滇西昌宁-孟连结合带石炭纪火山-沉积地层时代与构造环境

The age and tectonic environment of Carboniferous volcanic-sedimentary strata in Changning-Menglian zone,Western Yunnan Province

昌宁-孟连结合带内大面积出露石炭纪火山-沉积地层,对其时代的精确厘定及古构造环境的识别,有助于认识和恢复昌宁-孟连结合带乃至东古特提斯洋演化。通过详细的野外地质调查与剖面测制,查明玄武岩与碳酸盐岩之间为整合接触关系。对碳酸盐岩开展了详细的微相分析,识别出4种微相类型,分别为颗粒生屑灰岩(MF1)、似球粒泥粒生屑灰岩(MF2)、鲕粒灰岩(MF3)和含珊瑚碎屑礁灰岩(MF4),主体形成于热带-亚热带较温暖的古气候背景,代表了浅水高能环境。对碳酸盐岩与玄武岩之间的凝灰岩夹层开展锆石U-Pb定年研究,获得^(206)Pb/^(238)U年龄加权平均值为351.7±2.5 Ma,表明玄武岩的喷发时代为早石炭世。该套火山-沉积地层岩石组合包括具有OIB型地球化学特征的玄武岩、礁灰岩、塌积砾岩、凝灰岩等,将其构造环境厘定为洋岛-海山。该洋岛-海山是三江造山带洋壳残片的重要组成部分,记录了古特提斯洋盆构造演化及造山带形成过程的重要信息。

A large area of Carboniferous volcanic-sedimentary strata is exposed in the Changning-Menglian junction.The precise determination of its age and the identification of the paleo-tectonic environment are of great significance for understanding and restoration of Changning-Menglian zone and evolution of the Eastern Paleotethys Ocean.The detailed field geological survey and profile surveying indicate that the contact between basalt and carbonate rock is conformity.A detailed microfacies analysis was carried out on carbonate rocks,and four types of microfacies were identified,granular bioclastic limestone(MF1),spherulitic mud particles bioclastic limestone(MF2),oolitic limestone(MF3)and coral clastic reef limestone(MF4),indicating that the main carbonate rock was formed in a shallow-water high-energy environment under a warmer tropical-subtropical palaeoclimate background.Zircon U-Pb dating of the tuff interbeded between carbonate rock and basalt yields the average age of 351.7±2.5 Ma,indicating that the eruption of basalt was in the Early Carboniferous.This set of volcanic-sedimentary rocks include basalt,reef limestone,tuff,etc.with OIB-type geochemical characteristics,and its tectonic environment is defined as ocean island-seamount,which is an important part of oceanic crust fragments in the Sanjiang orogen and records important information about the tectonic evolution of the Paleo-Tethys oceanic basin and the formation process of the orogenic belt.