西藏桑耶地区晚白垩世中性侵入岩的成因及对新特提斯板块北向俯冲的指示意义  

Genesis and Geological Significance of Late Cretaceous Intermediate Intrusions in Sangye, Tibet

作　　者：邓科 王金贵 董玉杰 何林武 袁仁华 张泽国 陈守关 辛堂 DENG Ke;WANG Jingui;DONG Yujie;HE Linwu;YUAN Renhua;ZHANG Zeguo;CHEN Shouguan;XIN Tang(The Fifth Geological Brigade of Tibet Autonomous Region Geological Exploration and Development Bureau,Geermu,Qinghai 816099,China;Hebei Institute of Regional Geological Survey,Langfang,Hebei 065000,China)

机构地区：[1]西藏自治区地质矿产勘查开发局第五地质大队,青海格尔木816099 [2]河北省区域地质调查院,河北廊坊065000

出　　处：《现代地质》2023年第2期375-389,共15页Geoscience

基　　金：中国地质调查局“冈底斯—喜马拉雅铜矿资源基地调查”项目(DD20160015-09)。

摘　　要：冈底斯岩浆岩带呈近东西向沿雅鲁藏布江缝合带分布,是一条巨大的构造—岩浆岩带。南冈底斯中生代的岩浆活动经历了205~152 Ma、109~80 Ma、65~41 Ma和33~13 Ma 4个阶段,均是挤压环境的产物。但其触发机制一直存在争议。争议的焦点在于新特提斯洋和班公湖—怒江洋的俯冲方式。为解释冈底斯南缘中生代中酸性岩浆岩的成因及构造环境,本文以西藏自治区扎囊县桑耶地区的石英二长岩为研究对象,通过年代学、岩石地球化学、Lu-Hf同位素分析岩浆起源、岩石成因及其构造环境。LA-ICP-MS锆石U-Pb定年结果显示,岩体形成于晚白垩世(91~88 Ma)。岩石地球化学分析揭示其属准铝质高钾钙碱性系列,富集大离子亲石元素和轻稀土元素,亏损高场强元素和重稀土元素,无明显Eu负异常,锆石εHf(t)值为+10.6~+14.2。石英二长岩形成于斜长石和石榴石的过渡带,岩浆经历了角闪石、磷灰石及黑云母的分离结晶作用。其典型的微量元素比值特征指示岩浆来源于年轻俯冲洋壳的部分熔融,并有少量新生幔源物质加入,具典型的I型花岗岩特征,推断其形成于俯冲岛弧环境,表明晚白垩世时期冈底斯南缘处于新特提斯板块向北俯冲的构造背景之下。The Gangdese magmatic belt is a huge tectonic-magmatic belt,which is nearly E-W directed along the Yarlung-Zangbo suture zone.Mesozoic magmatism in the southern Gangdese experienced four stages:205-152 Ma,109-80 Ma,65-41 Ma and 33-13 Ma,all of which were the products of a compression environment.However,the triggering mechanism has been controversial.Focus of controversy lies in the subduction mode of the Neotethys and the Bangong-Nujiang Ocean.In order to explain the genesis and tectonic environment of Mesozoic intermediate-acid magmatic rocks in the southern margin of Gangdese belt,we analyzed the quartz monzonite in Sangye area(from Zhanang County,Tibet)and discussed the magmatic origin,petrogenesis and tectonic environment through geochronology,whole-rock geochemistry and Lu-Hf isotopes.LA-ICP-MS zircon U-Pb dating shows that the intrusion was formed in the Late Cretaceous(91-88 Ma).The rocks are metaluminous and high-K calc-alkaline.They are enriched in LILEs and LREEs,depleted in HFSEs and HREEs,with no obvious negative Eu anomaly.εHf(t)value is+10.6 to+14.2.Quartz monzonite magma was likely formed in the transition zone between the plagioclase and garnet stability fields.The magma may have experienced the fractionation of amphibole,apatite and biotite.The typical trace element ratios indicate that the magma was originated from partial melting of the young subducted oceanic crust and the addition of minor juvenile mantle-derived materials,which is I-type granite.We inferred that it was formed in the subduction-related island arc setting,indicating that the Late Cretaceous southern margin of Gangdese belt was influenced by the north-dipping subduction of the Neotethys.

关 键 词：地球化学特征 锆石U-PB测年 LU-HF同位素 晚白垩世 冈底斯南缘 

分 类 号：P588.1[天文地球—岩石学] P595[天文地球—地质学] P597