内蒙古锡林郭勒杂岩中泥盆纪地层的发现及地质意义

分布在锡林浩特—达青牧场一带的锡林郭勒杂岩主要由变质表壳岩、变质基性-超基性岩、花岗质片麻岩等组成,其中部分为前寒武纪地层和岩石,构成前寒武纪微陆块。本文对锡林浩特西部呼热木台敖包和白音陶勒盖一带锡林郭勒杂岩中副变质岩锆石LA-MC-ICP-MS U-Pb年代学进行了研究,原岩碎屑锆石年龄介于403~3077 Ma,其中~(206)Pb/~(238)U最年轻一组的年龄在403~420 Ma,代表了该变质岩原岩的沉积下限。结合其变质时代(337 Ma)及被早石炭世—晚石炭世早期岛弧侵入岩侵入的事实,该套地层主要形成在早泥盆世中期—早石炭世早期,不是前寒武纪地层。其原岩主要为一套正常沉积碎屑岩,缺少火山岩,不具弧前沉积建造特征。它是沉积在前寒武纪锡林浩特微陆块之上的一套地层,为早古生代造山后伸展背景下晚古生代贺根山洋盆南缘初始的沉积记录。

The discovery of Devonian strata in Xilin Gol complex of Inner Mongolia and its geological significance

Abstract: Xilin Gol complex occurs in Xilinhot-Daqingmuchang area and is composed of metamorphic supracrustal rocks, metamorphic basic-ultrabasic rocks and granitic gneiss, with some of them being Precambrian in age and constituting Precambrian block. Recent dating of the LA-MC-ICPMS U-Pb zircons from parametamorphic rocks of Xilin Gol complex in Huremutaiaobao and Baiyintaolegai area yieled ages of detrital zircons ranging from 403Ma to 3077 Ma with a group of 206Pb/238U ages in the youngest strata between 403 and 420 Ma, probably representing the lower limited age of the sedimentary rock. Combined with 337 Ma metamorphic age of the parametamorphic rocks which were emplaced by arc intrusive rocks of Early Carboniferous-early Late Carboniferous, the authors hold that the parametamorphic rocks were formed in middle Early Devonian-early Early Carboniferous, and their protolith belonged to normal sedimentary rocks without being subjected to volcanic activity, and hence it was not characterized by forearc deposition formation. Therefore, it comprised a set of strata deposited on Precambrian Xilinhot micro-block, representing initial deposition of the southern margin of Late Paleozoic Hegenshan Ocean basin with extension after Early Paleozoic collision orogenesis.