滇西保山地块南部公养河群研究新进展

保山地块是滇西特提斯多岛洋的重要组成部分,属滇-缅-马-泰微大陆的北端。出露的最老地层被称为公养河群,主要分布于保山地块南部酒房—旧城—大汪塘和镇安街—绕廊街—公养河—芒海—瑞丽2个片区,为一套厚度巨大的砂岩、长石石英砂岩-砂质板岩-泥质板岩组合,夹硅质岩、泥质硅质岩、薄层灰岩,具有典型的浊流沉积特征,部分地段有低级变质现象。公养河群中生物化石稀少,缺乏年代学依据,其时代被推测为震旦纪—中寒武世。近年来的1:25万、1:5万区域地质调查项目对公养河群进行了深入研究,在新发现的中酸性火山熔岩、凝灰岩夹层中获得了一批可靠的LA-ICP-MS锆石U-Pb年龄(925.0±9.2~499.2±2.1Ma)。结合岩性组合、沉积环境的差异,建议将公养河群自下而上细分为青白口—南华系大汪塘组、震旦系—中寒武统公养河组、上寒武统—奥陶系蒲满哨组3个组级岩石地层单元。主要介绍大汪塘组的分布、岩性组合、沉积环境、时代依据。

New progress in the study of Gongyanghe Group in southern Baoshan block, western Yunnan Province

Located at the northern end of the Yunnan-Myanmar-Malaysia-Thailand microcontinent, the Baoshan block is an important part of multi-island ocean in Paleozoic Tethys, western Yunnan Province. The oldest exposed stratum is known as the Gongyanghe Group on the Baoshan block. It is mainly exposed in two areas, i.e., Jiufang-Jiucheng-Dawangtang and Zhen''anjie-Raolangjie-Gongyanghe-Manhai-Ruili. It is a lithological association with huge thickness, consisting of sandstone, feldspar quartz sandstone, sandy slate and muddy slate, intercalated with siliceous rocks, argillaceous siliceous rocks, thin limestone, with the typical turbidity sedimentary characteristics and some low grade metamorphic features. Few fossils exist in the Gongyanghe Group, causing the lack of the chronological information, and hence its geological time was previously inferred to be Sinian to Middle Cambrian. In recent years, 1:250000 and 1:50000 regional geological survey conducted further research on Gongyanghe Group. A number of reliable LA-ICP-MS zircon U-Pb ages (925.0±9.2~500.0±1.0Ma) were obtained from newly discovered intermediate-acid volcanic lava and tuff interlayer. Combined with the differences of lithological association and sedimentary environment, it is suggested that Gongyanghe Group should be subdivided into 3 formations from bottom to top, namely Qingbaikou to Nanhua System Dawangtang Formation, Sinian-Middle Cambrian Gongyanghe Formation, and Early Cambrian-Ordovician Pumanshao Formation. This paper mainly describes the distribution, lithological association, sedimentary environment and geological epoch of the Dawangtang Formation.