Early Paleozoic Collisional Orogeny And Magmatism On Northern Margin Of The Qaidam Basin

The northwest-trending Early Paleozoic collisional orogenic belt located on northernmargin of the Qaidam basin is a part of the Central Orogenic belt and one of a few places inChina where HP-UHP metamorphic rocks exposed. Based on the data of 1:50 000 and 1:250000 regional mapping and the theories of plate tectonics and continental orogeny, this paperstudies basically the framework of the orogen, magmatism and metamorphism duringorogenic process and presents zircon U-Pb isotopic ages of igneous and metamorphic rocksfrom the orogen.The Early Paleozoic orogenic belt consists of three tectonic units: (1) the Yuqiahe-Shaliuhe HP-UHP metamorphic belt, which is predominantly composed of Neoproterozoicgranitoid gneisses and metamorphic supercrutal rocks (Yuqiahe/Shaliuhe Group complex)with minor HP-UHP metamorphic rocks such as eclogite;(2) the Tanjianshan ophiolitecomplex – volcanic arc belt, which consists of ophiolite complex, volcanic island arc,back-arc basin assemblage and arc granitoid rocks;(3) the Olongbruk micro-block, whichhas a double-layered structure, i.e. a metamorphic basement composed of the Delinghacomplex and a cover composed of the Quanji Group.The igneous petrotectonic assemblages formed in the orogenic process include: (1)ophiolite complex assemblage, composed predominantly of the ultramafic-mafic cumulates,minor metamorphic peridotite, pillow lava, cherts and leucocratic granites as well.Geological and geochemical characteristics indicate that ophiolite complex is classified as"SSZ"(supra-subduction zone) type;(2) island arc volcanic assemblage, a suit oflower-grade metamorphic calc-alkaline volcanics, composed mainly of intermediate-basicvolcanic rocks with minor acidic volcanic rocks and formed before ~450Ma;(3) back-arcbasin volcanic assemblage, associated with a suit of terrigenous flysch, which underwentmetamorphism at ~435Ma;and (4) arc granitoid rocks assemblage, a suit of calc-alkalinequartz-diorite, tonalite, granodiorite and granite, whose alkalinity increases toward continent,and formed during two stages of 465-435Ma and 435~400Ma, and accompanied bysyn-collisinal granites at later stage.The study of metamorphism indicates that the orogen has a paired-metamorphic beltstructure, i.e., co-existence of Yuqiahe-Shaliuhe HP-UHP metamorphic belt and Tanjianshanisland-arc lower-medium pressure metamorphic belt. Rocks in the HP-UHP metamorphicbelt, including pelites and granitic gneisses, was probably all or partly subducted into thedeep interior of the lithosphere and underwent HP metamorphism, but some eclogites wentup to the UHP metamorphic condition. Metamorphic temperatures and P-T-t paths ofHP-UHP metamorphic rocks are spatially diverse. Longitudinally, the P-T-t path of eclogitesfrom Yematan and Xitieshan differ from the one in Lüliangshan area. While the former hadbeen overprinted by granulite facies metamorphism during exhumation, the later onlyreached amphibolite facies metamorphism. Laterrally, the metamorphic temperature andpressure increase toward the geosuture. The island-arc belt underwent chiefly LP-MPamphibolite-greenschist facies metamorphism, and the metamorphic facies in the back-arcassemblage changed from high-amphibolite facies to greenschist facies from island-arc tocontinent.Based on the studies of petrology, geochemistry and zircon U-Pb isotopic dating, wehave established the sequence of chronotectonic events in the orogenic belt and the geneticrelationship among magmatism, metamorphism and orogenesis, and discussed the evolutionof the Early-Paleozoic Orogenic belt from the period of subduction of the oceaniclithosphere to that of the continent-continent collision.