北秦岭柳叶河盆地石炭系—二叠系含砾砂岩碎屑锆石LA-ICP-MS U-Pb年龄及其地质意义

通过北秦岭黑河地区柳叶河盆地碎屑锆石年龄研究,探讨盆地沉积物年龄结构,并进一步推断其物源及与秦岭造山带的盆山耦合过程。采自柳叶河盆地石炭系—二叠系含砾砂岩样品中的82颗碎屑锆石年龄可分为6组:260~542Ma、800~1100Ma、1320~1560Ma、1640~2250Ma、2300~2660Ma和2720~2870Ma。其中260~542Ma年龄段所占测点比例最大,占锆石总数的31%,其次为1640~2250Ma、2300~2660Ma和800~1100Ma年龄段。最老年龄值为2834±40Ma,最年轻者为264±5Ma。从碎屑锆石年龄结构对比及石炭纪地层交错层理推断,柳叶河盆地石炭系—二叠系物源区主要为当时的北秦岭山地。此外,碎屑锆石年龄结构对比表明,鄂尔多斯盆地南部与柳叶河原始盆地有相同的年龄谱特征,意味着它们具有同一物源区。北秦岭山地是在加里东造山作用后期影响下迅速隆升而形成的。其后,随着南侧勉略洋的打开,华北地台相对沉降及沿商丹带的走滑断陷使柳叶河盆地出现,仍旧处于隆升状态的北秦岭山地开始作为该盆地及其北侧鄂尔多斯盆地南部的主要物源区。

LA-ICP-MS U-Pb dating of detrital zircon from Liuyehe basin in North Qinling Mountains

Aimed at exploration of basin-mountain coupling relationship between North Qinling Mountains and Liuyehe basin, the authors employed the detrital zircons age dating of the Carboniferous-Permian sedimentary rocks from Liuyehe basin located in North Qinling Mountains. The U-Pb dating results of 82 zircons gave rise to six age groups in an age range of 264±5~2834±40Ma, 260~542Ma, 800~1100Ma, 1320~1560Ma, 1640~2250Ma, 2300~2660Ma and 2720~2870Ma. Among the groups, the 260~542Ma age group represents the largest proportion of the measured points, followed in succession by 1640~2250Ma group, 2300~2660Ma group and 800~1100Ma group. A comparative study of the age structure and the Carboniferous cross-bedding shows that the provenance of the Liuyehe basin during the Carboniferous-Permian was probably located in the North Qinling Mountains during the Late Paleozoic. In addition, the detrital zircon age structure comparison shows that the sedimentary rocks of the southern Ordos basin and the Liuyehe basin have the same age spectrum characteristics, which in turn means that they had the same provenance during the Permian. Under the influence of late Caledonian orogeny, North Qinling area experienced rapid uplift and eventually completed the evolution of basin-mountain conversion. Later, along with the Shangdan strike-slip fault, the Liuyehe basin began to form due to the opening-up of the Mianlue Ocean, while the North Qinling Mountains became its main provenance.