商丹洋早寒武世俯冲作用——来自甘肃关子镇蛇绿混杂岩古坡高镁安山岩的证据  

Early Cambrian subduction of the Shangdan Ocean:New evidence from the Gupo high-Mg andesite in the Guanzi Town ophiolite mélanges,Gansu Province

作　　者：毕志伟 吴纪修[2,3] 张亚光 王晓赛 王强 刘腾飞 王文 BI Zhiwei;WU Jixiu;ZHANG Yaguang;WANG Xiaosai;WANG Qiang;LIU Tengfei;WANG Wen(Hebei Regional Geological Survey Institute,Langfang 065000,Hebei,China;Institute of Exploration Techniques,CAGS,Langfang 065000,Hebei,China;China University of Geosciences(Beijing),Beijing 100083,China)

机构地区：[1]河北省区域地质调查院,河北廊坊065000 [2]中国地质科学院勘探技术研究所,河北廊坊065000 [3]中国地质大学(北京),北京100083

出　　处：《地质通报》2022年第5期740-753,共14页Geological Bulletin of China

基　　金：中国地质调查局项目《共和—略阳地区区域地质调查》(编号DD20190072)。

摘　　要：古坡一带新发现中寒武世高镁安山岩,位于秦岭造山带西段商丹缝合带关子镇蛇绿混杂岩中,其岩石组合为玄武安山岩、安山岩、辉石石英闪长岩等。这些岩石具有较高的SiO 2(54.27%~58.47%)、MgO(6.89%~8.99%)含量和Mg#值(61.29~67.05),较低的TiO_(2)(0.27%~0.99%)含量和TFeO/MgO值(0.88~1.13)。稀土元素总量较低(16.59×10-6~74.5×10-6),稀土元素配分曲线总体为平坦型或微右倾型,(La/Yb)N值为1.03~4.93,无明显Eu异常。岩石富集大离子亲石元素Cs、Rb、Ba、K、U等,亏损高场强元素Nb、Ta、Hf等,具有较低的Ti/V值。上述岩石地球化学特征与日本西南Setouchi岛弧火山岩带的sanukite(赞岐岩)类似,其岩浆可能为俯冲深积物熔融产生熔体与上覆地幔楔橄榄岩平衡反应成因。与古坡变质玄武安山岩密切共生的辉石石英闪长岩LA-ICP-MS锆石U-Pb年龄为514±4.5 Ma,为中寒武世。关子镇蛇绿混杂岩中古坡玄武安山岩/辉石石英闪长岩-安山岩岩石系列为形成于俯冲环境的高镁安山岩组合。研究结果为商丹洋在中寒武世俯冲作用提供了证据。The Gupo high-Mg andesite(HMA)is newly recognized in the Guanzi town ophiolite located in the Shangdan suture zone,Qinling Orogen.The Gupo high-Mg andestieis mainly composed of basaltic andesite,andesite,and pyroxene quartz diorite,which has relatively high contents of MgO(6.89%~8.99%),Mg#61.29~67.05),SiO 2(54.27%~58.47%),and low contents of TiO_(2)(0.27%~0.99%),K 2 O(0.09%~2.32%),Al 2 O 3(12.09%~18.38%),and TFeO/MgO(0.88~1.13).The contents of REE are also low(16.59×10-6~74.5×10-6).The distribution pattern of REE is flat and slightly right type(La N/Yb N=1.03~4.93)without obvious abnormity of Eu.In addition,the rocks are enriched with large ion lithophile element such as Cs,Rb,Ba,K,and U,depleted of high field-strength element such as Nb,Ta,and Hf with low value of Ti/V.All these geochemical characteristics are identical to the typical high-Mg andesite,and are similar to the Sanukite from the Setouchi arc volcanic belt in the SW Japan.The subducted oceanic crust and the deep sediments may produce the aquiferous fluid from melted subducted oceanic crust and the deep sediments may react with the overlying mantle peridotite to form the metamorphic basalt-andesite.LA-ICP-MS U-Pb zircon date obtained from the gabbro-diorite coexisting with the Gupo metamorphic basaltic andesite indicates that the high-Mg andesitic magmatism event happened at 514±4.5 Ma.The Guanzi town high-Mg andesites was probably formed from the fore-arc tectonic environment because of the oceanic crust subducted in the Middle Cambrian.This study provides the evidence of the Shangdan oceanic subduction in the Middle Cambrian.

关 键 词：高镁安山岩 俯冲作用 中寒武世 古商丹洋 甘肃省 地质调查工程 

分 类 号：P534.41[天文地球—古生物学与地层学] P588.144[天文地球—地质学]