藏东卡贡地区早侏罗世似斑状钾长花岗岩LA-ICP-MS锆石U-Pb年龄及地球化学特征

LA-ICP-MS zircon U-Pb age and geological characteristics of the Early Jurassic potassium porphyritic granites in Kargang area,eastern Tibet

用LA-ICP-MS技术测得卡贡地区似斑状钾长花岗岩锆石的206Pb/238U年龄为178±2 Ma(MSWD=3.7,n=20),属于早侏罗世的产物。岩石地球化学特征表明,岩石具富碱富钾(K2 O+Na2 O=8.02%~8.76%,K2 O/Na2 O=1.71~2.11)、低铝(Al2O 3=14.08%~14.73%,A/CNK=1.01~1.04),不含透辉石和刚玉分子较低的特征;岩石普遍富集Rb、Th、K、U等大离子亲石元素,明显亏损Ba、Nb、Sr、P、Ti等元素,高场强元素Zr、Nb、Ta含量极低,(Ga/Al)×10^4值为4.48~5.06,Rb/Sr值为1.84~2.29,均显示出岩浆分异程度较高的特征;岩石的形成与俯冲消减作用有关,其构造环境由挤压环境逐渐向相对伸展环境转变,伴随着地壳深部压力减小,幔源岩浆上涌,并与上覆地壳发生部分熔融作用而形成。

In this paper,LA-ICP-MS zircon U-Pb age test was carried out on the potassium porphyritic granites in the Kargang area,which shows that the 206Pb/238U age weighted average value of the rock mass is 178±2 Ma(MSWD=3.7,n=20),suggesting a product of Early Jurassic.The geochemical characteristics of the rocks show that the rocks have alkali-rich potassium(K2O+Na2O=8.02%~8.76%,K2O/Na2O=1.71~2.11),but low aluminum(Al2O3=14.08%~14.73%,A/CNK=1.01~1.04),without pyroxene and with lower corundum molecules.The rocks are generally characterized by rich Rb,Th,K,U and other large ionic pro-stone elements,obvious loss of Ba,Nb,Sr,P,Ti and other elements,and very low values of high field strength elements Zr,Nb,with(Ga/Al)×10^4 values of 4.48~5.06 and Rb/Sr ratios of 1.84~2.29,which shows the characteristics of high magma differentiation;in addition,the formation of rocks was related to subduction reduction,and its tectonic environment changed gradually from extrusion environment to relative stretching environment,accompanied by the decrease of deep crustal pressure,which led to the upwelling of mantle source magma and the partial melting of the overlying crust.