鄂尔多斯盆地西北缘奥陶纪凝灰岩锆石U-Pb年龄、Hf同位素特征及地质意义

Zircon U-Pb Ages and Hf Isotope Characterestics and Their Geological Significance of Tuff in Wulalike Formation in the Northwestern Ordos Basin

鄂尔多斯盆地西缘北段奥陶纪剖面中出露有火山凝灰岩夹层,它们是火山喷发的凝灰物质沉积的产物,其研究对于鄂尔多斯盆地奥陶纪年代地层格架的厘定和构造背景的确定有重要意义。本文报道了鄂尔多斯盆地西缘北段乌海地区哈图克沟剖面乌拉力克组顶部火山凝灰岩的锆石U-Pb年龄和Hf同位素特征。锆石LA-ICPMS U-Pb年龄为454.9±3.4Ma,SHRIMP U-Pb年龄为456.4±4.2Ma,结合古生物地层学,将乌拉力克组的沉积时代厘定为中奥陶世达瑞威尔阶晚期—晚奥陶世桑比阶。锆石原位微区Hf同位素εHf(t)=+4.02^+10.98,n(176Hf)/n(177Hf)值变化范围为0.282 619~0.282 963之间,二阶段模式年龄变化范围为729~1178Ma。根据凝灰岩与研究区周缘造山带的锆石U-Pb年龄及Hf同位素特征对比判断火山凝灰岩物质应主要来自于其西南方向的北祁连造山带和/或北秦岭造山带。鄂尔多斯盆地西缘和南缘广泛分布的火山凝灰岩具有一致的就位时间和物源,表明盆地西缘和南缘都受到了中—晚奥陶世北祁连和/或北秦岭洋壳俯冲引起的火山弧喷发的影响。

Thin layered and laminated tuff was developed in the Wulalike Formation in the northwestern Ordos Basin.This tuff bed was produced by volcanic ash fall deposits which can reclassify the Ordovician stratigraphic framework of Ordos Basin and provide importantconstrains on the plate tectonic setting of the northwestern Ordos Basin as well as major volcanism—magma activities. In this contribution,wecarried SHRIMP U Pb dating and Hf isotope on tuff sample collected from Zhuozishan area, located in Wuhai City,Inner Mongolia.LA ICP MS U Pb dating on zircons has yielded a weighted average of 4549±34Ma（MSWD=002）. SHRIMP U Pb dating on zircons from the tuff sample has yielded a set of concordant ages of 4332±33～4642±35Ma with the mean age 4564±42Ma（MSWD=19）. Accordingly,the main depositional period of Wulalike Formation,which is still controversial,is redefined to Darriwilian—Sandbian,Middle—Late Ordovician. In situ zircon isotope analysis yieldedεHf（t） = ＋402 ~ ＋1098,n（176Hf）/n（177Hf）ratios of 0282 619~0282 806 and two stage model age of 729~1178Ma. According to their regional distribution, it is thought that these tuffs derive from volcanic eruption in the North Qinling or/ and North Qilian region. The zircon age and magmas source are in accordance with the tuffs from Wuhai city to the tuffs（or K bentonites） from the southwestern Ordos Basin, which means all the western and southern Ordos Basin affected by the North Qilian or/ and North Qinling Early Paleozoic orogenic belt during Middle—Late Ordovician.