Geochemical characteristics of active backarc basin volcanism at the Southern end of the Mariana trough

Abstract

Major and minor chemistry, Sr, Nd and Pb isotope ratios, water content and hydrogen isotope ratios of the backarc basin volcanic rocks taken from the active spreading ridge and surrounding area between 12_400N and 13_150N in the southern part of the Mariana Trough indicate a complex regional interplay of suprasubduction-zone magmatic sources. In this area, active backarc spreading occurs along the eastern side of theMariana Trough, and discrete seamounts align on two parallel chains east of the backarc spreading center. The spreading-center rocks originated froma typical mid-ocean ridge basalt (MORB)-like source, influenced progressively southwestward by proximity to the arc magmatic source and range from basaltic to dacitic in composition. The volcanic rocks from the seamount chains are explained by the mixing of two types of magmatic sources; one is originally a depleted mantle similar to the arc source for magmas farther north and the other is the MORB-like source of the spreading ridge rocks. The influence of the arc source onmagma composition of the seamount chains is higher north of 13_N, indicating heterogeneity of the mantle wedge in this region and the merging of the two sources toward the southwest.