STUDY ON VOLCANIC ROCKS IN LONGKANG AND ZHANGZHA,JIUZHAIGOU,SICHUAN PROVINCE

The paper presents the petrologic characteristics,ages and its tectonics significance of the volcanic rocks in Longkang and Zhangzha,Jiuzhaigou,Sichuan Province.The volcanic rocks appearing alternately with marine carbonate and siliceous rock present stratiform and bedded rocks under spilitization,and so they should belong to paleo marine volcanic rocks.Their acidity is mainly basic and intemediate basic.While their alkalinity series contain alkalic ones and tholeiitic ones.The R Na type has both potassic and sodic type.Their REE and trace element distribution patterns intervene between centinental rift pattern and vicinal region of Shanxi,Gansu and Sichuan Proterozoic Eon paleo ocean basin pattern of Laji Mountains Cambrian Period.The volcanic rocks are assumed to be the products of small sized intercontinental ocean basin period.The investigation verifies that volcanic rocks in this place are of two periods.Those of one period possess strong spilitization.A large quantity of conodonts of Devonian Period are found in the carbonate appearing alternately with them.Accordlingly,the actual period of the volcanic rocks belongs to Devonian Period. Their K Ar isotopic age is 224-277 Ma,and represents the age of paleo ocean basin closure orogenesis and the age of volcanic rocks spilitization.Volcanic rocks in this period are formed in the early and middle stage of Southern Qinling Mountains ocean of Southern Qinling Mountains Devonian Period,whereas the rocks received metamorphism of greenschist facies and turned into spilite Keratophyre in the orogenesis stage of Southern Qinling Mountains'main body,middle Triassic Period,Indosinian movement main episode.However volcanic rocks of another period are very fresh,with K Ar isotopic age of 226 Ma.They are believed to be the products of Southern Qinling Mountains ocean reopening in early and middle stage of late Triassic Period.As last episode of Indosinian movement in late Triassic Period was not severe in southern Qinling Mountains'final orogenesis movement,the metamorphic rank of volcanic rocks is very low.