Petrogenesis Of Volcanic Rocks From The Southern Okinawa Trough And Its Implications For Crust-mantle Interaction In The Back-arc Basin

The Okinawa Trough(OT)is the only back-arc basin in the western Pacific which was formed in the continental crust basement and is still in the early stage of expansion.It has active volcanism and hydrothermal activities.Thus,the OT is an ideal place to study the interaction of multi-circle layers and the circulation of crust-mantle materials.In order to understand crust-mantle interaction of back-arc basin,the basaltic andesite,dacite and rhyolite in the southern OT were studied.The petrogenesis of the different types of volcanic rocks was revealed by the methods of element and isotope geochemistry of whole rocks and single minerals.The rhyolite from the Yonaguni Knoll IV hydrothermal field have high SiO2(>70.01 wt.%),K2O+Na2O(6.71?7.16 wt.%)contents and differentiation index(DI)values(DI>93),and are medium-K calc-alkaline in composition.The rhyolites enrich in LILEs and LREEs and strong depletion in HFSEs,with the most enriched isotopic compositions,including ?Nd(from-3.2 to-3.5),87Sr/86Sr ratios(0.705552 to 0.705713),?18O values(+8.1‰ to+8.6‰)and Pb isotopic compositions(206Pb/204Pb=18.568?15.589,207Pb/204Pb=15.641?15.656,and 208Pb/204Pb=38.548?38.904).Based on these characteristics,we propose that the rhyolites were most likely generated by the mixing of a mantle-derived basaltic magma and crustal felsic magma,followed by extensive fractional crystallization in shallow magma chamber.The genetic mineralogy of amphibole further proves that the crust-mantle mixed origin.The lack of exsolved hydrosaline fluid and the high oxidation conditions resulted in the metal elements and magmatic Cl remaining dissolved in the rhyolitic melt and/or partitioning into crystalline minerals upon eruption.Thus,the large volumes of rhyolitic magma intruded in the upper crust of Yonaguni Knoll ? hydrothermal field are a potential fertile source of these elements via leaching by heated sea waters.Apatite grains in the dacite from the CBVT are fluorapatites with F contents of 1.58?2.54 wt.%and Cl contents of 0.57?0.88 wt.%.The Cl contents of the apatites in the dacite were slightly lower than those in surrounding subduction related systems.Apatites have high in situ analyzed 87Sr/86Sr ratios(range from 0.70635 to 0.70697).Based on these characteristics,we propose that the dehydration of subducted slab released fluids into the overlying mantle wedge and triggered mantle melting,which produced primitive mafic magma with high Cl contents.Then,the mafic magma ascended into the crust,which has low Cl concentrations and high 87Sr/86Sr ratios.The mixing of a mantle-derived basaltic magma and a crustal felsic magma in a shallow magma chamber diluted the Cl content and elevated the 87Sr/86Sr ratio,thus producing the parental magma of the dacite.When the dacitic magma chamber was replenished by mafic magma at approximately 0.2 Ma,the high temperature from the injection would have caused the felsic melt to move upward and finally erupt.Basaltic andesites in the southern OT are enrichment of LILEs,depletion of HFSEs.The Sr-Nd-Pb-Hf isotope composition of the C5 basaltic andesite is the most depleted,with the characteristics of the“Indian MORB”lantle,while the Sr-Nd-Pb-Hf isotopes of the C4 and TVG9-1 basaltic andesite are more enriched,towards the EM2 endmember,mainly affected by a small amount(1%)of subducted sediments with high Nd/Hf ratio.The cold subducted Philippine Sea plate is characterized by older age and faster subduction rate,which is the reason of lack of contribution of the subduction sediment melt to the mantle source.Therefore,the initial magma formed by the interaction between the subduction plate and the overlying mantle wedge and its interaction with the thinned crust during the ascending process are the main mechanisms for the petrogenesis of basaltic andesite,rhyolite and dacite in the southern OT.Furthermore,the magmatism of the OT is a product of multi-layer interactions between the crust(subducted oceanic crust and thinned continental crust)and mantle(mantle wedge),and its magma evolution process is more complicated.