Neoproterozoic To Early Paleozoic Tectonic Evolution In The East Kunlun Orogen And The North Qaidam Orogen

The East Kunlun orogen and the North Qaidam orogen,located at the south and north margin of the Qaidam Block,respectively,are the important components of Central Orogenic Belt.These areas documented the multi-stage subduction/collisional orogenic processes.Thus,they are ideal places to study the tectonic evolution of the Tibetan plateau.Recently,many scholars have concentrated on its tectonic evolutionary history during Neoproterozoic to Early Paleozoic.However,some key scientific issues remain controversial,such as the detailed subduction and collision processes during the early Neoproterozoic,continental break-up process during the Neoproterozoic and continent-continent process during the early Paleozoic in the region.This seriously restricts the understanding of tectonic evolution of the East Kunlun orogen and the North Qaidam orogen.Therefore,this study carried out detailed mineralogical,petrological,geochemical and geochronological studies on the eclogites,intermediate magmatic rocks,granitic gneisses and metasedimentary rocks from the two orogenic belts.Combined with the regional geological data,we explored the contents of the tectonic affinities between the East Kunlun orogen and the North Qaidam orogen and adjacent blocks during the Neoproterozoic,and petrogenesis of Neoproterozoic granitoids and Early Paleozoic intermediate magmatic rocks,and protolith origin and metamorphic processes of the eclogites.A tectonic evolution model for the East Kunlun orogen and the North Qaidam orogen during the Neoproterozoic to Early Paleozoic has been proposed.The main conclusions of our study can be summarized as follows:(1)Zircon U–Pb dating results reveal that the protoliths of the granitic gneisses from the Xiarihamu and Yuka within the East Kunlun-North Qaidam orogenic belts were formed between 935 Ma and 928 Ma.Whole-rock geochemical data reveal that these granitic rocks are characterized by high-K calc-alkaline and peraluminous,with enrichment of light REE and depletion of heavy REE and HFSE(e.g.,Nb,Ta,and Ti),and significant negative Eu anomalies(Eu/Eu* = 0.30-0.52).The Xiarihamu and Yuka granitic gneisses show negative whole-rock εNd(t)values(–5.82 to –3.85 and–8.32 to –6.03,respectively)with two-stage Nd model ages of 2.05–1.89 Ga and2.25–2.06 Ga,respectively.Zircon Lu–Hf analysis show that they have low εHf(t)of-4.8 – +1.3 and-10.9 –-1.2,with two-stage Hf model ages of 2.12 Ga – 1.73 Ga and2.50 Ga ~ 1.88 Ga,respectively.The geochemical and Nd-Hf isotopic compositions of these granitic gneisses,coupled with the facts that they contain abundant Paleo-Mesoproterozoic inherited zircons and have typical Al-rich minerals(e.g.,muscovite),reveal that their protoliths are S-type granites that were mainly derived from partial melting of the Paleo-Mesoproterozoic metasedimentary rocks,and they most likely formed in syn-collisional settings during the amalgamation of Rodinia.(2)The East Kunlun orogen and the North Qaidam orogen are distinguished by the widespread distribution of eclogites.However,the protolith nature of these eclogites remains controversial.Zircon U–Pb dating results of these eclogites indicate that the protolith ages of the Yuka eclogites are 831–809 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 North Qaidam orogen 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.(3)The Mesoproterozoic metasedimentary rocks from the Shaliuhe Group in the North Qaidam orogen contain the youngest detrital zircon ages of 1228–1209 Ma.They display a major age peak at 1.9–1.8 Ga and the subordinate populations display ages of 1.6–1.2 Ga and 2.6–2.0 Ga.The detrital zircon U-Pb age spectra and Hf isotopic compositions of the Mesoproterozoic metasedimentary rocks in the North Qaidam orogen are similar to those of the rocks in the Qilian and Yangtze blocks,and they may share similar provenance areas.The youngest groups of detrital zircons for samples of the Jinshuikou Group and Shaliuhe Group that have clear oscillatory zoning and high Th/U ratios(> 0.1)yielded weighted mean ages of 710 Ma and 610 Ma,representing a maximum sedimentation age of their protolith.The Neoproterozoic metasedimentary rocks from the Jinshuikou Group in the East Kunlun orogen and Neoproterozoic metasedimentary rocks from the Shaliuhe Group in the North Qaidam orogen share similar age spectra,and yielded dominant age peaks at ca.1.0–0.9 Ga,and second age peaks at 2.6–2.1 Ga,1.8–1.1 Ga and 840–610Ma.The1.0–0.9 Ga zircon grains from the studied samples might be mainly derived from nearby coeval granitiod plutons.We consider that East Kunlun-Qaidam linked with Western Yangtze block during late Mesoproterozoic to middle Neoproterozoic.(4)The eclogites from the East Kunlun orogen and the North Qaidam orogen yielded metamorphic U-Pb zircon ages of 441–439 Ma and 439–436 Ma,respectively.On the basis of detailed petrological and mineral chemical studies,we used conventional thermobarometry to obtain the clockwise P–T paths of the eclogites in Yuka and Dulan.Combined with available regional geological data,we suggest that the eclogites from the East Kunlun orogen and the North Qaidam orogen experienced different subduction and exhumation processes.The syn-collision stage in the East Kunlun orogen and the North Qaidam orogen were initiated at ca.445–440 Ma and lasted to ca.420 Ma.The diorite porphyrites from Xiarihamu and diorites from Yuka formed at ca.412 Ma and ca.445 Ma,exhibiting sub-alkaline and metaluminous compositions.They display “arc-like” geochemical characteristics and depleted zircon Hf isotopic compositions.They were most likely generated by partial melting of lower crust,which was affected by the rising mantle-derived magmas that had been previously metasomatized by subduction-related fluids.the early Devonian(412 Ma)diorite porphyrites from Xiarihamu formed in a post-collisional extensional environment,probably associated with slab break-off.The late Ordovician(445 Ma)diorites from Yuka formed in a continental collision environment,related to slab rollback during initial collision.(5)Together with research results of this paper and available geological data,we suggest that the tectonic evolution of the East Kunlun orogen and the North Qaidam orogen since the Neoproterozoic to Early Paleozoic is as follows:(1)The early Neoproterozoic is oceanic subduction and continental collision stage,which generated extensive volcanic arc and syn-collisional magmatism,in response to the assembly of the Rodinia supercontinent.(2)The middle–late Neoproterozoic is a continental break-up stage,which formed voluminous mafic magmatism,in response to the break-up of the Rodinia supercontinent.In the late Neoproterozoic,rifting led to the opening of the Kunlun Ocean and South Qilian Ocean and passed into the stage of ocean evolution.(3)Early Paleozoic: The North Qaidam orogen has experienced oceanic subduction(530–445 Ma)and continent subduction/collision stage(445–420Ma)stages.Most Neoproterozoic mafic rocks were subducted to mantle depths and formed continental-type eclogites.The East Kunlun orogen has experienced oceanic subduction(550–440 Ma)and continent subduction/collision stage(440–420 Ma)stages.The East Kunlun-North Qaidam orogenic belts have experienced slab break-off and the exhumation of HP/UHP metamorphic rocks during 420–370 Ma.