柴达木盆地北缘都兰地区旺尕秀辉长杂岩的锆石LA-ICP-MS U-Pb年龄及地质意义

Zicon LA-ICP-MS U-Pb dating of the Wanggaxiu gabbro complex in the Dulan area,northern margin of Qaidam Basin,China and its geological significance

详细的地球化学研究表明,柴北缘旺尕秀辉长杂岩(包括辉长岩、辉绿岩和斜长岩)具有岛弧火山岩的特征,形成于岛弧环境。对辉长岩中锆石的阴极发光图像和微量元素的研究显示,所有的锆石均具有相对宽缓的振荡环带结构和轻稀土元素亏损、重稀土元素明显富集的典型基性岩浆锆石的特征。通过锆石LA-ICP-MSU-Th-Pb同位素分析,得到206Pb/238U加权平均年龄468Ma±2Ma,同时得到一个最大的谐和年龄522Ma±9Ma,表明旺尕秀辉长杂岩的形成时代应介于468～522Ma之间。岛弧型旺尕秀辉长杂岩时代的确定,结合本区已有的研究成果,表明柴北缘确实存在古生代洋盆事件,至少从中奥陶世开始发生板块的俯冲消减作用,从侧面说明柴北缘陆壳深俯冲的时代不应早于460Ma。

Detailed geochemical study shows that the Wanggaxiu gabbro complex from the North Qaidam has characteristics of island arc volcanic, likely formed in a sitting of an island arc. Cathodeluminescence investigations and trace elements analyses of zircons separated from the gabbro reveal that all zircons display relative wide oscillatory zoning, high abundances of REE and HREE, the REE patterns of depletion in LREE, and strong enrichment in HREE are of the features of zircons derived from basic magma. LA-ICP- MS U-Th-Pb isotopic analyses yield a ^206Pb/^238U weighted average age of 468 Ma±2 Ma, and a maximum concordant age of 522 Ma±9 Ma, implying that the Wanggaxiu gabbro complex was formed during 468-522 Ma. The determination of island arc type of Wanggaxiu gabbro complex, together with the previous studies on the Tanjianshan volcanics, suggesting the existence of Paleozoic o- ceanic event in the North Qaidam, and the slab subduction start at least from mid-Ordovician. And considering of this point, the age of the continental deep subduction of the North Qaidam should not be earlier than 460 Ma.