东昆仑百吨沟花岗岩岩石地球化学、锆石U-Pb年龄与Hf同位素特征及其构造意义

Geochemistry,zircon U-Pb chronology and Hf isotopes of the Baidungou granite from the East Kunlun Orogen and their implications

东昆仑造山带志留纪-泥盆纪的演化存在着争议,A型花岗岩常用来限定造山过程中构造转换的时间。百吨沟花岗岩岩体位于东昆仑造山带东段的五龙沟金矿集区,目前缺乏系统的研究,本文对其进行了岩石地球化学、锆石U-Pb年代学及Hf同位素研究。锆石U-Pb定年表明,该岩体结晶年龄为417.9±3.5 Ma。岩石地球化学分析结果显示,岩石高硅(SiO_(2)=71.2%~77.2%)、富碱(K_(2)O+Na_(2)O=8.23%~10.70%)、低MgO(0.04%~0.22%)和CaO(0.09%~0.95%),具有富集大离子亲石元素(Rb、K、Th、U)、亏损高场强元素(P、Ti)的特征,具有明显的负铕异常和"海鸥型"稀土元素配分模式,并且岩石具有较高的Ga/Al、TFeO/MgO值以及Zr饱和温度,表明该岩体为A型花岗岩。此外,锆石的εHf(t)=-0.6~1.9,对应的二阶段模式年龄t2DM介于1442~1284 Ma之间,推测该A型花岗岩主要源自早期具有相似Hf同位素的长英质火成岩部分熔融。该花岗岩具有A2型花岗岩的特征,指示其形成于后碰撞伸展环境。东昆仑造山带发育大量近同时的A2型花岗岩,反映其在晚志留世时进入后碰撞伸展阶段。

The Silurian-Devonian evolution of the East Kunlun Orogen(EKO)is controversial.A-type granites are often used to constrain the timing of tectonic transformation during the orogenic process.The Baidungou granite pluton,located in the Wulonggou goldfield in the eastern part of the EKO,has not been systematically studied in previous research.Therefore,this paper takes it as the research object to study its petrogeochemistry,zircon U-Pb geochronology and Hf isotopes.The zircon U-Pb dating shows the crystallization age of the intrusion is 417.9±3.5 Ma.Petrogeochemical studies reveal that the intrusion is characterized by high silicon(SiO_(2)=71.2%~77.2%)and alkali(K_(2)O+Na2O=8.23%~10.7%),low MgO(0.04%~0.22%)and CaO(0.09%~0.95%),enriched large ion lithophile elements(Rb,K,Th,U),depleted high field strength elements(P,Ti),obvious negative europium anomaly,and'gull type’REE patterns.Combined with high Ga/Al and TFeO/MgO values and Zr saturation temperatures of whole rocks,the intrusion is classified as A-type granite.In addition,the zirconεHf(t)values vary from-0.6 to 1.9 and two-stage Hf model ages range from 1442 to 1284 Ma,suggesting that this A-type granite was mainly derived from the partial melting of early felsic magmatic rocks with similar Hf isotopes.This granite exhibits characteristics of the A2-type,indicating that it formed in a post-orogenic extensional environment.The EKO developed a large number of A2-type granites nearly contemporaneous with this granite,reflecting it had entered the post-collision extension stage during the Late Silurian.