Genesis Of Magmatic Rocks In Western Kunlun Orogenic Belt And Implications On The Evolution Of Proto-Palaeo Tethys Ocean

The Tethys Ocean is a long-standing,latitudinally spreading palaeo-ocean located between the Raua Palaeo-continent in the north and the Gondwana Palaeo-continent in the south.The evolution of the Tethys Ocean is divided into three stages: the ProtoTethys Ocean,the Paleo-Tethys Ocean and the Neo-Tethys Ocean.The West Kunlun orogenic belt is the main tectonic unit of the Central Orogenic Belt and can be divided into the Northern Kunlun Terrane,Southern Kunlun Terrane,and Tianshuihai Terrane.These micro-terranes have experienced a series of important magmatic events during the Paleozoic-Mesozoic,recording the history of the Proto-Paleo Tethys Ocean from its origin to its annihilation,and are important vehicles for understanding the evolution of the Proto-Paleo Tethys Ocean and important mineralisation events.In this paper,the major and trace element geochemistry,geochronology and isotope studies were carried out on the Paleozoic-Mesozoic magmatic rocks of different blocks in the West Kunlun,the magmatic ages,geochemical characteristics and tectonic background were explored in depth.The main insights and results obtained are as follows.The Early Cambrian magmatic rocks are exposed in Zankan,Laobing,Hekelaner and Chahekou area,and are mainly composed of moderate-acidic intrusive rocks,with diorite-gabbro enclaves,which have been dated by zircon SHRIMP and LA-ICP-MS U-Pb age of 348 ～ 354 Ma.The Early Cambrian intrusions in the Tianshuihai Terrane are mainly I-type granites,which are the product of dehydration and melting under lowpressure conditions of a low-K basalt source,while the diorite-gabbro enclaves are the product of magmatic mixing between mantle-derived magmas and granites.Most of the granite samples in the Tianshuihai Terrane show Nd-Hf isotope decoupling,probably due to unbalanced melting in the source region.The isotopic signature of granites in the Tianshuihai Terrane implies the existence of two types of lower crust in the Tianshuihai Terrane.The Early Palaeozoic magmatism in the Tianshuihai Terrane may have been influenced by southward subduction of the Proto-Tethys Ocean.According to the palaeogeographic reconstruction and the age distribution of magmatism,the TSHT and the SKT were assumed to have been distributed between the northern margin of the East Gondwana continent and the Tarim Block.Thus,two branches of the Proto-Tethys Ocean coexisted,which was represented by the Kangxiwa Fault and Kudi ophiolite belt in the Early Palaeozoic.Furthermore,depending on the chronological statistics of micro-blocks in the northern margin of the East Gondwana continent,the subduction of the Proto-Tethys Ocean is diachronous,which initiated in the northwestern region of the East Gondwana continent and propagated eastwards.The Carboniferous Wuluate Formation consists mainly of pillow basalts interbedded with minor rhyolites,and is a bimodal volcanic assemblage.LA-ICP-MS zircon U-Pb dating of the basalts and rhyolites of the Wuluate Formation ranges from348 to 354 Ma,representing the age of eruption of the basalts and rhyolites.The basalts were formed by partial melting of spinel lherzolite to a degree of 5-10% and the source area shows a characteristic depleted mantle,which is consistent with the zircon εHf(t)values(+7.3 to +11.8).The modelling results show that the basalts have undergone crystalline differentiation of olivine,monazite,hornblende,plagioclase and magnetite to a degree of 40% to 95%.The rhyolites of the Wuluate Formation are developed by partial melting of lower crustal material under low pressure conditions.Combining geochemical data with regional geological features,this paper concludes that the volcanic rocks of the Wuluate Formation were deposited in a back-arc basin associated with the northward subduction of the Palaeo Tethys Ocean.The volcanic rocks of the Permian Qipan Formation are a suite of niobiumenriched basalts with LA-ICP-MS zircon U-Pb ages of 287±4.6 Ma.The basalts of the Qipan Formation were formed by dynamic melting of spinel lherzolite to a degree of5%-7% and underwent olivine,monazite and plagioclase crystallographic differentiation to a degree of 0%-50%.Zircons from basalts show the εHf(t)values of+0.2 to +11.5 are suggestive of a heterogeneous sourceand possible incorporation of enriched components.The abrupt transition from a depleted mantle in the Carboniferous,influenced by the subduction of the Palaeo Tethys Ocean,to a Permian enriched/primitive mantle in the West Kunlun area which was likely influenced by the incorporation of deep mantle material and the possible existing mixing of Oceanic Island Basalt(OIB)+ Island Arc Tholeiite(IAT)± Mid-Ocean Ridge Basalt(MORB)components in its source area.It is hypothesised that the basalts of the Permian Qipan Formation may be the result of interaction between the subduction zone and a mantle plume.Triassic magmatism is widely distributed in the Southern Kunlun Terrane and the Tianshuihai Terrane.The LA-ICP-MS zircon U-Pb ages of the Shazigou and Saluoyi pluton range from 213.7 ± 2.6 to 237.3 ± 2.8 Ma.The Shazigou and Saluoyi pluton are products of strong mixing of crustal-derived granitic magmas with mantle-derived basaltic magmas,and have undergone different degrees of plagioclase and K-feldspar crystal differentiation,respectively.Combined with regional data,the Triassic granites may have been developed in a Palaeo Tethys Ocean subduction to collision background.Combining the results of this paper with regional geological data,it is concluded that the West Kunlun Orogenic Belt developed a series of important PaleozoicMesozoic magmatic events,which recorded the evolutionary history from the initial subduction of the Proto-Tethys Ocean to the closure of the Paleo-Tethys Ocean,and formed a series of volcanic-sedimentary metamorphic iron ore,sedimentary manganese ore,pegmatitic lithium-beryllium ore and so on,during the complex and long oceanland transition and accretion-collision process.