The disaggregation of the Permian Quga Formation in Zhongba area, southwestern Tibet

On the basis of the recent 1∶50000 regional geological investigation in Zhongba area, southwestern Tibet, the so-called Permian Quga Formation was studied in eastern Zhongba microterrane, which is sandwiched between two branches of the western Yarlung Zangbo suture. Based on the results obtained, the authors hold that the former Quga Formation was disaggregated into three parts, i.e., the Ordovician-Silurian Ziqupu Group, the Devonian Nadeng'er Formation, and the Carboniferous-Permian Quga Group. The Ziqupu Group characterized by metamorphic carbonate rocks can be divided into two parts: the lower is named the Middle-Upper Ordovician Ziqudianzhan Formation which is composed of purplish red and gray marble and crystalline limestone, and the upper part is named the Silurian Ziqushi Formation which is composed of purplish red marble. The Nadeng'er Formation consists of gray phylitic slate and calcic schist with upward-increasing crystalline limestone, and is tentatively assigned to the Mayoumu Group. The Quga Group is subdivided into three parts. The lower part named the Gangzhutang Formation comprises gray, grayish green slate and siltstone with upward-increasing sandstone and limestone, and is dated as belonging to Carboniferous-early Early Permian. The middle part, Zhongba Formation, is characterized by pinkish red dolomites and variegated dolomitic bioclastic limestone, and belongs to the Early-Middle Permian, as evidenced by fossils of brachiopods, bryozoans and corals. The upper part named the Kazhale Formation is dominated by dark gray calcic slate intercalated and/or interbedded with limestone and sandstone, belonging to the Middle-Late Permian as shown by fossils. Some problems remain unsolved: whether the disaggregation of the Quga Formation could be applied to other areas out of Zhongba area or not; the lithologic character is remarkably different from that of other areas in spite of the similarity in biota.