The Study Of Petrological Geochemistry Of Basalts From Hyrothermal Fields, Mariana Trough

The Mariana Trough is an actively opening back arc basin that lies at the eastern edge of the Philippine Sea plate, and in the transition time from rift to sea floor spreading.This paper reports the result of a petological geochemistry study of eight basaltic samples from the Mariana Trough hydrothermal fields(near 18 N). The samples are basalt or basaltic andesite, belonging to cal-alkalic series; compared to MORE, they have higher Al2O3,Na20 and lower Ti02. They are enriched in LILE and LREE; REE patterns are right-leaned( LaN=10. 68-22. 64, (La/Yb) x = 1. 61~2.47), and have positive Eu anomaly. In PM normalizing plots seven samples are obviously depleted in Nb;HFSE/HFSE, LILE/HFSE, LILE/LIFE values have wide change between MORB and EMI. The values of Sr, Nd, Pb also have wide change (87Sr /66Sr-0.7029-0.7034, 143Nd/144H=0. 513072-0.513182, Nd=8.4-10.7, 206Pb/204Pb=17.83-18.29, 207Pb/204Pb=15.44-15.62, 208Pb/204Pb=37.45-38.18). All characters above show that the magma is derived from upper mantle altered or effected by subduction composition; mantle source has heterogeneity and it may be the combination of DMM and EMI. It is presented that the hydrous fluxing as well as adiabatic decompression plays an important role in melting of MORB-like mantle beneath Marina Trough.Compared with basalts from other back arc basins, it concludes that the magma varies from MORB-like to ARC-like with the evolution; during the earlier stage of evolution, there coexist the two MORB-like and ARC-like lavas, it may be the nature of the magma in back arc basins(especially in the earlier stage).