Geochemistry of Basaltic Lavas from the Mariana Trough: Evidence for Influence of Subduction Component on the Generation of Backarc Basin Magmas

摘要

This paper reports the results of a petrological and geochemical study of eight volcanic rock samples collected from the Mariana Trough (18 deg 00 min -18 deg 15 min N, 144 deg 35 min approx 144 deg 55 min E). The samples are basalts or basaltic andesites with geochemical features of the calc-alkaline series. They are enriched in LILE and LREE, with a La_N = 10.68-22.64 and (La/Yb)_N = 1.61-2.47, and have positive Eu anomalies. In trace element-normalized plots, seven of the eight samples are obviously depleted in Nb and enriched in Zr and Hf, but one sample does not show pronounced depletion in Nb. The ratios of HFSE/HFSE, LILE/HFSE, and LILE/LILE for these basaltic samples vary between N-MORB and EMI, suggesting that the magma may have been derived from an upper mantle source that was altered or affected by a subduction component. Compared with those from the Okinawa Trough, basaltic rocks from the Mariana Trough show a weaker Nb depletion, implying that the influence of a subduction component is relatively weaker than from the Okinawa Trough. This suggests that magma sources and dynamic processes of the Mariana Trough, in the transition from rift to seafloor spreading, are different from those of the Okinawa Trough, in the stage of a backarc rift.