青海共和盆地中三叠世花岗岩组合岩石成因:地球化学、锆石U-Pb年代学及Hf同位素约束

Petrogenesis of Middle Triassic granite association in the Gonghe basin,Qinghai:Constraints from geochemistry,U-Pb ages and Hf isotopic

青海共和盆地位于青藏高原东北缘,以往有关共和盆地及邻区早-中三叠世岩浆岩成因机制的认识分歧较大,且研究主要集中在露头岩石方面。本文以共和盆地干热岩GR1井深部花岗岩岩芯样品作为研究对象,对其进行岩石学、长石电子探针、主微量元素地球化学、锆石U-Pb年代学及Lu-Hf同位素研究。矿物组成及长石电子探针测试结果显示花岗岩主要为奥长花岗岩、英云闪长岩及花岗闪长岩。锆石U-Pb测年结果表明,奥长花岗岩的结晶年龄为236.5±1.7Ma,英云闪长岩的岩浆结晶年龄为241.6±3.0Ma。主、微量元素地球化学显示这些花岗岩主要为准铝质,属于高钾钙碱性系列。全岩Ta-Nb-Hf等不相容元素图解及锆石Hf同位素数据表明共和盆地的236.5~241.6Ma花岗岩显示火山弧及同碰撞花岗岩成分特征,说明中三叠世共和地区发生俯冲-碰撞转换。结合本文数据与区域背景资料,作者认为:共和盆地早-中三叠世花岗岩组合的形成与印支期宗务隆洋的南向俯冲作用密切相关,此时,在宗务隆-青海南山-西秦岭北缘存在统一的大陆边缘弧环境;在236~241Ma时发生俯冲-碰撞转换;晚三叠世时宗务隆-青海南山-西秦岭北缘已处于碰撞期和后碰撞期。

The Gonghe basin is located on the northeastern margin of the Qinghai-Tibet Plateau.The petrogenesis of Early-Middle Triassic magmatic rocks in the Gonghe basin and its adjacent areas was still controversial since past studies mainly focused on the outcropped rocks.This paper reports petrology,electron microprobe analysis(EMPA)on feldspar,the major and trace element geochemistry,zircon U-Pb chronology and Lu-Hf isotopic compositions of the granitoid core samples of 2450~3000 m GR1 Well,which is for the geothermal resources of the hot dry rock(HDR)in the Gonghe basin,Qinghai.The petrography and EMPA results reveal that the granitic pluton is mainly composed of trondhjemite,tonalite and granodiorite;and the U-Pb dating results show that the magmatic crystallization ages of the trondhjemite and tonalite are 236.5 Ma and 241.6 Ma,respectively.The major and trace element geochemical characteristics show these granites are mainly metaluminous and belong to the high-K calc-alkaline series.Incompatible elements(Ta,Nb,Hf)of the Middle Triassic granites(236.5~241.6 Ma)and the Hf isotopic characteristics of the zircons in them indicate that the granite association has the volcanic arc affinity and syn-collision affinity,which means the subduction-collision transition occurred in the Middle Triassic in the Gonghe area.Based on the data in this paper and regional backgrounds,we suggest that the formation of the Early-Middle Triassic granite association in the Gonghe basin is closely related to the southward subduction of the Zongwulong Ocean during the Indosinian period.There is a unified continental marginal arc environment along the Zongwulong-Qinghai Nanshan-northern margin of West Qinling belt before Middle Triassic;and after the subduction-collision transition occurred at236~241 Ma,the belt was in the collision and post-collision period in the Late Triassic.