Typical Non - Equilibrium Lithofacies Structure And Its Genesis In The Lava Of The Jianshan Formation In Xishan Of Beijing

Volcanic rocks are the evolutional results of the magma system under the non-equilibrium physical and chemical conditions. The non-equilibrium petrographic textures of volcanic rocks record the history of magma evolution, which has important significance for the realization of dynamic processes on volcanic activity, magma cooling and crystallization. Through the field work and petrography of lavas from Tiaojishan Formation in Western Hills of Beijing, the non-equilibrium petrographic textures of dark reaction rim of amphibole and amygdale are researched by polarizing microscope, electron microprobe, scanning electron microscopy and Fourier transform microscopic infrared spectrometer. According to the theory of non-equilibrium thermal dynamics, we preliminarily study the genesis on the textures of dark reaction rim of amphibole and amygdale in lavas, and obtain some conclusions.1. The textures of dark reaction rim of amphibole and amygdale of lava from Tiaojishan Formation in Western Hills are the typical non-equilibrium textures. The textures can reveal the dynamical processes during and after diagenetic evolution.2. The amphibole in trachyandesitic lavas from Tiaojishan Formation in Western Hills is titanian-magnesio-hastingsite. The parts of amphibole near the dark rim are rich in Ca, Si and Fe. The dark reaction rim of amphibole consists of magnetite, clinopyroxene, quartz and other minerals. The content of structural water changes from more to less, and then to more corresponding to measurement location from fresh parts to dark reaction rims of amphiboles by infrared spectrometer. The non-equilibrium textures of dark reaction rim of amphibole are formed by rapid depressurization, increasing temperature difference and volatile escape during volcanic eruption or migration of magma to the surface or near surface.3. The textures of amygdale of basaltic andesitic lavas from Tiaojishan Formation in Western Hills have difference forms. We observe two kinds of amygdale in the samples. The first type presents the ring or band structure. The edge of amygdale is mainly made up of epidote, and the center consists of calcite and quartz. The second type shows asymmetrical and messy distribution of minerals. From lava matrix to edge of amygdales, and then to epidote in amygdale, the contents of structural water change from more to less, and then to more. The mineral assemblage of amygdale may be caused by the superimposed hydrothermal or metamorphic events.