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《Sedimentary Geology and Tethyan Geology》 2018-03
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The discovery and geological significance of the Zhangjiatian Formation in the Longtangzhai area,Changning,western Yunnan

LI Guo-chang;WANG Wei;TIAN Ying-gui;FEI Zhi-bin;LI Chao;QIN Rong-fang;Yunnan Institute of Geology and Mineral Resources;  
The Zhangjiatian Formation newly built on the eastern margin of the Zhengkang landmass in the Longtangzhai area,Changning,western Yunnan is made up of the slope-basin clastic rock-siliceous rocklimestone-basalt formation,in which three types of sedimentary sequences can be recognized: distal low-density turbidity sequence composed of thin-bedded sandstones and mudstones,Bouma sequence representing proximal turbidity current deposits,and deep-sea static deposits composed of siliceous rocks,mudstone/siliceous mudstone and limestones. In the recent geological survey,abundant Tournaisian-Xiangbogian conodonts are collected in the limestones of the Zhangjiatian Formation. Especially, the occurrence of Visean-Tournaisianpelagic conodonts Gnathodusbilineatus and Scaliognathodusanchoralis indicates that a deep-sea trough basin was initiated during the Early Carboniferous. The Al/( A1 + Fe + Mn) ratios ranging between 0. 22 and 0. 40 suggest that the siliceous rocks in the Zhangjiatian Formation may be assigned to the thermally-affected biogenic siliceous rocks.Furthermore,the siliceous rocks have MnO/TiO_2 ratios ranging between 0. 13 and 0. 59,abnormal Ce value of 1. 0,( La/Yb)_N ratios between 0. 93 and 1. 93,( La/Ce)_N ratios between 1. 01 and 1. 09,and relatively flat NASCnormalized REE patterns. All these data have reflected the characteristics of continental-margin siliceous rocks.Referenced to the regional geological data,it is inferred that during the Late Palaeozoic,there once occurred a rift trough developed between the Zhengkang landmass and Gengma passive continental margin,which was associated with the tectonic evolution of the Changning-Menglian Ocean. The slope-basin turbidity deposits in the Zhangjiatian Formation just represent the sedimentary records of the tectonic events during this period.
【Fund】: 中国地质调查局地质调查工作项目(编号:12120113048700)
【CateGory Index】: P618.13
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