AN INVESTIGATION INTO THE MORPHOLOGY, PETROLOGY, AND GEOCHEMISTRY OF THE WEST ROTA CALDERA, MARIANA BACK-ARC BASIN

The West Rota caldera is located in the southern back-arc basin of the Izu-Bonin-Mariana arc system. During the Cook 7 expedition, which was sponsored by SCRIPPS Oceanographic Institute, the Melville passed over the caldera previously identified in the literature as 14?20’. However, West Rota had not previously been imaged, and only a couple of samples had previously been recovered from this caldera. This is the first detailed study of the West Rota caldera. West Rota was imaged using seabeam bathymetry and sidescan data collected from the Mellville. The bathymetric data was formatted using the computer program Generic Mapping Tools, as developed by the University of Hawaii School of Ocean and Earth Science and Technology, to produce the detailed geometry of the caldera. West Rota is 1.6km in height from the base of the volcano to the caldera rim, and the relief inside the caldera is about 1km. The caldera is 8km northwest to southeast by 6km northeast to southwest and is very distinct in that after the collapse of the caldera there was no subsequent volcanism. There were four successful dredge hauls from the region around and within West Rota. Petrographic analysis of the rocks from this location reveal a wide range of compositions, as well as some samples which show copper mineralisation and pervasive hydrothermal alteration. Vesicular basalts and basaltic andesites represent the more mafic end member, whereas altered dacite and crystal rich pumice characterize the felsic samples recovered. The petrography will be discussed in more detail along with the results of the geochemical analyses.