黔西南晚二叠世龙潭组物源分析及区域沉积古地理重建

Provenance of the Late Permian Longtan Formation in SW Guizhou Province and Implication for Reconstruction of Regional Sedimentation and Paleogeography

黔西南上二叠统龙潭组为一套以海陆交互相细屑岩为主的沉积地层,与下伏茅口组平行不整合接触。龙潭组砂岩碎屑组分以板片状—微晶状的玄武—安山质火山岩岩屑为主,并含少量霏细晶—隐晶质的酸性火山岩岩屑。泥质岩全岩主量元素以低SiO_2(28.3%~51.9%)、高TiO_2(1.0%~4.1%)和低Al_2O_3/TiO_2比值(3.9~6.9)为特征,与峨眉山高Ti玄武岩的Al_2O_3/TiO_2比值(集中在2.5~5.0之间)较为接近。龙潭组的碎屑锆石显示峰期年龄为~260Ma,与峨眉山大火成岩省的火山喷发期(258~263Ma)相吻合。锆石微量元素指示其与板内/非造山环境的岩浆活动有关,结合同时期北部陆相和南部深水盆地相的沉积物源研究,该结果表明龙潭组碎屑岩的源岩为峨眉山玄武质和长英质火山岩。基于此,本文对晚二叠世滇黔桂地区峨眉山源区—河流—滨岸—右江深水盆地的沉积古地理格局进行重建。

The Late Permian Longtan Formation in southwestern Guizhou Province is predominanted by fine grained clastic rocks of neritic facies. This sequence unconformably overlies the Middle Permian Maokou Formation. Sandstones of the Longtan Formation consist dominantly of basaltic andesitic volcanic fragments with lathwork microlitic structures, and minor felsic vitric silicic volcanic rock fragments. The mudstones are characterized by low SiO2（28.3%～51.9%） and high TiO2（1.0%～4.1%）, and have low Al2O3/TiO2 ratios （3.9～6.9）, very close to that of the Emeishan basalts （2.5～5.0）. Detrital zircons from the Longtan Formation fine grained sandstones show a unimodal age pattern with a peak around 260Ma, overlapping with the major eruption timing of the Emeishan large igneous province （258～263Ma）. In addition, trace elements of the detrital zircons indicate its relation to magmatic activities of intra plate/anorogenic magmas. Combined with the provenance data of contemporaneous deep marine sediments in the Youjiang Basin to the south and the terrestrial sediments to the north, our results demonstrate that the Longtan clastic rocks sourced from the Emeishan basaltic and felsic volcanic rocks. Therefore, this study reconstructed the sedimentary and paleogeographic pattern for the Late Permian Emeishan terrestrial neritic off shore Youjaing deepwater basin in the Dian Qian Gui region in SW China.