东昆仑大岔沟地区晚三叠世侵入岩年代学、岩石地球化学特征及Hf同位素研究

Geochronology,geochemistry,and Hf isotopic composition of the Late Triassic intrusive rocks from Dachagou area,East Kunlun

东昆仑祁漫塔格大岔沟地区晚三叠世侵入岩岩性主要为中细粒二长花岗岩和花岗闪长岩,锆石LA-MC-ICP-MS U-Pb定年表明,二长花岗岩的形成年龄为(233±2)Ma,属晚三叠世岩浆活动产物。岩石地球化学特征上,里特曼指数σ为0.85~2.59,为钙碱性-高钾钙碱性系列;相对富集大离子亲石元素Rb和K,亏损Ba、Ta、Nb、Sr、P和Ti等元素;轻稀土元素明显富集,轻重稀土元素分馏较强,具有较强的Eu负异常;花岗岩含角闪石且SiO_2与P_2O_5具明显负相关性,指示其属I型花岗岩。二长花岗岩锆石ε_(Hf)(t)值为–10.36^+1.6,二阶段Hf模式年龄变化范围为1164~1922 Ma,可能主要由中元古代地壳物质部分熔融形成,并有少量古元古代地壳物质和幔源物质加入,岩浆在演化过程中存在分离结晶作用。综合分析认为大岔沟地区晚三叠世可能处于碰撞-后碰撞转换阶段,加厚地壳的拆沉作用导致幔源岩浆底侵下地壳,使其部分熔融形成区内侵入岩。

The intrusive rocks in Dachagou,East Kunlun,consist primarily of monzonite granite and granodiorite.LA-ICP-MS zircon U-Pb dating yielded an age of(233±2)Ma for the monzonite granite,indicating the Late Triassic magmatism.The lithogeochemical characteristics of these rocks reveal a Rittmann Index(σ)of 0.85– 2.59;they belong to the calc-alkalic series and high-K calc-alkalic series.The rocks are enriched in large-ion lithophile elements(Rb,K)and light rare earth elements(LREE),and depleted in Ba,Nb,Ta,Sr,P,Ti and heavy rare earth elements(HREE).The lithological and geochemical characteristics of intrusive rocks indicate that they belong to I-type granite.TheεHf(t)(from–10.36 to+1.6)of zircons from monzonite granite,and the two-stage ages(1164– 1922 Ma),indicate that they are partial melting from the Mesoproterozoic crust,with little contribution of Paleoproterozoic material and mantle-derived magma.The fractional crystallization during magma evolution is clearly identifiable.In conclusion,the tectonic background of the Dachagou area in East Kunlun in Late Triassic was the transfer time from collision to post-collision.Intrusive rocks were related to mantle-derived magma underplate into the lower crust,which caused by thickened crust delamination.