TECTONIC ENVIROMENTS OF MAGMATISM IN EARLY PALEOZOIC IN THE NORTH ALTYN TAGH, CHINA

The North Altyn Tagh is located at the northewst area to Altyn Fault between Tarim and Qaidam Basins. Granitoids from this area formed mainly in Ordovician,and are classified as calc alkaline based on alkaline-lime index,and metaluminous and peraluminous on Shand's index. I and A-types are the principal granitoids,no S-type granitoids found here. Volcanic rocks occurring together with ophiolite suite in this area,mainly of Early Paleozoic,consist of pillow basalts and rhyolites,and smaller amounts of trachybasalts,basaltic trachyandesites and dacites,belonging to subalkali and showing bimodal distribution in SiO₂-Na₂O+K₂O plot. Major-and trace-element trectonic discrimination of the granitoids shows the evolution of tectonic environments from destructive active plate margin/volcanic arc in Early Paleozoic,the continental apeirorogenic uplift in Mesozoic and post orogenic in Cenozoic of North Altyn Tagh. Tholeiitic basalts had formed in several environments from MORB to island arc and within-plate. Acid volcanic rocks here of calc alkali series may belong to within plate environment only. The existing of "Altyn Ocean" in early Paleozoic is then deduced from the analyses above.