滇西昌宁龙塘寨地区张家田组的发现及地质意义

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

关键词:  古特提斯  镇康地块  沉积相  硅质岩  牙形石  裂陷海槽

Key words:  Paleo-Tethys  Zhengkang landmass  sedimentary facies  siliceous rock  conodont  rift trough

摘要:

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/TiO₂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。

Abstract:

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.