冈底斯岩浆弧曲水地区早中新世埃达克质花岗岩的成因及构造意义  

Petrogenesis and tectonic significance of the Early Miocene adakitic granites from the Quxu area,Gangdese magmatic arc

作　　者：秦圣凯 张泽明[1,2] 张荣辉 QIN Sheng-kai;ZHANG Ze-ming;ZHANG Rong-hui(Institute of Geology,Chinese Academy of Geological Sciences,Beijing 100037,China;School of Earth Science and Resources,China University of Geosciences,Beijing 100083,China)

机构地区：[1]中国地质科学院地质研究所,北京100037 [2]中国地质大学(北京)地球科学与资源学院,北京100083

出　　处：《岩石矿物学杂志》2023年第3期329-349,共21页Acta Petrologica et Mineralogica

基　　金：中国地质调查局地质调查项目(DD20221630);国家自然科学基金项目(41941016-2,U2244203,91855210)。

摘　　要：青藏高原拉萨地体南部广泛发育的渐新世-中新世埃达克质岩浆岩,是研究冈底斯岩浆弧后碰撞岩浆活动和地壳演化的理想载体。本文对冈底斯弧中段曲水地区的早中新世黑云母花岗岩进行了岩石学、地球化学、锆石U-Pb年代学和Hf同位素研究。全岩地球化学分析结果显示黑云母花岗岩具有高的SiO_(2)、Al2O3和K2O含量,属于中钾钙碱性、准铝质到弱过铝质岩石。微量元素富Sr,贫Y和Yb,富集轻稀土元素而强烈亏损重稀土元素,具有高的Sr/Y值(165~278)和(La/Yb)N值(26.6~39.7),具有典型埃达克质岩石的地球化学特征。锆石U-Pb年代学分析结果表明花岗岩的结晶年龄为21~19 Ma,Hf同位素分析结果显示,岩浆锆石εHf(t)值(-0.9~+12.7)大部分为正值且具有较大的变化范围。根据Sr/Y和(La/Yb)N值估算早中新世冈底斯弧的地壳厚度已达到70~80 km。综合本文和已有的数据表明,印度-亚洲大陆碰撞和碰撞后持续的汇聚作用以及大体积幔源岩浆的底侵共同导致冈底斯岩浆弧经历了显著的新生代地壳加厚;渐新世—早中新世,俯冲印度大陆板片的断离或加厚岩石圈地幔的拆沉作用引起软流圈地幔物质上涌,导致冈底斯弧加厚下地壳(新生地壳和古老地壳)发生强烈的部分熔融,形成了广泛分布的后碰撞埃达克质岩浆岩。The Oligocene to Miocene adakitic rocks are widely distributed in the southern Lhasa terrane,and their petrogenesis is crucial for understanding the post-collisional evolution of the Gangdese magmatic arc.Here,we conducted petrological,geochemical,zircon U-Pb geochronological,and in-situ zircon Hf isotopic studies of the Early Miocene biotite granites from the Quxu area in the middle segment of the Gangdese arc.The studied four biotite granites are classified as cale-alkaline,metaluminous to weak peraluminous granitoids,and show geochemical affinities to adakites,characterized by high SiO_2,Al_2O_3,K_2O and Sr contents,low Y and Yb contents,enrichment in LREE and depletion in HREE,and high Sr/Y(165~278)and(La/Yb)_N(26.6~39.7)ratios.Zircon U-Pb dating yields identical ages of 21~19 Ma,which are interpreted as the crystallization ages of the magmatic rocks.ZirconεHf(t)values ranges from-0.9~+12.7 and are mostly positive,reflecting variable contributions from juvenile and ancient crustal materials.The crustal thickness of the Gangdese arc in Early Miocene is estimated to be approximately 70~80 km based on the Sr/Y and(La/Yb)N ratios.Our presented new data,combined with previous data,supports the proposition that the Gangdese magmtic arc underwent significant Cenozoic crustal thickening resulted from the tectonic shortening during and after the india-Asia continental collision,as well as the addition of mantlederived magmas.We proposed that the breakoff of the deeply subducted Indian continent and/or delamination of the thickened Gangdese lithosphere resulted in the upwelling of asthenosphere and partial melting of the thickened lower crust,leading to the widespread occurrence of post-collisional adakitic rocks.

关 键 词：冈底斯岩浆弧 花岗岩 埃达克质岩 中新世 地壳加厚 

分 类 号：P588.121[天文地球—岩石学] P581[天文地球—地质学] P542