柴达木盆地北缘构造带早古生代造山后作用初探:泥盆纪沉积物物源示踪

A preliminary study on post-orogenesis of the North Qaidam tectonic belt duringthe Early Paleozoic by provenance analysis of the Devonian sediments

柴达木盆地北缘(柴北缘)泥盆系牦牛山组以发育冲积扇沉积体系的粗碎屑岩和中酸性火山岩的岩石组合为特征,记录了柴北缘构造带从碰撞造山进入造山后阶段的核部岩石剥露过程。尕海南山牦牛山组地层顶部和底部的凝灰岩锆石LA-ICP-MS U-Pb加权平均年龄分别为396.6±2.4 Ma(MSWD=0.39,n=14)和396.9±2.5 Ma(MSWD=0.052,n=14),指示了牦牛山组沉积时代大致为早泥盆世末期。不同出露点牦牛山组砂岩碎屑锆石LA-ICP-MS U-Pb年龄谱具有不同的峰值特征:尕海南山调查点A,样品20QH-19具有532~505 Ma、935~891 Ma、1136~1064 Ma和3110~1309 Ma多峰特征;尕海南山调查点B,样品20QH-21具有537~443 Ma、1073~798 Ma和2463~1785 Ma多峰及20QH-22以明显的单一峰值~420 Ma为特征;牦牛山调查点C,20QH-26以明显的~418 Ma单峰为特征。多样的年龄组分指示了牦牛山组沉积物质主要来源于柴北缘志留纪—泥盆纪同造山岩浆岩、寒武系—奥陶系滩间山群、沙柳河群及欧龙布鲁克地块。牦牛山组岩石组合特征、沉积体系发育、凝灰岩锆石U-Pb年龄及源区物质组成等指示了泥盆纪时期柴北缘地区处于原地堆积、快速沉积、分散的、不连通的盆地环境。同时,牦牛山组沉积物物源示踪了柴北缘构造带在泥盆纪时期经历的快速剥露过程并且可能已经进入造山后伸展垮塌阶段。

The Devonian Maoniushan Formation of the northern Qaidam basin is characterized by alluvial fan coarse-grained sediments and intermediate-acid volcanic rocks,recording core rocks exhumation process of the North Qaidam tectonic belt from collisional to post orogenesis.LA-ICP-MS U-Pb weighted mean ages of the upper and lower Maoniushan Formation tuff deposited around the Gahainanshan Mountain are 396.6±2.4 Ma(MSWD=0.39,n=14)and 396.9±2.5 Ma(MSWD=0.052,n=14),respectively,indicating the depositional time of the Maoniushan Formation is almost late Early Devonian.Different detrital zircons collected from different outcrops are characterized by diverse LA-ICP-MS U-Pb populations,that is,sample 20QH-19 collected from the field investigation A,which is located in the Gahainanshan Mountain,is characterized by multiple-peak ages dated at about 532~505 Ma,935~891 Ma,1136~1064 Ma and 3110~1309 Ma;sample 20QH-21 collected from the field investigation B,which is located in the Gahainanshan Mountain,is characterized by multiple-peak ages dated at about 537~443 Ma,1073~798 Ma and 2463~1785 Ma,and sample 20QH-22 is featured by a predominantly unique peak age dated at~420 Ma,whose pattern is similar to the sample 20QH-26 collected from the field investigation C.Diverse U-Pb age populations indicate that the detrital materials of the Maoniushan Formation were mainly sourced from the Silurian-Devonian syn-orogenesis igneous rock,the Cambrian-Ordovician Tanjianshan Group,the Shaliuhe Group of the North Qaidam tectonic belt and the Olongbuluke terrane.Rock assemblage characteristics,developments of depositional systems,U-Pb ages of tuff rocks and sediment dispersal patterns of the Maoniushan Formation indicate that autochthonous sedimentation,rapid deposition,scattered and disconnected sedimentary environments.Provenance analysis of the Maoniushan Formation sediments indicate that the source areas had undergone rapid exhumation process and probably evolved into the extensional collapse stage followed the post-orogenesis during the deposition of the Maoniushan Formation.