塔什库尔干塔吐鲁沟寒武纪岩体的厘定及其地球化学特征与构造意义

Dating and Geochemical Features of the Cambrian Pluton in Tatulugou of the Taxkorgan Landmass and Tectonic Implications

为研究塔什库尔干陆块的构造-岩浆演化,对塔吐鲁沟地区原中生代花岗岩体进行了LA-MC-ICP-MS锆石U-Pb同位素定年和岩石地球化学的研究。结果表明,塔吐鲁沟岩体实为以中晚寒武世侵入为主体的复式岩体,寒武纪花岗岩前三阶段锆石206Pb/238U年龄加权平均值分别为507.2±2.6Ma、506.9±1.6Ma和507.7±3.7Ma。里特曼指数δ为1.16~2.87,属钙碱性和高钾钙碱性系列,铝饱和指数A/CNK大都小于1.1(0.87~1.14),稀土总量(∑REE)为69.95×10-6~264.57×10-6,稀土配分曲线呈右倾型,具有弱到中等铕负异常(δEu=0.33~0.84),Th、K、La、Ce、Nd、Tb相对富集,Nb、Ta、Ti及P、Sr亏损,为与洋壳俯冲作用有关的准铝-弱过铝质I型花岗岩。岩浆初始来源可能为上地幔和加厚玄武质下地壳部分熔融,上侵过程中遭受上地壳混染,具有高硅富碱而低镁、铁、钛的演化特征。构造环境属于大洋俯冲期和俯冲向同碰撞转换期,且形成时代与原特提斯洋456±30~698.4Ma俯冲闭合过程是吻合的,预示着寒武纪岩体是原特提斯洋俯冲的产物,塔什库尔干陆块在中晚寒武世属于洋壳俯冲火山弧环境。

In order to study the tectono-magmatic evolution of the Taxkorgan landmass, we have made U-Pb isotopic dating of zircon by LA-MC- ICP-MS and analysis of petrochemistry to the former Mesozoic granitic pluton in the Tatulugou area. The results show that the Tatulugou pluton is a multi- ple-stage-intruded rock body, composed mainly of the middle-late Cambrian intrusions. The weighted averages of zircon ^206pb/^238U ages of the first three phases Cambrian pluton are 507.2±2.6Ma,506.9±1.6Ma and 507.7±3.7Ma, respectively. Their Rittmann indexes are 1.16-2.87, belonging to calc-al- kaline and high-K calc-alkaline series, of which most of aluminum saturation indexes are 1.10 （0.87-1.14）. The total amounts of rare earth elements （EREE） are （69.95-264.57）×10^-6, and the REE diagram shows rightward incline with weak to moderate negative Eu anomalies （δEu = 0.33-0.84）. The relatively high concentration of incompatible elements Th, K, Nd and Tb and depleted Nh,Ta,Ti,P and Sr suggest that this rock is weakly peraluminous or quasi-aluminous I-type granite associated with subduction of oceanic crust. The material source of the magma may be from partial melting of the upper mantle and thickened basaltic lower crust. During the rising process, the magma was contaminated by the upper crust, and thus bad the evolution characteristics of high Si, rich alkali but poor Mg,Fe, and Ti. This Cambrian pluton formed in an oceanic subduction environment and in the transition period from subduction to syncolfisional setting, consistent with the processes of subduction and closure of Proto-Tethys during （456±30）-698.4Ma. These lines of evidence suggest its formation is attributed to the subduction of Proto-Tethys, and the Taxkorgan landmass might be in an environments of ocean crust subduction and volcanic arc in the Middle-Late Cambrian epoch.