燕山中段大红峪组火山岩的古风化特征

燕山中段 (主要是指北京市平谷县内 )大红峪组火山岩 ,在火山活动的间歇期 ,存在古风化现象。古风化使火山岩的成分、结构遭受改变 ,以致影响对岩石岩性的确定及同位素年龄的测定等。不同的地质环境 ,其风化情景不同。文中论述了火山岩的三种风化带及其不同的风化环境特征 ,并与现代风化带进行了对比。研究大红峪组的古风化现象 ,有利于更好地认识当时的地质环境。

The Palaeoweathering of Volcanic Rocks of Dahongyu Formation in the Middle Yanshan Ranges

The Dahongyu formation,outcropping in the upper Precambrian of Jixing section in Yashan tecton stratigraphic zone,consists of sandstones,volcanic rocks and dolomites from bottom to top.There are two types of the volcanic rocks in Dahongyu formation:trachybasalt and trachyte.This article describes the palaeoweathering of the volcanic rocks,mainly the trachybasalt. The trachybasalt or trachyte is divided into the brown one and the green one by color.The contact between the brown and the green is transitional.The brown one,lain above the sea level,had been slightly weathered or not.This study is to describe the palaeoweathering characteristics within the volcanic crater above the sea level,the volcanic shoreline and the volcanic slope beneath the sea level. In the course of the palaeoweathering,the rock-forming minerals of trachybasalt or trachyte had been hydrated:chloritization,sericitization or carbonatization were derelopep.Finally,the new weathering minerals stable in palaeosurface condition had been formed.Based on their geochemical behavior,some elements were carried away, others were concentrated.