Chronology And Geochemistry Of The Volcanic Rocks Of Yuzan Formation In The Northern Margin Of Yili Block, Xinjiang Province

In the northern margin of Yili block, west Tianshan, Xinjiang, there are a lot of Paleozoic volcanic rocks, the author distinguish the Late Devonian volcanic rocks form the Dahalajunshan Formation which had been previously classified into the Early Carboniferous. According to the latest zircon LA-ICPMS U-Pb dating and comparison of the region, "Yuzan" Formation is proposed to name these Late Devonian volcanic rocks. Meanwhile, petrology and geochemistry studies indicate the magma sources and petrogenesis; the author illustrates the northern margin of the Yili block Late Devonian volcanic rocks of the tectonic setting and its dynamic mechanism and its time on the North Tianshan oceanic subduction constraints.In this paper, we present detailed researches about petrology, geochemistry and geochronology on intermediate-acidic volcanic rocks which occur in the northern Nileke area. Combined with the publishing data by Leng et al, we derived the following conclusion:Based on the previous studies and this article received the zircon LA-ICPMS U-Pb data, the original "Dahalajunshan Formation" is further disintegration of the new Upper Devonian Yuzan FormationZircon LA-ICPMS U-Pb dating results show that zircon ages of the two rhyolite are 369.7±4.9Ma and 363.4±2.5Ma respectively, indicate that the existence of the northern margin of the Yili block Middle Paleozoic volcanic magmatic events, the time of which is the Late Devonian. Yuzan Formation volcanic rocks belong to sub-alkaline rocks association with calc-alkaline evolutionary trend, mainly composed of andesite, dacite, rhyolite and tuff. They are enriched in large ion lithophile elements (Ba, Rb, Th, K, e.g.) and LREEs, depletion in high field strength elements (HFSEs, e.g., Nb, Ta, Ti).It suggests that they have a blood relation with the continental margin volcanic-arc rocks geochemical properties. Trace element geochemistry diagrams further show that these rocks formed in subduction zones above the active continental margin. Andesite-dacite originated in the subduction zone of the mantle wedge metasomatized by subduction fluid, the depth of magmatic origin is 30-50km,which may be on the process of assimilation of upper crustal contamination; rhyolite is likely related primarily to the upper crust remelting, the depth of the magmatic origin is less than 30km, which experiences different degrees of fractional crystallization during evolution.According to the studies about petrology, geochemistry, magma evolution and structure place, the northern margin of the Yili block Late Devonian volcanic rocks is related to the subduction of the North Tianshan ocean towards Yili plate in the Late Devonian. The discovery of the Late Devonian volcanic rocks provide a precise basis that the time of the southward subduction of the North Tianshan ocean began in Late Devonian.