A Study On The Volcanic Geolology And Rock Of The Ashi Volcano In The West Kunlun Mountains

The Ashikule Basin is located in Yutian county of Xinjiang Autonomous Region, eastern Kunlun, south of the Tarim Basin. The basin lies between81°26′E-81°46′E and35°37′N-35°50′N, with an area of740km2. The Ashikule Basin is located in the ’arc’ intersection of the NE trending Altyn Tagh fault and NW trending Kangxiwar fault, has frequent tectonic activity, and hosts more than10volcanoes, including the Ashi volcano, which is the latest active volcano in the Ashikule volcanic group. Since this area has very harsh natural conditions, research level on this area is pretty low. Selecting the Ashi volcano, this work carried out a detailed and systematic study from four aspects:volcanic geology, petrology, geochemistry and microstructure.The research results show that the Ashi volcano is composed of a volcano cone and lava flows. The volcano cone is made up of the early cinder cone and late splash cone. There are several fumaroles on the north side of the cone, and overflow outlet of lava flow developed on the south side. Lava flow is distributed around the cone, about33km2in area. According to topography and superposed relationship, this lava flow can be divided into4parts.In extrusive rock of the Ashi volcano developed vesicular structure, amygdaloidal structure and porphyritic texture. Matrix content is about44.7%-86.2%, mainly as a glassy, aphanitic, microcrystalline (plagioclase, pyroxene, ilmenite). Phenocrysts content is between1%-15%, mainly plagioclase (mainly feldspar) and pyroxene (including augite, bronzite and hypersthene), and occasionally small amounts of olivine, titanium iron ore and quartz. SiO2content is53.85%-56.60%,57.23%-60.20%; K2O content of3.58%-4.39%; total alkali content of6.74%-7.81%; the value of K2O/Na2O value is1.17-1.35, belonging to shoshonitic series, base-neutral, neutral rock series, alkaline series, sub-alkaline series, lithology latites. The Ashi volcano is characterized by highly enriched trace elements K, Rb, Sr, Ba, U, Th, Pb, LREE, depleted in Nd, Ta, Ti, P element, and all samples have consistent distribution patterns of trace elements, suggesting that the magma sources are the same. Lower Mg#(23.65-33.91), Cr (24.5×10-60.7×10-6) and Ni (11.0×10-6-52.2×10-6) values and the U, Th, Pb, and LREE enrichment demonstrate that magma is non-primitive magma but suffered from crustal contamination.The phenocryst-liquid equilibrium temperature is1091℃-1186℃, and the equilibrium pressure is6.0Kbar-10.2Kbar, indicating that the depth of the magma chamber is about18km-30.6km. Clinopyroxene crystals pressure varies greatly, and the corresponding magma chamber depth is within the crust, suggesting that magma during the ascent process had stayed in the crust.This thesis utilizes Crystal Sizes Distribution (CSD) theory to statistically treat pyroxene micro-phenocryst in trachyandesite of the Ashi volcano, and generates CSD maps. These CSD figures show significant differences between different samples on slope and intercept, and the largest difference is up to one order of magnitude. According to the difference of slope and intercept, CSD maps can be divided into two types. This thesis assumes that tfachyandesite in the study area formed from the same magma chamber. In general, for a magma chamber, is is believed that the growth rates of phenocrysts are the same. Assuming micro-pyroxene phenocrysts have a same growth rate, it can be concluded that different slopes in CSD diagram illustrate different magma residence times; different intercepts explain different nucleation rates of micro-phenocryst, that is, there are different undercooling degrees in the magma chamber. From the above analysis, the following conclusions can be drawn:(1) Trachyandesite of the Ashi volcano formed at different eruptive times;(2) Undercooling degrees are different in the magma chamber at different eruptive times;(3) There are different rising rates of magma at different eruptive times.