云南思茅雅口地区中三叠统芒怀组底部火山岩层地质特征及其意义

长期以来人们认为 ,广泛分布于滇西澜沧江两岸的芒怀组底部岩层是一套具底砾岩性质的正常沉积的砂砾岩。近年来经笔者在雅口地区的调查和综合对比研究 ,发现该岩层实为一套火山碎屑岩。雅口一带的芒怀组底部属一套碰撞型富钾英安 -流纹质火山碎屑岩和熔岩 ,由下向上含有砾径不等的棱角状花岗质集块和角砾 ,其特征显示为快速破碎、原地堆积、表生固结的形成过程。底部与雅口中细粒二长花岗岩岩体直接接触 ,接触面呈较平缓的波状起伏 ,界线清楚 ;火山岩中集块和角砾的成分与该岩体岩性一致 ;岩体内部发育有与芒怀组火山岩成分相当的隐爆角砾岩及次火山脉岩。因此 ,芒怀组底部与雅口岩体的接触关系应该是火山喷出不整合 ,岩体本身受火山破震作用的影响成为火山通道之一。区域对比分析认为 :芒怀组与下伏地层之间的接触关系也应为火山喷出不整合。该认识将对进一步深化滇西的基础地质研究具有积极意义

GEOLOGICAL CHARACTERISTICS AND SIGNIFICANCE OF VOLCANIC ROCKS AT THE BOTTOM OF THE MIDDLE TRIASSIC MANGHUAI FORMATION IN YAKOU REGION, SIMAO CITY, YUNNAN PROVINCE

The layer at the bottom of the middle Triassic Manghuai Formation in the western Yunnan, has long been considered as normal sandy gravel of basal conglomerate characteristics. However, recent field investigation and comparative study in Yakou region have discovered that this layer is in fact composed of a series of pyroclastic rocks. This layer can be assigned to a series of collision type K-rich dacite—kaliliparite and lava, which contain angular granitic agglomerates and rubbles of different sizes. The ch