内蒙古突泉盆地中侏罗统万宝组碎屑岩LA-ICP-MS锆石U-Pb测年、地球化学特征及对源区的制约

Zircon U-Pb dating and geochemical characteristics of detrital rocks and their constraints on the provenance of Middle Jurassic Wanbao Formation in Tuquan basin

突泉盆地中侏罗统万宝组碎屑岩中砾石、岩屑及重矿物组合研究显示,母岩主要为流纹岩、安山岩、花岗岩、砂泥质岩石和少量中低变质岩;碎屑岩地球化学特征显示,源区形成于大陆岛弧和活动大陆边缘环境。在万宝组砂岩样品中,利用LA-ICP-MS定年方法获得100个单颗粒碎屑锆石U-Pb年龄介于171Ma±7Ma～1221Ma±23Ma之间,其中1个年轻的年龄为171Ma±7Ma,可近似代表其沉积时的最新年龄,2个较老年龄(1221Ma±23Ma,504Ma±10Ma)可能来自于较古老结晶基底。其余年龄在214Ma±5Ma～359Ma±9Ma之间,为三叠纪—石炭纪的岩石年龄。结合区域地质特征认为,盆地东缘和南缘古隆起在万宝组沉积时已经存在,下伏的大石寨组、哲斯组和古生代侵入岩为盆地提供了主要物源。

The detrital rocks of Middle Jurassic Wanbao Formation mainly include conglomerates,sandstones and mudstones.Among the sandstones,lithic sandstones are dominant,and feldspathic lithic sandstones and debris feldspar sandstones take the second place.This kind of distribution implies characteristics of near source sedimentation.The rock assemblage comprising gravels,debris and heavy minerals implies that the parent rocks of the clastic rocks consisted of rhyolite,andesite,granite,argillo-arenaceous rocks and some intermediate to low-grade metamorphic rocks.Geochemical characteristics of the clastic rocks indicate that the rocks in the provenance region were mainly felsic and formed in the environment of continental island arc and active continental margin.100 U-Pb data were obtained by LA-ICP-MS method for the zircons in detral rocks from Wanbao Formation in this study,and the majority of the grains from the sandstone samples in Wanbao Formation are clustered in the ages of 171Ma ±7Ma ～1221Ma ±23Ma.Among them,the younger age（171Ma±7Ma） represents the oldest sedimentary age,two of the older age（1221Ma±23Ma,504Ma±10Ma） may display a source of crystalline palaeobasement,and the other ages（between 214Ma±5Ma and 359Ma±9Ma） belong to the periods between Triassic and Carboniferous.According to these geological characteristics,the authors hold that the east and the south paleouplifts near Tuquan basin had already existed when sedimentary strata in Wanbao Formation were formed,and the provenances of Wanbao Formation were mainly from Dashizhai Formation,Zhesi Formation and Paleozoic intrusive rocks.