扬子板块北缘城口地区新元古代龙潭河组凝灰岩锆石U-Pb年代学、地球化学特征及其构造意义  

ZIRCON U-Pb GEOCHRONOLOGY,WHOLE ROCK GEOCHEMISTRY AND GEOLOGICAL IMPLICATION OF THE NEOPROTEROZOIC LONGTANHE FORMATION TUFFS IN THE CHENGKOU AREA NORTHERN MARGIN OF THE YANGTZE PLATE

作　　者：曹芳 杨文光[1] 秦溱 麦源君 李德亮 朱利东[1] CAO Fang;YANG Wenguang;QIN Zhen;MAI Yuanjun;LI Deliang;ZHU Lidong(Institute of Sedimentary Geology,Chengdu University of Technology,Chengdu,Sichuan 610059,China;Chongqing Institute of Geological Survey,Chongqing 401122,China)

机构地区：[1]成都理工大学沉积地质研究院,四川成都610059 [2]重庆市地质调查院,重庆401122

出　　处：《矿物岩石》2024年第3期52-63,共12页Mineralogy and Petrology

基　　金：国家自然科学基金项目(批准号:41972118)。

摘　　要：扬子板块北缘城口-房县断裂出露新元古代龙潭河组火山-沉积序列,被认为是新元古代Rodinia超大陆聚合和裂解的产物,记录了构造事件的重要信息,是探讨扬子板块北缘构造演化的理想产物,对于理解新元古代扬子板块北缘构造演化格局具有重要意义。本文对城口地区龙潭河组凝灰岩开展了锆石SHRIMPU-Pb年代学、全岩主、微量元素地球化学测试分析,测试结果表明锆石具典型的环带状结构,锆石Th/U比值全部大于0.4,为岩浆锆石,锆石U-Pb年龄加权平均值分别为871.0±25.0Ma,819.0±8.1Ma,813.0±11.0Ma,798.0±15.0Ma,表明城口龙潭河组凝灰岩可能为青白口纪中-晚期岩浆活动产物。岩石地球化学特征显示岩石轻稀土元素富集,重稀土分布平坦,轻重稀土分异较强,原始地幔标准化微量元素蛛网图上明显富集Rb,Ba,Th,U,K等大离子亲石元素(LILEs),并且强烈亏损Nb,Ta,Ti等高场强元素(HFSEs),与弧岩浆的地球化学特征相一致;Zr/TiO_(2)与Nb/Y图解显示岩石类型与活动陆缘组合相一致,Th/Tb与Ta/Yb图解也显示出强烈的大陆亲缘性,表明龙潭河组凝灰岩可能形成于活动陆缘岩浆弧的俯冲背景下。综合分析已有的地质学、地球化学和年代学研究,在新元古代Rodinia超大陆裂解的背景下,扬子板块北缘在871~798Ma仍处于以俯冲作用为主的构造活动,可能Rodinia超大陆裂解在扬子板块北缘的构造活动中响应较弱。The volcano-sedimentary sequence of the Neoproterozoic Longtanhe Formation around the Chengkou-Fangxian fault on the northern margin of the Yangtze Plate is considered to be the product of the convergence and fragmentation of the Neoproterozoic Rodinia supercontinent.This formation records sedimentary,magmatic,and tectonic events in this area.Therefore,studying Longtanhe Formation tuffs can shed light on the tectonic evolution of the northern margin of the Yangtze Plate during the Neoprotero-zoic.In this paper,zircon SHRIMP U-Pb geochronology,whole rock geochemistry are used to study the Longtanhe Formation tuffs in the Chengkou area.The analytical results show that the zircons in the tuffs have a typical ring structure,with Th/U ratios greater than O.4,indicating magmatic origins.The weigh-ted average zircon U-Pb ages obtained are 871.0±25.0 Ma,819.0±8.1 Ma,813.0±11.0 Ma,and 798.0±15.O Ma,suggesting that the Longtanhe Formation tuffs in the Chengkou area were volcanic products of magmatic activity in the Middle to Late Qingbaikou Period.Geochemical analysis of the tuffs shows that the rocks are enriched in light rare earth elements,with a flat distribution of heavy rare earth elements and strong differentiation between light and heavy rare earth elements.The spider diagram of standardized trace elements in the primitive mantle shows obvious enrichment of large ion lithophile elements(LILEs)such as Rb,Ba,Th,U,and K,and strong depletion of high field strength elements(HFSEs)such as Nb,Ta,and Ti,consistent with the geochemical characteristics of arc magmas.The Zr/TiO_(2) and Nb/Y diagrams show that the rock type is consistent with an active continental margin association,and the Th/Tb and Ta/Yb diagrams also show strong continental affinity,reflecting that the Longtanhe Formation tuffs may have formed in a subduction-related magmatic arc setting.Comprehensive analysis of existing geological,geochemical,and chronological data suggests that the northern margin of the Yangtze Plate was still undergoing subduction

关 键 词：新元古代 扬子板块北缘 龙潭河组 凝灰岩 俯冲作用 构造意义 

分 类 号：P581[天文地球—岩石学] P595[天文地球—地质学]