新疆西昆仑东南部泉水沟岩体的年龄、成因及构造意义

Geochronology,Petrogenesis and Tectonic Significance of Quanshuigou Pluton from Southeastern West Kunlun Mountain in Xinjiang,China

泉水沟岩体位于西昆仑造山带和巴彦喀拉褶断带夹持区域,对追溯西昆仑地区的区域发展演化具有重要指示意义.通过高精度同位素年代学、岩石矿物学与地球化学等手段,准确限定岩体的形成时代并查明其地球化学特征.LA-ICP-MS锆石U-Pb定年获得该岩体中花岗闪长岩的年龄为217.6±0.9Ma,其中的暗色包体黑云母石英闪长岩年龄为217.4±0.7Ma,均属晚三叠世(T3).花岗闪长岩具有富硅(SiO2=67.21%~68.85%)、富碱(全碱变化于6.36%~6.76%,里特曼指数δ=1.56~1.86)、富钾(K2O=3.50%~3.96%,K2O/Na2O=1.20~1.41),而贫TiO2、MnO和P2O5(<1%)的特点;岩体的铝饱和指数A/CNK为0.88~1.05;总体上属高钾钙碱性准铝质-弱过铝质I型花岗岩.岩石富集轻稀土,轻重稀土元素分馏明显((La/Yb)N为1.03~11.05),具有铕负异常(δEu为0.42~0.73);富集Rb、Th、K和LREE等大离子亲石元素,而贫Ba、Sr、Ti、P等元素.岩石微量元素地球化学特征显示,泉水沟花岗闪长岩及其包体为下地壳变质玄武岩熔融的产物,在岩浆演化过程中岩体的分离结晶作用以斜长石、辉石和磷灰石为主,而角闪石分离结晶不明显.区域构造环境演化表明,晚三叠世时随着古特提斯洋向北消减至最终闭合,在康西瓦断裂带两侧发生大规模陆块碰撞挤压,致使地壳垂向增生加厚引起了强烈的岩浆活动,从而在下地壳发生重熔作用形成了泉水沟岩体,也表明该区当时已处于洋陆拼合后的碰撞造山新阶段.

Lying in the transitional region of the West Kunlun area and Bayankala folded fault belt of Xinjiang, the Quanshuigou pluton has indicative significance for area evolution. In order to determine the formation time, geochemical characteristics and tectonic setting of the Quanshuigou pluton, geochronology, mineralogy, petrology and geochemistry were studied. Combined with the cathodoluminescence (CL) images and element U, Th and Pb features of zircon, this study yields two ages of 217.6±0.9 Ma and 217.4±0.7 Ma for granodiorite and quartz diorite using the LA-ICP-MS zircon U-Pb dating, belonging to the Late Triassic. The geochemical analyses show that the granodiorites are characterized by high SiO2 (67.21%-68.85%), high alkali (Na2O+K2O=6.36%-6.76%, Rittman index δ=1.56-1.86), high potassium (K2O=3.50%-3.96%, K2O/Na2O=1.20-1.41), and low TiO2, MnO and P2O5 (content <1%);and aluminum index A/CNK=0.88-1.05, indicating a transitional composition between metaluminous and peraluminous I-type granite series. The pluton is enriched in LREE, depleted in HREE ((La/Yb)N=1.03-11.05), with negative Eu anomaly (δEu=0.42-0.73). While the trace elements are characterized by enrichment of Rb, Th, K and LREE, and depletion of Ba, Sr, Ti, P. The chemical signatures indicate that the pluton experienced the highly fractionated process and partial melting. The Quanshuigou diorite with characteristic source-mixed granite from mantle and crust should be likely derived from metabasalt in lower crust, and subsequently underwent the fractional crystallization of plagioclase, pyroxene and apatite without amphibole. In the Late Triassic, the Paleo-Tethys Ocean was gradually evolved from subduction toward north to the complete closure, and at the same time regional tectonic stress was transformed from subduction process to collision compressional action environment. Based on comprehensive analysis of the genetic type of the pluton and evolution of regional tectonic environment, it is suggested that the Quanshuigou pluton formed just after this tectonic stress transfromation, which shows that this area was in continent-continent collision orogenic regime in the Late Triassic.