Petrogenesis of volcanic rocks from Saipan and Rota, Mariana Islands, and implications for the evolution of nascent island arcs

High-silica rhyolites in Saipan have an Ar-Ar age of 45.1Ma which is 'proto-arc' and synchronous with boninitic lava eruptions elsewhere in the Izu-Bonin-Mariana arc. These rhyolites are transitional between approximately 48Ma boninitic lavas and post-38Ma 'first-arc' andesites and dacites from Saipan and Rota that have typical subduction-related compositions. The petrogenesis of the whole arc suite is outlined and the difference between the proto-arc volcanism of Saipan and other fore-arc locations is that the crust ceased extending 3 to 4 million years earlier beneath Saipan as mantle-upwelling, fore-arc extension and shallow melting was replaced by counter-flowing asthenospheric mantle (i.e. normal mantle wedge convection), crust stabilisation and deeper melting. Listed are major, trace and REE analyses of the rhyolites, andesites and other lavas, plus EPMA of coexisting ortho-and clino-pyroxenes in a websterite xenolith.