500 km as three eclogite-bearing terranes. In this study, we report an integrated study of zircon grains from mica-schists accompanying the eclogites, in terms of mineral inclusions, U–Pb age systematics andP–Tconditions. The presence of coesite is identified, as inclusions within the metamorphic domain of zircons, which provides unambiguous evidence for subducted terrigenous clastic rocks of the Proto-Tethys Ocean exhumed from coesite-forming depths. U–Pb dating of the metamorphic zircons yields a concordia age of 426.5 ± 0.88 Ma, which is likely to be the time of ultrahigh-pressure metamorphism in the Kehete terrane.P–Tcalculations suggest that metapelite may have experienced a clockwiseP–Tpath with peakP/Tconditions of 685 ± 41 °C and >28 kbar, and equilibrated at 482–566 °C and 5.6–8.9 kbar during subsequent exhumation. The high-pressure – ultrahigh-pressure (HP-UHP) metamorphic belt within the EKO may have formed by collision between the Qaidam Block and the South Kunlun Block, as a consequence of the closure of the Proto-Tethys Ocean.

The Qilian Orogen, which lies between the Alashan Block in the north and the Qaidam Block in the south, records the entire tectonic evolution of Proto-Tethys Ocean from its spreading, subduction, closing, marginal accretion to continental collision. There are three different types of parallel ophiolite belts extending in the Qilian Orogen, from south to north, including: (1) the South Qilian ophiolitic complex belt with oceanic plateau-type, MOR-type and backarc-type ophiolites in the south; (2) the Tuoleshan MOR-type ophiolite belt in the middle; and (3) the Zoulangnanshan back-arc-basin (BAB)-type ophiolite belt in the north. The southern oceanic plateau-type ophiolite belt is represented by Lajishan-Yongjing ophiolite, which is a typical ophiolite fragment of oceanic plateau originated from the Cambrian mantle plume activity during ca. 525 similar to 500Ma. The middle ophiolite belt extends along Aoyougou, Yushigou, Binggou and Yongdeng, representing the ophiolite complex of the MOR-type oceanic crust with the ages of 550 similar to 495Ma. The northern ophiolite belt contains forarc-type and BAB-type ophiolites. The forarc-type ophiolite, represented by the Dachadaban ophiloite, formed in response to the processes from subduction initiation, forearc extension to back-arc basin development during 517 similar to 487Ma; the BAB-type ophiolite, represented by the Jiugequan and Laohushan ophiolites, are the SSZ-type ophiolite and the products of back arc basin extension during Ordovician (490 similar to 445Ma). The three ophiolite belts are the products formed in different tectonic settings during the history of Qilian Ocean from Neoproterozoic to Early Paleozoic, and thus is of great significance in understanding the tectonic evolution of Proto-Tethys Ocean in Qin (ling)-Qi (lian)-Kun (lun) Orogenic Belt. The timing and distribution of the three ophiolite belts and arc volcanics constrain a northward subduction polarity.

Generation of arc volcanic rocks is a complicated process, potentially involving some or all of oceanic slab subduction, mantle wedge metasomatism, and melting of both the subducted slab and the mantle wedge. Interaction between melts of different sources plays important roles in the subduction factory and the formation of magmatic arcs and is still controversial. To elucidate the mantle-melts interaction process, we present a study of the sodic adakite-like lavas (445-444 Ma) and basalt-basaltic andesite (basaltic) enclaves (similar to 453 Ma) in the early Paleozoic intra-oceanic arc system from the South Qilian Accretionary Belt, Qilian Orogen. Basaltic enclaves in the Muli adakite-like lavas have high Mg# (62-69) high Cr and Ni, and radiogenic Sr and unradiogenic Nd isotopic compositions (I-sr = 0.706327-0.706497; epsilon(Nd)(t) = -3.2 similar to - 2.0). They are considered to represent primitive magmas derived from a highly metasomatized mantle wedge. The adakite-like lavas are dacitic with adakite-like compositions, enriched Sr-Nd isotopic compositions (I-sr = 0.705597-0.706747; epsilon(Nd)(t) = -1.7 similar to + 0.4) and significantly variable zircon Hf isotopic compositions (epsilon(Hf)(t)= +0.9 similar to + 17.8), most likely a hybrid magma produced by mixing primitive basaltic melts of metasomatized mantle wedge and melts of subducted oceanic crust. The Muli adakite-like lavas and basaltic enclaves occurred in an intra-oceanic island arc setting with an unusually hot thermal structure, which induced melting of subducted slab (including sediments and mafic oceanic crust) and metasomatized mantle wedge. The geodynamic mechanism associated with slab melting is plausibly either or both of the juvenile nature of South Qilian Oceanic Crust, and/or the heating of a slab edge by upwelling hot asthenospheric mantle during subduction initiation. (C) 2019 Elsevier B.V. All rights reserved.

The East Kunlun Orogen, the northwestern part of the Central China Orogenic Belt, is a long-lived accretionary orogenic belt that records the evolution and eventual destruction of branches of the Tethys Ocean, from the Cambrian to the Triassic. Here we report an Early Paleozoic eclogite belt that extends for similar to 500 km within the East Kunlun Orogen. This belt consists of eclogite blocks, metasedimentary rocks and minor serpentinite blocks, accompanied by ophiolites (530-460Ma) and concurrent arc volcanic sequences and granitic plutons. Geochemical data show that the eclogites have normal mid-ocean ridge basalt-to ocean island basalt-like compositions. U-Pb dating of metamorphic zircons from eclogites and their surrounding rocks gave peak and retrograde metamorphic ages of 430-410Ma. Coesite pseudomorphs in garnet, quartz exsolution rods in omphacite and P-T calculations suggest that some eclogites experienced ultrahigh-pressure (UHP) metamorphic conditions at 29-30 kbar and 610-675 degrees C; these could represent oceanic crust subducted to and exhumed from coesite-forming depths (100-120 km). The UHP metamorphic eclogite belt in the East Kunlun Orogen may represent the final closure of the Proto-Tethys Ocean (opening at similar to 580Ma, subduction initiating at similar to 520Ma) at similar to 430-410Ma in the East Kunlun, with the formation of the Pan-North-China Continent in the Early Paleozoic and expansion of the Paleo-Tethys Ocean in the south.

>Ultrahigh-pressure(UHP) metamorphism plays important roles in continental dynamics. Understanding of this process has been enhanced by the discovery of coesite and micro-diamond from supracrustal rocks [1]. Such UHP mineral inclusions provide unequivocal evidence that the least dense part of the lithosphere