YOUNG BASALTS OF THE SOUTHERN MARIANA FORE-ARC (YK08-08, LEG 2, DIVE 1096): ARE THEY RELATED TO THE ARC, FORE-ARC OR BACK-ARC BASIN MAGMAGENETIC SYSTEMS?

The Mariana Arc system in the western Pacific is an excellent example of an intraoceanic arc-trench system. The southern Mariana arc exhibits tectonic complexities not found elsewhere in this system. These complexities are related to the collision of the Caroline Ridge in Late Miocene time, which separated the Mariana and Yap trenches and formed a young, North-dipping subduction zone at the south end of the ever-widening Mariana Trough back-arc basin. These tectonic complexities have also led to the development of a tear in the subducting slab. In this area, the back-arc basin is unusually close to the trench (~ 110 km) and the magmatic arc is poorly developed, as a broad magmatic welt instead of discrete large volcanoes. Shinkai dive 1096 in July 2008 traversed a ~ 900m thick section of fresh, young pillow basalts in the fore-arc above the slab tear (12.1^oN, 143.86^oE). This is the only place in the Mariana fore-arc where such fresh and young lavas have been found. These rocks are subaphanitic vesicular olivine basalts. Their geochemical composition shows two stratigraphic groups: homogeneous sequences of upper basalts (Mg# = 61.4 – 66.8) and lower basaltic andesites (Mg# = 49.4 – 51.1). The high Mg# of the upper basalt suggests near-equilibrium with mantle peridotite, (Ni = 54 - 118 ppm, Cr = 95 - 220 ppm), whereas the lower basaltic andesite group is fractionated (Ni = 16 - 26 ppm, Cr = 8 - 14 ppm). The major and trace element contents of upper and lower basalts suggest they are related by fractional crystallization of a temporally common mafic melt. Their flat Rare Earth Element patterns indicate significant melting of spinel peridotite. Petrographically, these basalts resemble back-arc basin basalts. However, their major and trace element contents are more similar to arc basalts from south of 16^oN. Their position approximately above the slab tear suggests they could be related to this tear which could explain their eruption so close to the trench.