南秦岭蚂蝗沟黑云斜长片麻岩LA-ICP-MS锆石U-Pb年代学及其大地构造意义

DETRITAL ZIRCON U-Pb DATING BY LA-ICP-MS AND ITS GEOLOGICAL SIGNIFICANCE IN MAHUANGGOU, SOUTHERN QINLING MOUNTAINS

秦岭各变质岩群的研究是探讨秦岭造山带形成演化的重要内容,针对产出于南秦岭蚂蝗沟的黑云斜长片麻岩进行岩石学分析,其主要矿物为石英、斜长石、黑云母,并含有少量石榴石。样品轻稀土富集,重稀土亏损,具有Eu较强烈负异常和Ce基本无异常的特点,微量元素总量差异不大,且Nb,Ti显著亏损,应用尼格里指数对Si投影图和La/Yb对ΣREE投影图判断其为副变质岩,原岩为杂砂岩且具太古后碎屑沉积岩的特点。应用LA-ICP-MS锆石U-Pb定年方法测试,结果显示锆石共有四期年龄,其谐和年龄分别为780Ma±8.8Ma,608Ma±2.8Ma,450Ma±5.5Ma,205.1Ma±2.3Ma,分别是对晋宁运动第二幕、加里东运动早期、加里东运动晚期和印支运动的响应。四期年龄分别独立形成于不同区域,在最后一次构造事件发生时才共同沉积再经历弱变质作用形成黑云斜长片麻岩,变质年龄为202Ma±5.5Ma,与最后一期构造热事件同是印支运动作用的结果,为秦岭造山带构造演化提供证据,对认识其构造运动具有一定的重要意义。

Geochemical study of metamorphic rocks （mainly biotite-plagioclase gneiss） in Qin- ling Mountains plays an important role in analyzing the evolution of Qinling orogenic belt. The mineral assemblage of the gneiss is composed of quartz, plagioclase, biotite and a small amount of garnet. It is showed that the gneiss is characteristic of enrichment in I.REE and depletion in HREE, with obvious negative Eu anomaly,and without Ce anomaly. Meanwhile,trace elements Nb andTi have a striking depletion. Plots of （al＋fm）-（c＋alk） vs Si and plots of La/Yb vs ∑REE indicates that the gneiss belongs to parametamorphic rocks, and its original rocks are apog- rite,characteristic of post Archean clastic sedimentary rocks. LA-ICP-MS U-Ph zircon dating shows that there are four concordia diagrams of these tested zircons：780 Ma±8.8 Ma,608 Ma± 2.8 Ma,450 Ma±5.5 Ma,205.1 Ma±2.3 Ma,correspond to the second phase of Jinning move- ment, early stage of Caledonian event, late stage of Caledonian event and Indo-China movement re- spectively. The original materials of the above mentioned four stages formed at different places and only deposited and metamorphosed during the last tectonic event. The research data provided evidence for structural evolution of Southern Qinling orogenic zone.