Petrogenesis Of Ankaramite In Devonian Beitashan Formation On The Southern Margin Of Altay Orogen

This work includes field trip on the southern margin of Altay Orogen and analysis of the Devonian Beitashan formation, in order to understand the types of volcanic rocks and their distribution. Bulk rock geochemistry were analyzed to discuss the basaltic-andesite on the button of Beitashan formation, at the same time, EPMA and LAICPMS were used to analyze the clinopyroxene phenocryst and inelt inclusions in it, respectively, to understand the petrogenesis of Laoshankou Ankaramite.The basaltic-andesite on the button of northwest part of Beitashan formation is Boninite, while the pyroxene-phyric basalt on the button of Beitashan formation in Laoshankou district is ankatamite. The distribution of Ankaramite and Boninite imply that Beitashan was in a partly spreading fore-arc environment in middle Devonian.Diopside is a liquidus phase in the main stage of the magma chamber, and it’s reversed zoning patterns suggest a process of new generated magma recharging were toke place in the magma chamber. The clinopyroxene phenocrysts with melt inclusions lining along their growth zone is a visual proof of magma mixing. The chemical composition of the melt inclusions and the bulk rock of Ankaramite suggest that Ankaramite erupted from the magma chamber directly without significant differentiation in a upper magma chamber. The composition of the melt inclusions imply that three types of melt once exist in the magma chamber. The first type is ankaramitic melt, which is similar to the bulk rock. The second type is high-Al basaltic melt, which is possibly evolved from ankaramitic melt. The third type is picritic melt, which is possibly the new generated magma that recharged into the magma chamber.The magma chamber that diopside grew in is possibly the place that magma mixing toke place. Endiopside is possibly carried into the magma chamber by the new generated magma, which derives from the melting of the refractory harzburgite, and show close relation ship with N-MORB. Beitashan formation is an island arc relic that accreted to the Altay Orogen, and the two types of basaltic volcanic rocks on its’button suggest that Beitashan district was in a partly spreading fore-arc environment. At the same time, the petrogenesis of Ankaramite in Laoshankou imply that significant amount of new generated magma were uplifted in Middle Devonian. Partly spreading for-arc environment and the uplifting of athenosphere goes well with the model of ridge-subduction, therefore, the Ankaramite in Laoshankou district is possibly is product of ridge subduction.