古新世特提斯喜马拉雅南亚带石英砂岩成因及其构造意义:以藏南岗巴地区古新统基堵拉组为例

特提斯喜马拉雅南亚带作为印度被动大陆北缘的主要构造单元,相较于其他类似构造单元发育着多套特殊的石英砂岩,意味着被动大陆边缘物源区陆源碎屑供应能力的多次变化,而引起印度被动大陆北缘石英砂岩沉积的构造背景和构造意义尚不明确。以特提斯喜马拉雅南亚带的岗巴地区古新统基堵拉组石英砂岩为例,通过砂岩碎屑成分分析、古流向恢复、重矿物分析和碎屑锆石年代学方法,对基堵拉组石英砂岩的沉积学及物源区特征,以及蕴含的成因和构造意义进行探讨。从沉积相分析结果来看,在早古新世岗巴地区所属的板块为印度被动大陆边缘,处于新特提斯洋的海岸线附近,以滨岸相为主,显示了一种浅海陆棚到陆相的变化。从砂岩岩相学的结果分析,基堵拉组的陆源碎屑物主要是成熟度极高的石英砂岩,同时古水流近NNE方向。从碎屑锆石年代学数据分析结果可知,基堵拉组的碎屑锆石年龄特征与早白垩世德干高原地区相吻合。故认为基堵拉组石英砂岩的形成是由于印度北缘的陆源碎屑供应量突然增多与被动大陆边缘物源区构造抬升导致,而引起被动大陆边缘物源区构造抬升的原因主要与德干大火成岩省形成相关。最终认为石英砂岩的发育成因与印度大陆北缘德干大火成岩省形成时构造隆升所导致的稳定克拉通再活化有关。

Genesis and Tectonic Significance of Quartz Sandstones in the Southern Subzone of Tethyan Himalayas: A Case Study on the Paleocene Jidula Formation in Gamba Area,Southern Tibet

The Tethyan Himalayas is the main tectonic unit of the northern Indian passive margin. Compared to other similar tectonic units, there are many suites of special quartz sandstones, implying that the terrigenous supply capacity had changed overtime. Tectonic setting and significance of these quartz sandstones are yet to be clear. In this paper, the quartz sandstones (Jidula Formation) in Gamba of the Tethyan Himalayas were analyzed for their detrital compositions, paleocurrent, heavy minerals and detrital zircon ages. Sedimentology and provenance characteristics of these quartz sandstones, their formation and tectonic significance were discussed. According to sedimentary facies analysis, in the Early Paleozoic Gamba area belonged to the Indian passive continental margin of the Neotethys. Littoral facies dominate and show a change from shallow shelf to continental facies. Sandstone lithofacies suggest that the terrigenous clastic material of the Jidula Formation comprises mainly quartz sandstone with very high maturity, and the paleocurrent direction was near NNE. Detrital zircon dating results suggest that the detrital zircon ages of the Jidula Formation coincide with those from the Early Cretaceous Degan Plateau. It is suggested that the formation of quartz sandstones (Jidula Formation) was led by the sudden increase of terrigenous clastic supply in the northern Indian plate margin and the tectonic uplift of the passive continental margin, which was in turn caused mainly by the formation of the Deccan large igneous province (LIP). We concluded that the origin of the quartz sandstones is related to the reactivation of stable cratons caused by the up-doming of the Deccan LIP on the northern Indian plate margin.