西藏墨竹工卡地区早侏罗世花岗岩地球化学、岩石成因及其地质意义

Geochemistry,Petrogenesis and Geological Significance of Early Jurassic Granite in Mozhugongka Area,Tibet

为了研究南冈底斯晚三叠世-早侏罗世时期岩浆岩的成因类型和构造背景,针对墨竹工卡地区的松多黑云母二长花岗岩体进行岩相学、年代学、全岩地球化学研究.LA-ICP-MS锆石U-Pb定年结果显示,松多黑云母二长花岗岩结晶年龄为190.2±2.9 Ma,形成于早侏罗世.在地球化学组成上,黑云母二长花岗岩具有低TiO2(0.68%~0.75%),富SiO2(65.22%~66.13%)、Al2O3(16.26%~16.73%)、Na2O(4.05%~4.29%)、K2O(3.96%~4.24%)的特点,显示钾玄岩系列和弱过铝质(A/CNK=1.04~1.11)的主量元素地球化学特征;在微量元素蛛网图上,具有富集Rb、Th、K、Zr、Hf元素和亏损Ba、Nb、Ta、Sr、Ti、P元素的特征;锆石饱和温度介于805~835℃,FeOT/MgO比值高,样品显示出具有部分A型花岗岩特征.结合前人研究表明,晚三叠世-早侏罗世时期南冈底斯岩浆岩构造背景与新特提斯洋北向俯冲有关,松多黑云母二长花岗岩形成于新特提斯洋板片北向俯冲引起的弧后伸展环境;其成因与软流圈上涌导致幔源岩浆底侵引起下地壳的部分熔融有关.

In order to explore the petrogenesis types and tectonic setting of the Late Triassic-Early Jurassic magmatic rocks in the southern Gangdese,we report detailed petrography,geochronology,and whole-rock geochemistry of the Songduo biotite monzogranite plutons in the Mozhugongka area.The zircon LA-ICP-MS U-Pb dating results show that the age of the Songduo biotite monzogranite plutons is 190.2±2.9 Ma,indicating that it was formed in the Early Jurassic.In terms of geochemical composition,the Songduo biotite monzogranite plutons have low TiO2(0.68%-0.75%),high SiO2(65.22%-66.13%),Al2O3(16.26%-16.73%),Na2O(4.05%-4.29%),and K2O(3.96%-4.24%)contents,belonging to the shoshonite series and weaklyperaluminous(A/CNK=1.04-1.11);In the spidergram,the samples show enrichment in Rb,Th,K,Zr and Hf,and depletion in Ba,Nb,Ta,Sr,Ti and P.The whole-rock zircon saturation temperature varies from 805 to 835℃,with high FeOT/MgO ratios.The Songduo pluton has an affinity with A-type granite.In combination with pervious research results,it shows that the Late Triassic-Early Jurassic magmatic rocks in central and southern Gangdese were formed in magmatic arc related to the northward subduction of the Neo-Tethys oceanic slab.We suggest that the Songduo biotite monzogranite plutons generated in a back-arc extensional environment during the subduction of the Neo-Tethys oceanic slab,and the formation of granite is related to the partial melting of the lower crust caused by the intrusion of mantle-derived magma.