鄂尔多斯盆地西南部汭水河地区上三叠统碎屑锆石U-Pb年代学特征及其地质意义

U-Pb Chronological Characteristics of Late Triassic Sediment in Southwestern Ordos and it’s Tectonic Significance

鄂尔多斯盆地西南部延长组物源及其演化是进行区域原盆恢复和认识区域构造演化及华北克拉通西部破坏的关键。本文以鄂尔多斯盆地西南部汭水河地区晚三叠世延长组砂岩为研究对象,采用锆石LA-ICP-MS测年方法,通过砂岩样品中碎屑锆石U-Pb年龄及其变化,结合区域背景及古水流资料,系统地研究了盆地西南部延长组物源及其演化。研究认为,晚三叠世早期(231～225Ma),盆地西南部延长组沉积物源主要为盆地东北部阜平杂岩及盆地北部的海西期岩浆岩,部分搬运来的物质与来自秦祁造山带的剥蚀物质一起经宝鸡—武都裂陷槽流入松潘—甘孜地区;晚三叠世中期(225～218Ma),来自秦岭造山带的挤压开始增强,盆地西南部延长组地层中来自南缘秦岭造山带的物源开始增加,盆地西南部沉积物源开始与松潘—甘孜地区出现明显差别,宝鸡—武都裂陷槽开始关闭;至晚三叠世晚期(218～210Ma),秦岭与祁连造山带完成拼贴,宝鸡—武都裂陷槽最终完全关闭。

To make sure the sedimental sources and it’s development of Yanchang Formation is the key of restoring prototype basin , regional tectonic evolution and breakage of western north-china craton in late triassic on southwest edge of Ordos Basin. Taking some sandstone sample of Yanchang Formation from Ruishuihe section as research object, the detrital zircons ages and it＇s development were analyzed from sandstone samples located in the Southwestern Ordos basin,with the method of LA-ICP-MS,trace element compositions and cathodoluminescence. Combining regional background,tectonic events and paleocurrent survey,the sedimental sources and it＇s development were comprehensively analyzed, on the basis of the detrital zircons age spectrum and it’s change. The results indicate that the study area was under the regional dextral strike slip dynamic environment in late Triassic. It can be divided into three stages in late Triassic, early stage （231~225Ma）,middle stage（225~218Ma） and late stage （218~210Ma）,respectively. In the early stage,there are mainly sediment provenance from Hercynian provenance on north margin of Ordos and Fupin complexes on northwestern margin of Ordos. Meanwhile, some of them and sediment provenance from Qinqi orogenic belts were influxed into Songpan-Garze basin through Baoji-wudu aulacogen. In the middle stage,there are mainly sedment provenance from Qinqi orogenic belts on southwestern margin of Ordos,Hercynian provenance on north margin of Ordos, Fupin complexes on northwestern margin of Ordos. Meanwhile,Baoji-wudu aulacogen began to close. In the late stage,Baoji-wudu aulacogen was closed.