• bimodal magmatism;
  • hydrothermal mineralization;
  • Mariana Arc;
  • pumice;
  • Quaternary volcano;
  • submarine caldera

Abstract  West Rota Volcano (WRV) is a recently discovered extinct submarine volcano in the southern Mariana Arc. It is large (25 km diameter base), shallow (up to 300 m below sealevel), and contains a large caldera (6 × 10 km, with up to 1 km relief). The WRV lies near the northern termination of a major NNE-trending normal fault. This and a second, parallel fault just west of the volcano separate uplifted, thick frontal arc crust to the east from subsiding, thin back-arc basin crust to the west. The WRV is distinct from other Mariana Arc volcanoes: (i) it consists of a lower, predominantly andesite section overlain by a bimodal rhyolite-basalt layered sequence; (ii) andesitic rocks are locally intensely altered and mineralized; (iii) it has a large caldera; and (iv) WRV is built on a major fault. Submarine felsic calderas are common in the Izu and Kermadec Arcs but are otherwise unknown from the Marianas and other primitive, intraoceanic arcs. 40Ar–39Ar dating indicates that andesitic volcanism comprising the lower volcanic section occurred 0.33–0.55 my ago, whereas eruption of the upper rhyolites and basalts occurred 37–51 thousand years ago. Four sequences of rhyolite pyroclastics each are 20–75 m thick, unwelded and show reverse grading, indicating submarine eruption. The youngest unit consists of 1–2 m diameter spheroids of rhyolite pumice, interpreted as magmatic balloons, formed by relatively quiet effusion and inflation of rhyolite into the overlying seawater. Geochemical studies indicate that felsic magmas were generated by anatexis of amphibolite-facies meta-andesites, perhaps in the middle arc crust. The presence of a large felsic volcano and caldera in the southern Marianas might indicate interaction of large normal faults with a mid-crustal magma body at depth, providing a way for viscous felsic melts to reach the surface.