陕南勉略宁地区黑木林-硖口驿新元古代镁铁质岩锆石U-Pb年代学、地球化学特征及其构造意义  

Zircon U-Pb geochronology,geochemistry and tectonic significance of Heimulin-Xiakouyi Neoproterozoic mafic rocks in Mianxian-Lüeyang-Ningqiang area,southern Shaanxi Province

作　　者：刘静远 裴先治[1,2] 李瑞保[1,2] 裴磊[1,2] 李佐臣[1,2] 刘成军[1,2] 赵少伟 王盟[1,2] 陈有炘[1,2] 周海[1,2] 赵杰 LIU Jingyuan;PEI Xianzhi;LI Ruibao;PEI Lei;LI Zuochen;LIU Chengjun;ZHAO Shaowei;WANG Meng;CHEN Youxin;ZHOU Hai;ZHAO Jie(School of Earth Science and Resources,Chang’an University,Xi'an,Shaanxi 710054,China;Key Laboratory of Western China’s Mineral Resources and Geological Engineering,Ministry of Education,Xi'an,Shaanxi 710054,China)

机构地区：[1]长安大学地球科学与资源学院,陕西西安710054 [2]西部矿产资源与地质工程教育部重点实验室,陕西西安710054

出　　处：《地质学报》2022年第7期2421-2440,共20页Acta Geologica Sinica

基　　金：国家自然科学基金项目(编号41872233和41872235);陕西省自然科学基金项目(编号2019JM-312、2020JM-229和2020JQ-353);中央高校基本科研业务费资助项目(编号300102270202、300102279108和300102270107)联合资助的成果。

摘　　要：对于扬子板块西北缘新元古代早—中期(835~720 Ma)岩浆岩形成的构造背景尚存在较大争议。本文对陕南勉略宁地区碧口微地块内黑木林-硖口驿SSZ型(俯冲带上盘型)蛇绿混杂岩中的辉长岩、辉长闪长岩和变安山质玄武岩分别进行了锆石LA-ICP-MS U-Pb年代学、岩石学及岩石地球化学研究。研究结果表明,木瓜园辉长岩和变安山质玄武岩属岛弧拉斑玄武岩系列,稀土元素球粒陨石标准化配分曲线均呈右倾型,具有微弱的Eu负异常,微量元素原始地幔标准化图解显示其富集大离子亲石元素(LILE)和轻稀土元素(LREE)而亏损高场强元素(HFSE,如Nb、Ta等),与岛弧岩浆岩相似。黑木林、硖口驿辉长闪长岩属钙碱性系列。黑木林辉长闪长岩具与Bonin弧前玻安质岩石类似的地球化学特征,具较高的SiO_(2)含量(54.27%~56.79%)、MgO含量(11.10%~17.04%)和较低的TiO_(2)含量(0.08%~0.19%),富集大离子亲石元素Rb、Th、U和Pb,亏损高场强元素Nb、Ti,以及弱的Zr、Hf正异常等特征。硖口驿辉长闪长岩MgO含量较低(4.24%~8.91%),略低于玻安质岩石,其N-MORB(正常洋中脊玄武岩)标准化微量元素蜘蛛网图与Bonin弧前玻安岩类似,富集LILE、亏损HFSE,具低的Ti/Zr比值等特征,属于过渡类型的类玻安质岩石。锆石LA-ICP-MS U-Pb测年结果显示,黑木林-硖口驿SSZ型蛇绿混杂岩的原岩形成时代为835~830左右。其中木瓜园辉长岩、黑木林和硖口驿辉长闪长岩的结晶年龄分别为835±4 Ma、830±2 Ma和830±11 Ma。结合前人关于扬子板块西北缘新元古代早—中期构造演化过程的研究,认为扬子板块西北缘在新元古代早—中期(950~720 Ma)处于长期的洋壳东南向俯冲的活动大陆边缘环境并发育俯冲增生造山作用,且新元古代早期(835~830 Ma)在扬子板块西北缘发生了洋内俯冲,形成黑木林-硖口驿SSZ型蛇绿混杂岩,最终增生拼贴就位于碧口微地块。The tectonic background of the early to middle Neoproterozoic(835~720 Ma) magmatic rocks in the northwestern margin of the Yangtze block is still in debate.We report the zircon LA-ICP-MS U-Pb ages,petrological and geochemical data of the gabbro,gabbro-diorite,and meta-andesitic basalt from the Heimulin-Xiakouyi suprasubduction zone(SSZ) ophiolitic mélange in Bikou Terrane,Mianxian-Lueyang-Ningqiang area,southern Shaanxi Province.The results show that the Muguayuan gabbro and meta-andesitic basalts belong to tholeiitic basalt series with right-dipping chondrite-normalized REE patterns,weak negative Eu anomalies.The primitive mantle normalized incompatible element patterns show the enrichment of LILE and LREE and depletion of HFSE(such as Nb,Ta,etc.),which is similar to island arc magmatic rocks.The Heimulin and the Xiakouyi gabbro-diorite belong to calc-alkaline series.The Heimulin gabbro-diorite displays similar geochemical characteristics to the Bonin fore-arc boninite,with relatively high SiO_(2) content(54.27%~56.79%),MgO content(11.10%~17.04%),and low TiO_(2) content(0.08%~0.19%).The trace elements of the whole rock have the characteristics of enrichment of LILE(Rb,Th,U and Pb),depletion of HFSE(Nb,Ti),and weak Zr-Hf positive anomalies.The Xiakouyi gabbro-diorite also shares similar geochemical signatures to boninite,with relatively lower MgO content(4.24%~8.91%).In addition,the N-MORB(normal mid-ocean-ridge basalt)-normalized trace element spider diagrams are similar to Bonin fore-arc boninitic rocks,which is enriched in LILEs and depleted in HFSEs,and show low Ti/Zr ratios,indicating that it could belong to transitional type of boninite-like rocks.Zircon LA-ICP-MS U-Pb dating results show that the age of Heimulin-Xiakouyi SSZ ophiolitic mélange is about 835~830 Ma.The crystallization age of Muguayuan gabbro,Heimulin and Xiakouyi gabbro-diorite are 835±4 Ma,830±2 Ma and 830±11 Ma,respectively.Combined with previous research on the tectonic evolution of the early to middle Neoproterozoic in the northwest

关 键 词：扬子板块西北缘 碧口微地块 新元古代 SSZ型蛇绿混杂岩 玻安岩 

分 类 号：P534.3[天文地球—古生物学与地层学] P597.3[天文地球—地质学] P587