扬子西缘洋岛型与岛弧型火山岩岩石成因与构造意义:从板内裂谷到洋陆俯冲  

Petrogenesis and tectonic significance of OIB-and arc-type volcanic rocks in the western Yangtze Block:From intracontinental rifting to subduction.

作　　者：张继彪 丁孝忠[1] 刘燕学[1] ZHANG Jibiao;DING Xiaozhong;LIU Yanxue(Institute of Geology,Chinese Academy of Geological Sciences,Beijing 100037,China)

机构地区：[1]中国地质科学院地质研究所,北京100037

出　　处：《地学前缘》2021年第4期250-266,共17页Earth Science Frontiers

基　　金：中国地质调查局项目“全国陆域及海区地质图件更新与共享(DD20190370)”;中国地质调查局项目“江南造山带区域地质调查片区总结与服务产品开发(121201111120117)”。

摘　　要：扬子西缘晚中元古代—早新元古代岩浆岩对扬子陆块构造演化以及Rodinia超大陆的汇聚和裂解至关重要。本文获得扬子西缘会理群天宝山组玄武质凝灰岩和盐边群渔门组角闪安山岩SHRIMP锆石U-Pb年龄分别为(1035±15)Ma和(884±9)Ma。天宝山组玄武质凝灰岩属于碱性玄武岩系列,富集大离子亲石元素和高场强元素,具有正的εNd(t)值(4.6),表现出与洋岛玄武岩相似的地球化学特征。天宝山组火山岩来自以石榴子石和尖晶石为稳定区的地幔橄榄岩1%~5%的部分熔融。渔门组角闪安山岩属于钙碱性岛弧玄武岩系列,以富集大离子亲石元素和轻稀土,亏损高场强元素为特征,具有明显的Nb、Ta、Ti负异常,εNd(t)值为1.1~2.8。渔门组火山岩来自以石榴子石和尖晶石为稳定区的地幔橄榄岩5%~15%的部分熔融。构造背景判别图解指示天宝山组玄武质凝灰岩形成于被动大陆边缘板内裂谷盆地,而渔门组角闪安山岩形成于活动大陆边缘岛弧环境。根据本文获得的年代学及地球化学数据,笔者认为扬子西缘与大陆裂谷相关的天宝山组火山岩和与板块俯冲有关的渔门组火山岩,记录了新元古代早期构造动力学背景由板内裂谷转为洋陆俯冲的变化。The late Mesoproterozoic to early Neoproterozoic magmatism in the western margin of the Yangtze Block played an important role in the geodynamics and crustal evolution of the Yangtze Block during the assembly and breakup of the Rodinia supercontinent.In this study,new SHRIMP zircon U-Pb dating yielded ages of(1035±15)and(884±9)Ma for the Tianbaoshan basaltic tuff and Yanbian hornblende andesite in the western Yangtze Block,respectively.The Tianbaoshan basaltic tuffs are of alkali basalt,exhibit OIB-like geochemical affinity and enrichments of LILE and HFSE,and have a positiveεNd(t)(4.6)value,with geochemical characteristics similar to ocean island balsalt.They are presumably derived from low-degree(1%-5%)partial melting of the upwelling asthenospheric mantle in an intra-plate rifting setting in the garnet+spinel stability field.The Yanbian hornblende andesites have calc-alkaline arc rocks signature,characterized by the enrichments of LILE and LREE and depletion of HFSE,significantly negative Nb,Ta and Ti anomalies,and positiveεNd(t)(1.1-2.8)values.They are derived from 5%-10%partial melting of mantle wedge in a subduction setting in the garnet+spinel stability field.On the basis of these results,we propose that the late Mesoproterozoic rifting-early Neoproterozoic subduction related volcanic rocks record a tectonic switch,from a continental-rifting basin to an convergent environment in the western Yangtze Block.

关 键 词：中—新元古代 扬子板块西缘 锆石U-PB年龄 岩石成因 构造背景 

分 类 号：P597[天文地球—地球化学] P588.1[天文地球—地质学]