Petrogenesis And Tectonic Setting Analysis Of Aqishan Formation In Xinjiang

Aqishan Formation lies in the Yamansu island-arc belt of Late Paleozoic ditch-arc belt of Jueluotage-Hongqishan in Junggar plate, located in the southeast of this work area. While predecessors have in-depth researched for Jueluotage area of eastern Tianshan, the study for the Aqishan Formation is still in very low-level. Therefore, this paper according to systematically research the petrography, rare earth elements, trace elements geochemistry of the volcanics of Aqishan Formation, scientifically demonstrates the formation of volcanic magmatic process, magma nature and mantle source district features. Based on these researches we get some initial understanding as follows:1. The main volcanic rock types of Aqishan Formation are dacite dust tuff, dacite glass tuff, dacite glass debris tuff, dacite crystal glass tuff, dacitic crystal tuff, dacite crystal debris tuff, dacite debris tuff, dacite complex tuff, dacite breccia tuff, dacite breccia-bearing glass tuff, dacite breccia-bearing crystal tuff, dacite breccia-bearing complex tuff, dacite breccia tuff ,andesite crystal debris tuff , andesite debris tuff, andesite complex tuff, andesite and basatic trachyte andesite. The main lithologies are dacite pyroclastic rock, andesite and trachyte andesite basaltic lava. The main rock-forming minerals are clinopyroxene and plagioclase.2. Dacite samples are collected from Aqishan Formation volcanics, its formation age is 341.7±2.7Ma by SHRIMP U-Pb zircon determination in the centre of Beijing ion probe, belonging to early Viseen stage of Early Carboniferous.3.The rare earth elements pattern of Aqishan Formation volcanics is enriched in light-REE, obvious positive Eu anomaly, and enriches LILE and correspondingly depletes high-field strength elements, Nb, Ta and Ti are obvious depleted. These are of the petrology and geochemical indication of island arc volcanics, and reflect the volcanic rocks character of B-type subduction environment (Gill, 1981).4.Aqishan Formation volcanics belongs to comagmatic region. when the magma is rising, it should be hybridized a little by the crust substance, and the fractional crystallization of the main mineral are olivine and clinopyroxene. 5.Through geochemical analysis, Aqishan Formation volcanics show characteristics of typical oceanic crust subduction type adakite, and it also proves that Aqishan Formation volcanics are of the island arc volcanic environment from another perspective. Furthermore, according to the definition of adakite, it can be inferred that the magma source of Aqishan Formation volcanics is formed by the oceanic crust plate subduction melting.