Geochemistry and Geological Implications of Two Sets of Devonian Volcanic Rocks in South Margin of East Junggar

Zhao Hao; Liao Qun'an; Luo Ting; Tian Jinming; Zhang Meng; Wang Liangyu; Liu Hongfei

Abstract：The East Junggar has always been one of study hotspots on the Paleozoic ocean-continent transformation and tectonic evolution of the northern Xinjiang.However,studies of volcanic rocks in the south margin of the East Junggar are relatively weak since studies focus more on the origin and structural attributes of Carboniferous volcanic rocks,resulting in a lack of understanding of the tectonic setting and magmatic evolution of the Devonian volcanic rocks.Here we report a detailed petrologic,geochemical and geochronologic analyses on the Devonian volcanic rocks from the Beitashan and Wulubasitao Formations in the northern of the Karamaili ophiolite,to examine its formation environment and magma origin.Combining with the previous works in this region,our study reveals that the volcanic rocks of the Beitashan Formation from the Karamaili formed during the Early Devonian to Middle Devonian (404 Ma),which is characterized by intermediate-high K,Ti and Fe,low Al2 O3 levels,enriched in LREE and LILE,and relatively depleted in HFSE (Nb,Ta).These rocks was probably formed in a continental marginal arc construction environment and the magma is originated from a depleted mantle wedge metasomatismed by melt and fluid from subduction sediment.For the Wulubasitao Formation,the volcanic rocks formed during the Middle Devonian (MoQinulla),characterized by poor alkali,low K,Ti,Fe and high Al,indicating typical island arc volcanic rocks.Isotope and trace element characteristics show it was derived from depleted mantle wedge metasomatismed by fluid from subducted plate.According to the differences of two volcanic rocks features as well as regional geology,we speculate that the subduction of the Karamaili ocean transformed from continental marginal arc to island arc during the Middle Devonian.