Ages and geological significance of the dark dykes emplaced in the Karamay pluton and adjacent area,in western Junggar,Xinjiang,NW China:Evidence form LA-ICP-MS zircon chronology and Ar-Ar amphibole chronology

There are large number of dark diorite porphyrite dykes emplaced in the Karamay pluton and its adjacent area,western Junggar,Xinjiang,NW China.They are considered to be formed by magma filling into the existing fractures,which can be briefly classified into three groups with different strikes.The LA-ICP-MS U-Pb dating for zircons from a diorite porphyrite dyke sample yields 206Pb/238U age of 303.1±1.2Ma.The Ar-Ar plateau age and inverse isochron diagram age of amphiboles from the same sample,are 312.1±2.8Ma(1120～1400℃)and 313.6±6.9Ma respectively.The zircon 206Pb/238U age of host rock is determined,by LA-ICP-MS zircon U-Pb dating,as 319.0±1.0Ma,and zircon 206Pb/238U age of a granite porphyry vein is 315.3±1.0Ma.These chronology data show that the formation of the dykes in this area is not Permian,but Late Carboniferous,and that before filling of those dykes,the granitic magma intrusion occurred in western Junggar area,resulting in Karamay pluton and granite porphyry veins.The above dating data of the dykes and the host rock imply that the fractures,filled by the dykes,occurred from 315Ma to 303Ma.This study gives the new temporal constraint for further research in both Late Paleozoic geodynamical environment and magmatic intrusion of western Junggar,Xinjiang,NW China.