滇东北昭通地区峨眉山玄武岩钕-锶-铅同位素特征——峨眉山地幔柱源区性质与Rodinia超大陆事件的耦合关系  

Nd-Sr-Pb isotopes of Emeishan basalt in the Zhaotong area of northeastern Yunnan—Coupling relationship between source of Emeishan mantle plume and Rodinia supercontinent

作　　者：王晓峰 熊波 戚戎辉 刘军平 关学卿 吴嘉林 WANG Xiaofeng;XIONG Bo;QI Ronghui;LIU Junping;GUAN Xueqing;WU Jialin(Yunnan Institute of Geological Survey,Kunming 650216,Yunnan,China;MNR Key Laboratory of Sanjiang Metallogeny and Resources Exploration and Utilization,Kunming 650051,Yunnan,China)

机构地区：[1]云南省地质调查院,云南昆明650216 [2]自然资源部三江成矿作用及资源勘查利用重点实验室,云南昆明650051

出　　处：《地质通报》2021年第7期1084-1093,共10页Geological Bulletin of China

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》子项目《云南省系列地质图件数据处理与洋板块地质研究》(编号:DD20190370)、《云南乌蒙山区1:5万龙树街、塘房、昭通县、鲁甸县4幅区域地质调查》(编号:121201010000150002-02)

摘　　要：滇东北昭通地区峨眉山玄武岩Nd-Sr-Pb同位素的最新研究结果表明,该区玄武岩样品普遍具有1000~900 Ma的亏损地幔Nd模式年龄,其源区可能为来自下地幔类似地幔集中带(FOZO)组分(10%~40%)的地幔橄榄岩和来自古老再循环洋壳的类似EM1组分(60%~90%)不同比例混合而成。样品满足Dupal异常边界条件,暗示峨眉山玄武岩岩浆形成及侵位时与目前所处的纬度有较大差异,二者可能有较大的空间距离。据此,提出新的峨眉山地幔柱形成模式:1000~900 Ma,源于Rodinia超大陆事件中衍生的洋壳发生了俯冲消减作用,洋壳经部分熔融后的固态残留物在660 km地幔过渡带中堆积,形成较冷的难熔“巨石体”。晚二叠世(约260 Ma),这些“巨石体”(很可能是榴辉岩相的超高压变质岩)受重力驱动作用进一步下沉到核幔边界“D”层时,发生钙钛矿→后钙钛矿的矿物相转变,这是一个强烈的放热反应,导致下地幔及俯冲板片堆积体自身的部分熔融,引起深部高温富铁、富钛的熔融体上涌进入地幔,形成地幔柱。这些岩浆大规模的上涌、喷发,形成了扬子陆块西缘规模巨大的峨眉山大火成岩省(LIPs)。此时古地理位置还处于南半球的某个位置,古特提斯大洋关闭后,这些携带南半球特有的地球化学烙印(Dupal异常)的玄武岩随着扬子板块一路向北漂移,到达今天的位置。The latest Nd-Sr-Pb isotopes study of Emeishan basalt in the Zhaotong area of northeastern Yunnan Province shows that the basalt samples generally have a 1000~900 Ma Nd model age of depleted mantle.The source could be generated by mixing of different proportions of a recycled ancient oceanic crust component(EM1-like,60%~90%)and a peridotite component from the lower mantle(FOZO-like component,10%~40%).The sample satisfies the Dupal anomaly boundary condition,suggesting that the latitude of Emeishan basalt magma formation and emplacement is quite different from the current latitude,and there may exist a large space distance between them.Therefore,a new model for the formation of the Emeishan mantle plume is proposed.During 1000~900 Ma,the oceanic crust derived from the Rodinia supercontinent event subducted and subsided,and the remnants of the oceanic crust piled on the 660 km mantle transition zone to form relatively cold refractory megaliths.In the Late Permian(~260 Ma),these megaliths(probably eclogite facies super-compressive metamorphic rocks)further subsided down to the“D”layer of the core-mantle transition,and resulted in the transformation from perovskite to post-perovskite,which was a strong exothermic reaction resulting in partial melting of lower mantle and subducting plate accumulations themselves,leading to upwelling of high temperature iron and titanium to form the mantle plume.The large scale upwelling and eruption of these magmas formed the large-scale Emeishan Igneous Province(LIPs)in the west of Yangtze block.At this time,the paleogeographic location was still in a certain position in the southern hemisphere.After the closure of the Paleotethys,these basalts bearing the unique geochemical imprint of the southern hemisphere(Dupal anomaly)drifted northward along with the Yangtze plate and reached the present position.

关 键 词：峨眉山玄武岩 Nd-Sr-Pb同位素 地幔柱 RODINIA超大陆 洋壳的俯冲、再循环 

分 类 号：P588.14[天文地球—岩石学]