天宝山组地层问题初议

<正> 川西地区天宝山组一名的广泛使用是在1:20万西昌幅区测(1966)以后,四川省地层表称其“主要分布于会理城附近,会理顺河、天宝山、岔河及德昌乐跃一带;会东拖落村亦有出露。……北厚南薄,岩相变化较大,难于对比。”本文在四川省地层表定义的天宝山组为前震旦系最高层位(假整合覆于凤山营组之上,角度不整合伏于苏雄组之下)的基础上讨论天宝山组及相应地层的分布,自南至北分述如下(图1):

Preliminary opinion of the strata of the Tianbaoshan Formation

Tianbaoshan Formation, which overlays the Fengshanying Formation diaconformably and underlies the Suxiong Formation unconformably, is the highest stratigraphical unit of the Pre-Sinian in Sichuan Province. It can be divided into three members: the lower part is composed of rhyoporphyry and decite, the middle one rhyoporphyry and quartz rhyoporphyry, and the upper one tuffaceous sandstone, tuffite as well as sericite-schist. A dating of 906.7 ± 18.5 m.y. by means of Rb-Sr isochron has been done with decite of the lower member of he formation. Based on this as well as other data, it is thought that the volcanics formed about 1050-900 million years ago.The distribution of Tianbaoshan Formation has been discussed. In Dechang-Huili region, the outcrops are very piecemeal. Actually, the previous "Tianbaoshan Formation" in several places is Limahe as well as Fengshanying Formations. Between the sediments of these two, there is a suite of volcanics which, can be correlative with that of the upper member of Xiaohe-kou Formation in Dongchuan district, Yunnan Province. The so called "Tianbaoshan Formation" west of Anninghe Fault should belong to Yanbian Group, the products of Yanbian eu-geosyncline. In Mianning-Ganluo-Shimian area, the .corresponding strata have been found, namely, Yangluogou Formation along Xiaoxiangling Mt, rhyolite of Lugu at Lugu district and volcanics of Sanhe at Suxiong district. The distribution of Tianbaoshan Formation and its equivalents indicates the boundary of a fault-block.