扬子板块西南缘基性侵入岩锆石年龄及地球化学特征--Columbia超级大陆裂解的响应

Geochemical characteritics and U-Pb age of the zircons from mafic intrusion in the southwestern margin of the Yangtze plate: Response to break-up of the Columbia supercontinent

云南易门狮子山铜矿区广泛出露辉绿、辉长岩体。前人对区内基性岩浆岩研究程度较低,缺乏辉绿岩的形成时代资料。本文采用锆石激光剥蚀法(LA-ICP-MS)对云南易门狮子山铜矿一带的辉绿岩进行了年代测定及地球化学分析,首次获得锆石U-Pb年龄加权平均值为1858±18 Ma(MSWD=0.90,n=13),代表了该辉绿岩的形成和侵位时代。辉绿岩全岩地球化学具有类似于OIB的特征,SiO2含量(49.1%~49.8%),高Ti2O(2.68%~3.22%);低MgO(4.85%~5.51%),碱含量(4.86%~5.29%),其原始岩浆来源于富集型地幔,构造背景形成于伸展构造环境,表明约1858 Ma扬子板块处于碰撞后板内伸展构造环境,应为Columbia超大陆裂解的产物。易门狮子山地区被辉绿岩侵入接触的原"东川群"地层的沉积时代应大于辉绿岩的侵位时代,结合岩性组合及区域资料,应划入中太古界元江群。

Objectives: On the basis of 1:50000 regional geological survey,it is discovered that pyroxene and gabbro are widely exposed in the shizishan copper orefield,Yimen County,Yunnan Province. The studies on basic magmatic rocks in the area are relatively less and the diabase formation time data are absent. In this paper,we carried out zircon laser denudation( LA-ICP-MS) dating and geochemical analysis on the diabase in Shizishan Copper orefield,Yimen County.Results and Conclusions: The weighted average age of zircon U-Pb is 1858±18 Ma( MSWD = 0.90,n = 13).It represents the formation and emplacement of the diabase. The whole rock geochemical characteristics of the diabase are similar to those of the OIB,and show that the diabase formed in a extensional tectonic environment. So,the Yangtze plate was in the post-collisional intraplate extensional tectonic environment at 1858 Ma ago,which should be related to the break-up of Columbia supercontinents. The sedimentary time of the "Dongchuan Group"intruded by the diabase in the Shizishan orefield must be earlier than that of the diabase emplacement. Combined with the lithologic assemblage and regional data,the "Dongchuan Group"should be divided into the Mesoarchaean Yuanjiang Group.