Magmatic Processes Inferred from Chemical Composition and Texture of the Lavas in Ashikule Volcanic Cluster, Western Kunlun, China

Abstract:

Petrology, chemical composition, texture, magma temperature and pressure of volcanic rocks at Ashikule volcanic cluster (AVC), Western Kunlun, China were studied to investigate the magmatic processes before eruption. Whole rock geochemistry of AVC lavas shows that Ashikule volcanic rocks have a wide compositional range, including phonotephrite, basaltic trachyandesite, trachyandesite, trachyte and acid rhyolite, but are mainly trachyandesite. K₂O/SiO₂ ratios were greater than 1, and they fall into shoshonite series in K₂O vs. SiO₂ diagram. Oxide diagrams show that intermediate and acidic volcanic rocks have a similar magmatic evolution, which was mainly related to the fractional crystallization of pyroxene, plagioclase, apatite, ilmenite, etc. Acidic magma was the product of magmatic crystallization in the last stage. Trace elements indicate that magma underwent mixing of crustal components in the process of crystallization before eruption. Xishan phonotephrite shows different evolution series compared with intermediate and acidic volcanic rocks, revealing the differences of their magmatic sources. The phenocrysts in Ashikule volcanic rocks have textures of reverse zoning, serious resorption, reaction rims, etc., suggesting they crystalized from an open system, and magma mixing happened among them. The equilibrium temperature and pressure of phonotephrite are estimated as 1146 - 1316℃ and 0.38 - 1.7 GPa, respectively, by using thermobarometer, corresponding to a depth of 14 - 62 km. They are 1062-1215 ℃ and 0.22 - 1.18 GPa, respectively, for intermediate rocks, and the depth is 8 - 43 km. Acidic rocks have a lower magmatic temperature 778 - 889 ℃. Study on volcanic and tectonic activities shows that tectonic movements control the magma activity, the times of area accelerated uplift are consistent with those of the eruption episode. Active tectonics led to multiple magma upwelling, and each of the rising magmas undergone their own evolution. Therefore, there were many different components of magma beneath this area, namely, multiple magma capsules with different compositions contemporaneously.