鄂尔多斯盆地南缘中晚三叠世物源演变及其地质意义*

针对鄂尔多斯盆地南缘中晚三叠世物源构成转化及盆山耦合机制不清的问题,选取了铜川地区金锁关剖面和周至柳叶河剖面的延长组砂岩为研究对象,运用岩石学、碎屑锆石U-Pb年代学、地球化学的方法,探讨鄂尔多斯盆地南部延长组的物源构成和变化,并探寻其构造耦合机制。结果表明,金锁关剖面上三叠统碎屑锆石可分为5个年龄段,分别是237-330 Ma、390-480 Ma、870-1230 Ma、1740-1980 Ma、2070-2732 Ma,中三叠统碎屑锆石共具有4个年龄段,分别是240-290 Ma、1760-1840 Ma、2250-2300 Ma、2350-2700 Ma;盆地南端柳叶河地区上三叠统碎屑锆石共具有5个年龄段,分别是244-310 Ma、360-600 Ma、800-1300 Ma、1700-2100 Ma和2450-2550 Ma。通过物源对比发现,中三叠世鄂尔多斯盆地南缘的物源来自于华北克拉通、兴蒙造山带和阿拉善地区,晚三叠世沉积砂体的物源来自华北克拉通、阿拉善、兴蒙造山带、西秦岭、北秦岭以及祁连造山带,且岩石学特征和源区构造背景的转变均支持这一认识。这种物源的转变,与中三叠世盆地南部秦岭造山带的活化以及盆地样式的转变有关。

Provenance evolution of southern margin of Ordos Basin during the Middle-Later Triassic and its geological implication

In this study, the sandstone samples obtained from two profiles including the Jinsuoguan of Tongchuan and the Liuye river of Zhouzhi area, are selected as the research object. It aims at understanding the Middle-Late Triassic source composition and transformation in the southern margin of Ordos Basin and the coupling mechanism of basin and mountain. By means of the petrography, detrital zircon U-Pb geochronology and geochemistry analysis, the changes of the source composition in the southern margin of the Ordos Basin are investigated, and tectonic coupling mechanism between the basin and mountain are explored. It shows that five groups of ages for the Upper Triassic detrital zircons in the Jinsuoguan profile are found: 237-330 Ma, 390-480 Ma, 870-1230 Ma, 1740-1980 Ma, 2070-2732 Ma. Four groups of age are found in the Middle Triassic detrital zircon: 240-290 Ma, 1760-1840 Ma, 2250-2300 Ma, 2350-2700 Ma. Five groups of age are discovered in the Upper Triassic detrital zircon found in the northern Ordos Basin: 244-310 Ma, 360-600 Ma, 800-1300 Ma, 1700-2100 Ma and 2450-2550 Ma. Based on the comparison of sediment source, it confirms that the Middle Triassic sediment provenance of the south margin of the Ordos Basin is from the North China Craton, Xing’anling-Mongolian Orogenic Belt and the Alxa region. The Late Triassic provenance of the sand bodies is from the North China Craton, Alxa region, West Qinling, the Northern Qinling and Qilianshan Orogenic Belt. The deduction is also supported by petrological characteristics and the transformation of the tectonic setting in the source area. The Middle Triassic provenance transformation is related to the activation of the Qinling Orogenic Belt in the south margin of Ordos Basin and the transformation of the basin pattern.