GRANITOIDS,VOLCANIC ROCKS AND CHERTS FROM NORTH ALTYN TAGH,NW CHINA: IMPLICATION FOR THE TECTONIC ENVIRONMENT DISCRIMINATION

Granitoids,volcanic rocks and cherts,mainly of early Paleozoic,in northern Altyn Tagh(Fig.1) are analyzed here for the purpose of the determination of the tectonic environments and their development since Early Paleozoic.(1) Granitoids The collection of 128 main|element petrochemical data from North Altyn Tagh area shows that most of the granitoids here are granites and granodiorites of calc|alkaline series.They consist mainly of metaluminous and peraluminous in Shand's index,and only one of them,which belongs to Mesozoic,is peralkaline.Most of the granitoids plot in the IAG CAG CCG fields in the Maniar and Piccoli's diagrams (1989) for tectonic discrimination of granitoids.IAG (Island arc granitoids)and CAG (Continental arc granitoids) can be distinguished for Early and Late Paleozoic granitoids,and maybe some CCG (Continental collision granitoids) for Early Paleozoic.Granitoids of Mesozoic and Cenozoic inherited the characteristics of those of Paleozoic.Destructive active plate margin (pre|plate collision)and Anatectic magmatism(syn|orogenic,S|type granites)are distinguished (Fig.2)for Paleozoic granitoids using de la Roche R 1| R 2 multicationic diagram (Batchelor and Bowden,1985).Mesozoic and Cenozoic granitoids inherited the characteristics of Anatectic magmatism (syn|orogenic)of these early ones,and post|orogenic(A|type)granites occurred in Mesozoic.The mechanism for magma formation is mainly partial melting.