西秦岭天水地区龙王洞蛇绿混杂岩U–Pb年龄与俯冲-碰撞时限  

U–Pb isotopic age of Longwangdong ophiolitic mélange in the Tianshui Area, West Qinling Orogenic Belt and its time limit of subduction–collision

作　　者：时超 辜平阳[1,2] 彭璇 潘晓萍[1,2] 田绒 SHI Chao;GU Pingyang;PENG Xuan;PAN Xiaoping;TIAN Rong(Xi’an Center of China Geological Survey,Geosciences Innovation Center of Northwest China,Xi’an 710119,Shaanxi,China;Centre for Orogenic Belt Geology,China Geological Survey,Xi’an 710119,Shaanxi,China;Chinese Academy of Geological Sciences,Beijing 100083,China)

机构地区：[1]中国地质调查局西安地质调查中心/西北地质科技创新中心,陕西西安710119 [2]中国地质调查局造山带地质研究中心,陕西西安710119 [3]中国地质科学院,北京100083

出　　处：《中国地质》2024年第6期1960-1971,共12页Geology in China

基　　金：陕西省自然科学基础研究计划项目(2023–JC–ZD–14,2022JQ–286);中国地质调查局项目(DD20190069,DD20221636,DD20243411,DD20240029)联合资助。

摘　　要：【研究目的 】西秦岭天水地区龙王洞蛇绿混杂岩是一套典型的蛇绿岩体,是秦岭商丹缝合带西延的重要证据,对其进行岩石学、年代学和地球化学的研究有助于更好地理解该地区的构造演化过程。【研究方法 】本文通过岩石学、全岩地球化学对该套蛇绿岩进行分析,确定其主要由变辉长岩、变玄武岩、辉石岩等组成。【研究结果 】通过LA–ICP–MS锆石微区原位U–Pb同位素测年,获得变辉长岩的^(206)Pb/^(238)U加权平均年龄为(474.6±6.7)Ma,指示该蛇绿混杂岩形成于早奥陶世。岩石地球化学分析表明,变玄武岩属于高钛、富镁、富铁、拉斑系列;稀土元素分布模式呈现稀土总量偏低、Eu负异常不明显的平坦型曲线;微量元素分布模式为Cs、Th和Nb相对亏损的平坦型曲线,Zr/Nb比值低,总体表现出与E–MORB相似的特征。【结论 】变玄武岩地球化学特征指示龙王洞蛇绿混杂岩可能形成于岛弧区拉张环境,结合区域资料进一步认为其形成于俯冲带上弧后盆地环境,进一步暗示了西秦岭天水地区在早奥陶世处于商丹洋的大洋俯冲构造背景。This paper is the result of geological survey engineering.Objective The newly discovered Longwangdong ophiolitic mélange as a typical ophiolite suit in the Tianshui area of West Qinling Orogenic Belt witnessed the westward extension of Shangdan suture zone in Qinling Orogenic Belt.Studying its petrology,geochronology,and geochemistry will help improve our understanding of the tectonic evolution of this region.Methods This study demonstrates through petrological and whole−rock geochemical analyses that this ophiolite suite is mainly composed of amphibolite,metamorphosed basalt,and pyroxenite.Results LA–ICP–MS zircon U–Pb dating shows the meta−gabbro was formed at(474.6±6.7)Ma,constraining the formation time of the mélange.The whole−rock geochemical analysis reveals that the meta−basalts are subalkalic tholeiite in composition and rich in Ti,Mg and Fe.The total REE of the meta−basalts are low and characterized by flat type distribution pattern without obvious Eu anomalies.Their trace element distribution yields flat patterns with negative anomalies of Cs,Th,Nb,and lowed Zr/Nb ratios,consistent with that of the E−MORB.Conclusions The geochemical features of the meta−basalts suggest the Longwangdong ophiolitic mélange was potentially generated in the extended volcanic arc environment,which was considered to be the back−arc basin based on the regional geological data.It further implies that Shangdan Ocean experienced a period of subduction process in the Early Ordovician in the West Qinling region.

关 键 词：蛇绿混杂岩 LA–ICP–MS锆石U–Pb测年 早奥陶世 构造环境 西秦岭 地质调查工程 商丹缝合带 

分 类 号：P597.3[天文地球—地球化学] P548[天文地球—地质学]