Geochemical And Tectonic Implications Of Bimodal Volcanic Rocks In The Eastern Beishan Area,Gansu

Ganquan formation in the east of Beishan Gansu,exposed in the southern of the Central Asian Orogenic Belt.Located between the Hongliuhe-Niujuanzi-Xichangjing ophiolitic melange belt and the Zhangfangshan-Huitongshan ophiolite melange belt.Therefore,it is very important to determine the time limit of subduction of the Ancient Asia Ocean,especially in the southern section.It has played an important role incontrolling the formation and evolution of the basin in sorthern China and wasconsidered as the key area for the final closure of the Ancient Asian Ocean.It hasattracted the attention of many scholars and made important progress?Xiao W J et al.,2004,2009;Wang J R et al.,2016;Xu B et al.,2001;Su B.X.,et al.,2009;2010;2011;Li Shan et al.,2013,2009,2010,2011??However,there is still a great deal ofcontroversy over the closure time of the Ancient Asian Ocean.This paper selects the andesite-rhyolite bimodal volcanic rocks in the east of Beishan as the research object.Through petrology,isotopic chronology and geoche-mistry,the research on the genesis and formation of tectonic dynamic processes of the ganquan Formation volcanic rocks has mainly achieved the following understa-nding:1.Andesite-rhyolite volcanic rocks combination,the rhyolite SiO₂=75.12-75.88wt%,the andesite SiO₂=54.66-63.14 wt%.In addition,K₂O/Na₂O ratio,TiO₂ andMgO content have significant differences.Combination the existence of the both has obvious SiO₂ component discontinuities and belongs to the bimodal volcanic rocks combination?Hildreth,1981?;The zircon U-Pb age of the Rhyolite LA-ICP-MS was272.4±1.2 Ma?MSWD=0.016?,indicating that the bimodal volcanic rocks formed in the Early Permian and also indicated the era of the formation of the ganquan forma-tion.The ganquan formation originally belonged to the Late Carboniferous.2.Rhyolite has high silicon,high K?K₂O=4.41-4.21 wt%;Na₂O=4.00-4.07 wt%?,medium Al?Al₂O₃=13.14-13.53 wt%?,low Mg?0.26-0.37 wt%;Mg^#=21.86-31.26?,characteristics of low Ti?0.13-0.14wt%?,A/CNK=1.04-1.07>1,and 10000*Ga/Al>2.6,belonging to type“A”?granite?rhyolite.The rocks are enriched with K,Th,Nd,U,Pb,etc.;they are depleted of Ba,Sr,P,Nb,Ta,Ti.The depleted of Ba and Sr isrelated to the fractional crystallization of the plagioclase residue or magma ascending process in the source area;the depleted of Nb,Ta and Ti,and the LILE enrichmentand strong negative Eu anomaly??Eu=0.48-0.50<1?.Indicated that the basalticmagma formed by decompression and melting at the lower overlying asthenosphere heats the middle and lower crust,and promotes the formation of partial melting of the crust.Andesite with SiO₂?54.66-63.14wt%?,enriched Na and depleted K?Na₂O=3.25-4.32 wt%,K₂O=1.26-1.59 wt%?,high Al?Al₂O₃=14.36-16.12 wt%?,high MgO?3.47-4.26 wt%?,Mg^#=41.50-43.69,rocks enriched in LREE,K,Ba,Pb,U,etc.;weakly enriched Nd,Zr,Hf,depleted of Ta,Nb,Ti.It reveals that andesite originates from the basalt magma formed by partial melting of the ascending and descending dome of the asthenosphere,and in the process of underplating,mixing of crusts and/or crustal materials occurs;3.The discovery of bimodal volcanic rocks reveals that the eastern Beishan area and its neighbors have entered the post-collisional intracontinental tension riftenvironment in the Early Permian,The closure of oceanic basins represented by the Hongliuhe-Niujuanzi-Xichangjing and Zhangfangshan-Huitongshan Ophioliticmelange belt should occur earlier than the Early Permian.