The Days Of Beijing Xishan Area Bun Mountain Group Volcanic Rock Lithofacies Typical Structure And Its Reason Analysis

The texture and composition characteristics of the phenocrysts in the magma recorded the rate and process of the evolution and eruption of magma chamber from deep source and the transformation process after the positioning. The volcanic rocks of Tiaojishan Formation in the Western Hills of Beijing are the products of the later Jurassic magmatic activity. On the basis of the field geological investigation and systematic sampling of volcanic rocks of Tiaojishan Formation which are collected from Yanchi and Anjiazhuang toQingshuijian along the road in Tiaojishan basin, the identification and research of their petrography by LV100pol Nikon polarizing microscope, and the analysis of minerals of trachyandesitic lava under the electron probe, The preliminary conclusions are summarized as follows:1. The following rock types have been initially distinguished:(?)the lava include the group of trachyandesitic lavas (trachyandesitic lavas, amygdaloidal trachyandesitic lavas, pyroxene trachyandesitic lavas, hornblende trachyandesitic lavas) and basaltic trachyandesitic lavas;(?)the pyroclastic rocks include trachyandesitic breccia lava, andesitic agglomerate lava, volcanic breccia and tuffaceous siltstone. In addition, the rocks have experienced a metasomatic alteration. The mineral assemblage of alteration is epidote, chlorite, calcite, quartz, sericite and pyrite. They often metasomically change the native phenocryst, matrix minerals and glass or form the amygdale of volcanic rocks or fill the fissures into late-staged veinlets.2. The studies of chemical composition of minerals of trachyandesitic volcanic rocks of Tiaojishan Formation show that, the plagioclase in trachyandesitic volcanic rocks is high albite, clinopyroxene is augite, hornblende is titanian-magnesio-hastingsite which belongs to calcic amphibole. The metamorphic minerals of trachyandesitic volcanic rocks of Tiaojishan Formation are mainly epidote, chlorite, calcite, quartz, pyrite, etc., among which chlorite is ferric magnesio-chlorite, and epidote is the typical one.3. On the basis of the results of field geology, petrography and mineralogy, we discusses the geneses of textures of volcanic rocks and the dynamics of volcanic eruption of Tiaojishan Formation in Western Hills of Beijing, and their relationship with regional metamorphism. The texture changes of trachyandesitic volcanic rocks reflect the evolution of magma in the chamber and the change of crystallization condition of magma erupted to the surface. Volcanic rocks of Tiaojishan Formation in Western Hills of Beijing generally experienced a low-grade metamorphism.