The discovery and geological significance of the Zhangjiatian Formation in the Longtangzhai area,Changning,western Yunnan

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/TiO₂ 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.