东昆仑马尼特地区片麻状花岗闪长岩锆石U-Pb年代学、地球化学及其构造背景

Zircon U-Pb Geochronology and Geochemistry of the Gneissic Granodiorite in Manite Area from East Kunlun,with Implications for Geodynamic Setting

为确定东昆仑马尼特地区片麻状花岗闪长岩的形成时代、源区性质和构造背景,对其进行了锆石U-Pb年代学、地球化学和锆石Hf同位素研究.本次测试的片麻状花岗闪长岩锆石LA-ICP-MS(laser ablation inductively coupled plasma mass spectrometry)U-Pb加权平均年龄为495.6±1.1Ma(MSWD=0.13),属于晚寒武世.马尼特片麻状花岗闪长岩SiO_2含量为61.47%~63.99%,Na_2O、K_2O和CaO含量分别为2.91%~3.64%、0.93%~2.31%和4.29%~6.52%,全碱ALK=3.92%~5.69%,铝饱和指数A/CNK=0.83~0.97,属准铝质钙碱性系列岩石.岩石具有富集大离子亲石元素(Rb、K)和不相容元素(Th、U),相对亏损Nb、Ta、Zr、Ti高场强元素的特征,Nb/Ta、La/Nb、Th/Nb、Th/La等比值显示出岩石具有壳源特征.岩石具有高的εHf(t)值(12.2~15.0),Hf两阶段模式年龄在506~662Ma范围内,其岩浆源区初始物质主要来源于新生地壳.岩石在微量元素Rb-(Y+Nb)构造判别图落入火山弧花岗岩区域,在R1-R2构造判别图落入板块碰撞前消减区花岗岩区域.结合岩石成岩年龄、地球化学特征以及区域构造演化,推测其应形成于原特提斯洋俯冲的构造环境,属于大洋洋壳向南俯冲的产物,即柴达木地块和万宝沟大洋玄武岩高原之间的洋壳同时向南、北发生双向俯冲消减.

In order to determine the formation time,magma source,and tectonic setting of the gneissic granodiorite in Manite area,East Kunlun,zircon U-Pb dating,Hf isotope data and geochemistry of the gneissic granodiorite are studied in this paper.The chronology indicates that the magmatic zircon LA-ICP-MS（laser ablation inductively coupled plasma mass spectrometry）U-Pb weighted mean age of the Manite gneissic granodiorite in the East Kunlun is 495.6±1.1Ma（MSWD=0.13）,belonging to the Late Cambrian.The gneissic granodiorites have SiO2 of 61.47%-63.99%,Na2O of 2.91%-3.64%,K2O of 0.93%-2.31%,CaO of 4.29%-6.52%,ALK of 3.92%-5.69% and A/CNK=0.83-0.97.Chemically,they are metaluminous and belong to calc-alkaline rock series.The gneissic granodiorite is characterized by the enrichment of LILEs（Rb and K）and incompatible elements（Th and U）,and depletion of HFSEs（Nb,Ta,Zr and Ti）.The ratios of Nb/Ta,La/Nb,Th/Nb and Th/La show crustal characteristics.TheεHf（t）values of zircons from the gneissic granodiorite range from 12.2-15.0,their Hf two-stage model ages vary from 506 to 662Ma.The initial materials are mainly derived from the juvenile crust.The gneissic granodiorite falls into the volcano arc granite area with the determination of trace elements in Rb-（Y＋Nb）tectonic discrimination diagram and falls into the zone of pre-plate collision with the characteristics of active continental margin before the collision.Combined with the geochronological data,geochemical characteristics and the regional tectonic evolution,it is concludedthat the gneissic granodiorite in the Manite area was formed in the environment of Proto-Tethys Ocean crust subduction,presumably it belongs to the southward subduction of oceanic crust.The crust between the Qaidam massif and Wanbaogou oceanic plateau occurred bidirectional subduction to the south and the north.