RADIOLARIANS FROM HAGONG FORMATION IN ZHONGDIAN, NW YUNNAN,AND ITS TECTONO-PALEOGEOGRAPHIC IMPLICATIONS
Feng Qinglai1, Zhang Shitao1, Ge Mengchun1, Yan Chengmin2, Yu Hua2, Duan Guoxi3, Bao Junyue3
1. China University of Geosciences, Wuhan 430074; 2. Regional Geological Survey, Geology and Exploration Bureau of Yunnan Province, Yuxi, Yunnan 653100; 3. No. 3 Geological Team, Geology and Exploration Bureau of Yunnan Province, Dali, Yunnan 671000
The Dege-Zhongdian microplate, located between the Garze Litang and Jinshajiang Belts, was considered as a part of the Yangtze plate and a whole carbonate platform in Late Paleozoic, which could be tectonically divided into the Xiangcheng island arc, the Yidun back-arc basin and the Zhongzan block in Late Triassic. Three members of the Upper Triassic Hagong Formation in Zhongdian, NW Yunnan, are as follows in ascending order: terrestrial conglomerate and sandstone, neritic siltstone and mudstone with a lot of chaotic blocks, and deep-sea turbidites with interlayers of silicolites. The sedimentary m閘ange could be linked with the Upper Triassic m閘ange in West Sichuan. They were deposited in the Late Triassic Yidun back-arc basin and all chaotic blocks came from the margin of the basin. Except the Late Paleozoic limestone blocks, there are some siliceous blocks from which the radiolarians in ages of Middle Devonian, Early Carboniferous and Middle Permian are also found in Zhongdian. The results indicate that the Dege-Zhongdian microplate must have a complicated tectono-geographic feature in Late Paleozoic, including shallow water carbonate platform and deep-sea basin, instead of a simple carbonate platform. In addition, it was commonly accepted that the region east of Jinshajiang belt became continent after Triassic. However, the Jurassic radiolarians were discovered from the siliceous interlayers in the top part of the Hagong Formation in Zhongdian, which comprehends that there might be a marine basin during Jurassic in the studied area.