鄂尔多斯盆地西南部上古生界碎屑锆石U-Pb年龄及其构造意义

The Upper Paleozoic detrital zircon U-Pb geochronology and its tectonic significance in southwestern Ordos Basin

通过对鄂尔多斯盆地西南部晚古生代山西组1段和下石盒子组8段碎屑锆石进行LA-ICP-MS U-Pb测年分析,结合周缘地层年龄结构和地质历史事件,进而追寻盆地沉积物物源,推断盆地与造山带的盆山耦合过程。研究表明105个岩浆成因的碎屑锆石可分为4个年龄组段:(1)260~340 Ma,占总数的21.9%,推断物源主要来自北秦岭和西秦岭构造带;(2)370~470 Ma,占总数的24.8%,反映物源主要来自北秦岭、西秦岭构造带和北祁连造山带;(3)1600~2000 Ma,占总数的32.4%,指示物源来自北秦岭造山带、北祁造山带和华北板块;(4)2300~2600 Ma,占总数的15.2%,物源分别来自华北板块基底结晶岩系、北祁连构造带、北秦岭构造带和西秦岭构造带。研究区总体上具有来自北秦岭造山带、西秦岭造山带、北祁连造山带、兴蒙造山带及华北板块基底五个物源区,其中兴蒙造山带、北秦岭造山带和北祁连造山带为主要物源区。古生代碎屑锆石年龄证实了鄂尔多斯盆地西南部奥陶纪被动大陆边缘形成,志留纪—泥盆纪转化为陆-陆碰撞造山带,石炭纪—二叠纪逐渐由造山带转化为沉积盆地。

Aimed at exploration of provenance and basin-mountain coupling relationship in southwestern Ordos Basin,the authorsselected detrital zircons from the first member of Shanxi Formation and the eighth member of Shihezi Formation of the late Paleozoic in southwestern Ordos Basin and conducted LA-ICP-MS detrital zircon U-Pb isotope analysis.The result reveals thatthe105analytical points of the magma-genetic detrital zircons yielded four age groups:(1)The first group which ranges from260to340Ma accounting for21.9%of the total analytical points,indicating that the provenance might have come from the NorthQinling orogenic belt and the West Qinling orogenic belt;(2)The second group which ranges from370to470Ma accounting for24.8%of the total analytical points,suggesting that the provenance was mainly derived from the North Qinling orogenic belt,theWest Qinling orogenic belt and the North Qilian orogenic belt;(3)The third group which ranges from1600to2000Ma accountingfor32.4%of the total analytical points,implying that the provenance might have come from the North Qinling orogenic belt and theNorth Qilian orogenic belt;(4)The last group which ranges from2300to2600Ma accounting for15.2%of the total analyticalpoints,indicating that the provenance was derived from the North Qinling orogenic belt,the West Qinling orogenic belt,the NorthQilian orogenic belt and the crystalline basement of the North China plate.This study indicates that the provenance of southwesternOrdos Basin had multiple origins:the North Qinling orogenic belt,the West Qinling orogenic belt,the North Qilian orogenic beltand the crystalline basement of the North China plate,mainly from the North Qinling orogenic belt and the North Qilian orogenicbelt.The Paleozoic detrital zircons have also proved that the southwestern Ordos Basin passive continental margin was developedduring the Ordovician,then it evolved into continent-continent collisional orogenic belt during the Silurian-Devonian,and finallygradually evolved into a sedimentary basin during the Carboniferous-Permian period.