扬子西南缘中-新元古代两期岩浆活动年代学及地质意义  

Geochronology and Geological Implication in Two Episodes of Meso-Neoproterozoic Magmatism in the Southwestern Yangtze Block

作　　者：张继彪 丁孝忠[1] 刘燕学[1] 张恒[1] Zhang Jibiao;Ding Xiaozhong;Liu Yanxue;Zhang Heng(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：《地球科学》2020年第7期2452-2468,共17页Earth Science

基　　金：“江南造山带区域地质调查片区总结与服务产品开发”项目(No.121201111120117);全国陆域及海区地质图件更新与共享项目(No.DD20190370)。

摘　　要：扬子陆块西南缘发育一系列中-新元古代岩浆岩,对认识扬子陆块构造演化具有重要意义.对会理地区天宝山组流纹岩和盐边地区辉长岩进行SHRIMP锆石U-Pb年代学、地球化学研究.天宝山组流纹岩时代为1011.9±8.9 Ma,辉长岩时代为910.6±4.7 Ma.天宝山组流纹岩具有高硅、高FeOt/MgO、高钾等特征;稀土含量(∑REE=292×10-6~401×10-6)较高,表现出轻稀土富集重稀土弱亏损的特征[(La/Yb)N=1.77~6.74],Eu负异常明显(δEu=0.43~0.56),与A型花岗岩相似;天宝山组流纹岩来自古老的地壳物质的部分熔融,形成于大陆裂谷环境.盐边群辉长岩稀土含量(∑REE=54×10-6~98×10-6)较低,轻重稀土分异较弱[(La/Yb)N=1.46~4.72],Eu具有轻微的异常(δEu=0.81~1.31);岩石具有明显的Nb-Ta、Ti负异常,无Zr-Hf正异常;地球化学数据显示辉长岩来自被俯冲板片释放的流体/熔体交代的地幔楔部分熔融,形成于岛弧环境.两期岩浆活动指示了扬子西南缘1000~910 Ma之间构造动力学背景发生了转变,由伸展背景转变为挤压背景.Late Mesoproterozoic to Early Neoproterozoic igneous rocks occur in the southwestern Yangtze block,which had a great bearing on the evolution history of the Yangtze block during the late Mesoproterozoic to early Neoproterozoic.This study reports SHRIMP zircon U-Pb ages and geochemistry data for gabbros that intruded in the Yanbian Group,and that of rhyolites from the upper Tianbaoshan Formation of the Huili Group in the southwestern Yangtze block.The rhyolites were dated at1011.9±8.9 Ma and the gabbros were formed at 910.6±4.7 Ma.The rhyolites in the Tianbaoshan Formation were characterized by high SiO2 and K2 O contents and high FeOt/MgO ratio.The contents of rare earth elements of rhyolites are high(∑REE=292×10-6-401×10-6),and characterized by LREE-enriched and HREE-depleted patterns[(La/Yb)N=1.77-6.74]with typical depletion of Eu(δEu=0.43-0.56),consistent with the geochemical characteristics of A-type granites.The geochemistry indicates that the rhyolites were derived from the partial melting of previous crust and formed in a continental rift setting.The gabbro shave low rare earth elements(∑REE=54×10-6-98×10-6)and characterized by slightly LREE-enriched and HREE-depleted patterns with unconspicuous Eu anomaly[(La/Yb)N=1.46-4.72,δEu=0.81-1.31]and the trace element patterns with typical depletion of Nb–Ta and Ti but no enrichment of Zr–Hf.The gabbros were derived from the subduction–modified lithospheric mantle wedge and formed in an arc setting.In view of the two episodes of magmatism in the study region,we propose that the tectonic properties changed from a continental rift setting to a compression setting in the southwestern Yangtze Block at 1000-910 Ma.

关 键 词：扬子西南缘 中-新元古代 SHRIMP锆石U-PB年龄 地球化学 大地构造背景 

分 类 号：P548[天文地球—构造地质学] P597[天文地球—地质学]