滇西昌宁龙塘寨地区张家田组的发现及地质意义

在滇西镇康地块东缘龙塘寨地区新发现的张家田组为一套斜坡-盆地相碎屑岩-放射虫硅质岩-灰岩-玄武岩建造,可见浊流远端低密度流序列、鲍马序列、深水的静水沉积序列,具典型浊流沉积特征。灰岩中采获大量的杜内阶至祥播阶牙形石分子,特别是维宪阶和杜内阶的深水盆地相远洋型分子Gnathodusbilineatus和Scaliognathodusanchoralis类群的出现,说明早石炭世地史时期该区已形成深海槽盆。硅质岩Al/(A1+Fe+Mn)比值为0. 22~0. 40,说明张家田组硅质岩为受热水作用影响的生物成因硅质岩; MnO/TiO_2比值为0. 13~0. 59,Ce异常值在1. 0左右,无明显异常,(La/Yb)_N值为0. 93~1. 93,(La/Ce)_N值为1. 01~1. 09,表现出大陆边缘型硅质岩的特征。结合区域地质资料,本文认为晚古生代时期,在东侧昌宁-孟连洋演化的同时,在镇康地块与耿马被动大陆边缘之间曾发育有一个裂陷海槽,而张家田组斜坡-盆地相浊流沉积正是这一构造事件的沉积记录。

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.