Characteristics of Mesoproterozoic Granites and Their Geological Significances from Middle Tianshan Block,East Tianshan District,NW China

Abstract

A suit of pluton is identified,consisting of the typical rock association: gneissic diorite,gneissic granodiorite,gneissic monzonite granite,gneissic K-feldspar granite,from metamorphic rocks of the Xingxingxia Group,Middle Tianshan Block,Tianshan Orogenic Belt,NW China.16 zircons from Middle Tianshan gneissic granodiorites were dated using SHRIMP U-Pb method.A weighted average apparent age of(1 453±15)—(1 458±40) Ma(207Pb/206Pb) attained,is equal to the late Mesoproterozoic era,which represents the crystal age of the source rocks of gneissic granodiorite.The petrochemical and geochemical characteristics of the intrusive rocks shows a high-K calc-alkaline and I type granites,formed in convergent environment.It is a magmatism evidence for the existence and the closing event of the Proterozoic Tianshan Ocean according to the results of the SHRIMP U-Pb,which gives a good minimum ceiling of the age of Xingxingxia Group.Besides,the discovery of a residual zircon SHRIMP U-Pb age of(2 991±22) Ma indicates the basement time of the Middle Tianshan may cast back to the Archaean era.