扬子地台东南缘上奥陶统五峰组斑脱岩锆石U-Pb年龄及其地质意义

Zircon U-Pb age of bertonite of Upper Ordovician Wufeng Formation in Southeast Yangtze platform and its geological significance

扬子板块东南缘思南地区上奥陶统顶部—下志留统底部地层中普遍发育一层斑脱岩。选取思南县罗湾沱上奥陶统五峰组剖面进行LA-ICP-MS锆石U-Pb测年,获得其年龄加权平均值为449±11Ma,限定了该区域奥陶系—志留系地层沉积年龄,为扬子板块东南缘思南地区奥陶系—志留系界线附近的火山喷发事件、地层年代学研究提供了较可靠的证据。449±11Ma的年龄与扬子板块北西大巴山一带的年龄数据高度吻合,但略晚于华北克拉通西南缘奥陶系顶部斑脱岩的年龄(449.0~465.8Ma)。其所代表的火山活动可能与华夏板块向扬子板块俯冲有关。晚奥陶世—早志留世晚期,多期次的火山喷发事件影响了当时的海洋化学条件、生态圈环境,造成中—晚奥陶世以来的生物灭绝事件及冰川启动。

A layer of bentonite is usually developed in upper Ordovician and lower Silurian strata of southeast Yangtze platform in Sinan area.Sampling was done in Wufeng Formation of upper Ordovician and Longmaxi Formation of lower Silurian in Luowantuo of Sinan,high precision U-Pb dating was conducted and as a result,the stratigraphic sedimentary age of Wufeng Formation and Longmaxi Formation in this area were constrained.The data obtained provide reliable evidence for volcanic eruption and stratigraphic chronology near Ordovician and Silurian boundary of southeast Yangtze platform in Sinan area.The age data are correlated well with the data obtained in Dabashan of northwest China,which is a little later than the age of Ordovician bentonite in southeast of the North China Craton(449.0~465.8Ma).The volcanic activity was probably related to the subduction of Cathaysian plate to Yangtze plate.During Middle-Late Ordovician and late Early Silurian,many volcanic eruptions influenced the chemical condition and ecological environment of the ocean in that period and caused the extinction and glacier initiation since middle-late Ordovician.