Influence Of Subduction Component On Magma Genesis In Back Arc Basins

The Mariana Trough and the Okinawa Trough are active back arc basins which lie at the western edge of the Pacific Plate. This paper reports the results of a Sr, Nd and Pb isotopic composition analysis of seven basalt rock samples collected from the Mariana Trough. We contrasted the characteristics in geochemistry of basalt samples from the Mariana Trough and the Okinawa Trough. Taking example for the Mariana Trough and the Okinawa Trough, we studied the influence of subduction component on magma genesis in back arc basins. With the difference of tectonic locality and degree of back arc basins evolution, the influence of subduction component on magma genesis in back arc basins is different.All basalt samples from the Mariana Trough and the Okinawa Trough belong to subalkalic series, which straddle the boundary between low-K and medium-K fields. To compare with N-MORB, they are distinguished by: (1) Higher Al2O3, lower TiO2 and FeOT, and lower TiO2/K2O; showing no clearly correlation between MgO or DI and oxide; (2) Enriched in LREE, depleted in HREE, with high (La/Yb)N, (La/Sm)N, (Gd/Yb)N; (3) High Pb and Sr contents; Enriched in LILE, depleted in HFSE; Ratios of incompatible elements such as K/Yb,Ba/La,Pb/Ce,La/Yb,Sr/Nd are high; (4) High ratio of 87Sr/86Sr, 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb, low ratio of 143Nd/144Nd. These characteristics in geochemistry indicate the influence of subduction component on magma genesis in back arc basins.With the difference of tectonic locality and degree of back arc basins evolution, the characteristics in geochemistry of the basalt samples are different. From central section to northern of the Mariana Trough, to central section and southern of the Okinawa Trough, the contents of FeOT and P2O5 increase, but TiO2 decrease; Ratios of (La/Yb)N, (La/Sm)N and (Gd/Yb)N increase, whereas TiO2/K2O and Ce/Pb decrease; The degree of enrichment of LILE and depletion of HFSE are increase; Isotopic ratios of 87Sr/86Sr increase with 143Nd/144Nd decreasing; The ratios of 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb of the northern samples are higher than these of central section of the Mariana Trough. All these difference of the characteristics in geochemistry indicate that the influence of subduction component on magma genesis in back arc basins is different.The isotopic composition of Sr, Nd, and Pb suggest that the magma have derived from a mixture of Depleted Mantle and Enriched Mantle type-â…¡which the main composition is subduction component. With the model of mixing of two end-members, the mixing ratios of subduction component in the central section and northern of the Mariana Trough and in the central section and southern of the Okinawa Trough are 4%, 7%, 14% and 16%, respectively.All the study suggests that a subduction component has significant influence on magma genesis in back arc basins. It also implies that the influence of a subduction component in the transition from rift to seafloor spreading is different from that in the stage of a back arc rift.