The Regional Differences And Generation Of Basalts In The Okinawa Trough

Okinawa Trough is a typical back-arc basin located in the western Pacific, which formed by extension of continental crust caused by the northwestward subduction of Philippine Sea plate underneath the Eurasian plate, within which there are extensively distributed faults, intense volcanic activity, and multiple rock types including basalt, andesite, dacite and rhyolite. The magmatism in the Okinawa Trough is bimodal, being comprised dominatly of acidic volcanic rocks and mafic basalts. This paper bases on "The Interactions of Magmatism and Hydrothermal Systems" (2013CB429702), the sub-topics of "Hydrothermal Activity and Its Metallogenic Mechanism in typical back-arc basin" of the National Key Basic Research and Development Plan ("973" Plan). The systematacial analysis of regional differences of basaltic magma activities are carried out in terms of, previous-analyzed and published geochemical data of basalts in the middle and southern Okinawa Trough, and combines with tectonic setting to further explore the genetic mechanism. The main research results are as follows:(1) The mantle source beneath Okinawa Trough may have been "Indian" mantle domain.The mantle sources in Okinawa Trough, middle Ryukyu arc, and KBS and SBS(Sekibisho and Kobisho) are similar to depleted mantle that produces N-MORB. The degree of mantle melting for Okinawa Trough rangs from about 7% to 20%. The degree of mantle melting for southern Okinawa Trough rangs mainly from 15% to 20%, and 10% to 20% for the middle Okinawa Trough.(2) The basaltic initial magma in Okinawa Trough was significantly affected by the subduction component, and the elements with different compatibilities have different contribution to mantle source, subduction-mobile elements such as the contribution of Rb, Ba, Th, U, K and Pb to mantle source rang from 50% to 95%.(3) The basalts in Okinawa Trough are productions of fractional crystallization of basaltic initial magma which dominately experiences olivine, pyroxene and plagioclase fractionation, and undergoes little or no crustal contamination during its upwelling to eruption from the magma source.(4) Geochemical discrepancies of basalts in Okinawa Trough and adjacent fields reflect the mantle beneath them are different, and the mantle beneath the southern Okinawa Trough, KBS and SBS are more depleted mantle than that beneath the Ryukyu arc and middle Okinawa Trough. The contribution of subduction component to mantle sources is also distinct for middle and southern Okinawa Trough, middle Ryukyu arc, KBS and SBS:southern Okinawa Trough is higher than middle Okinawa Trough, middle Okinawa Trough is similar to KBS and SBS, and southern Okinawa Trough is similar to middle Ryukyu arc.(5) The Geochemical characteristic differences between magmatic sources result from distinct degree of mantle melting, except for mantle heterogeneity in middle and southern Okinawa Trough, the degree and depth of mantle melting in the southern Okinawa Trough are greater than that in middle Okinawa Trough. The difference for effection of subduction component on basaltic magma may be caused by the depths of subducting plate beneath the eruption sites in southern and middle Okinawa Trough. The contribution of subduction component to mantle sources is similar at a given depth of subducting plate, and is higher for shallower depth. The higher H2O content resulted from addtion of subduction component result in higher degree of mantle melting in the southern Okinawa Trough.