Off-ridge alkaline magmatism and seamount volcanoes in the Masirah island ophiolite, Oman

Abstract

The Masirah ophiolite offers an unique opportunity to study well preserved small seamount structures. Obducted seamounts have not been described up to now, and from the present-day ocean floor they are almost exclusively known from bathymetric studies. The thin oceanic crust of the Masirah ophiolite was formed at a ridge-transform intersect in Upper Jurassic time. It was overprinted and reworked by a major intra-oceanic tectono-magmatic event at mid-Cretaceous time, that has been well dated owing to the presence of interstratified sedimentary rocks (late Hauterivian to early Barremian, c. 130-125 Ma). This mid-Cretaceous magmatism produced alkaline volcanic rocks ranging in chemistry from alkalibasalts to rhyolites. Volcanism occurred in a NW-SE extensional regime. Small, elongate submarine volcano structures (seamounts) developed within widespread alkalibasaltic pillow lava and pillow breccia deposits, which are interfingered with deep-marine pelagic sediments. The volcanoes reached a maximum of a few kilometres in diameter and a few hundred metres in height. The seamounts are built up of basic to acid subvolcanic stock- or sheet-like intrusions, several generations of dikes, vent agglomerates and pyro- to epiclastic deposits. The latter range from coarse breccias to finely stratified lapilli and record explosive volcanism in a deep marine environment. In the magma chambers under the volcanoes local differentiations to trachytic and rhyolitic members took place. The alkaline rocks show a pronounced ocean island basalt (OIB) character indicating the considerable contribution of a mantle plume source (hotspot). As cause of the volcanism we propose a combination of original transform setting followed by drift past the Marion hotspot during the major plate tectonic reorganization between Greater India, Madagascar and Africa starting in mid-Cretaceous time.