Evidence for a Paleo-Spreading Center in the Oman Ophiolite: Mantle Structures in the Maqsad Area

Abstract

Mantle diapirs about ten kilometers in size have been recognized in the Oman ophiolite. A detailed study of the best preserved of these diapirs, cropping out in the Maqsad area, has been undertaken in order to understand melt migration processes in the asthenosphere beneath oceanic spreading centers. New results include the accurate location of the paleo-spreading axis related to the Maqsad diapir. It is evidenced by a 1-2 km wide dunitic corridor striking parallel to the sheeted dyke complex, bearing witness, in its structure and petrology, to pervasive soaking of the mantle by a basaltic melt; most of the upwelling flow is channelled at shallow depth within and beneath this horizon; away from it, the horizontal asthenospheric flow displays a clear divergent pattern. The zone of diverging flow extend 10-15 km on both flanks of the presumed paleo-ridge axis. The relations between the asthenospheric flow trajectories and a large mylonitic shear zone suggest that the Maqsad diapir corresponds to a late spreading event largely contemporaneous with early emplacement tectonics. Dykes facies and intrusion pattern at the periphery of the zone of diverging flow suggest that it ascended through a previously accreted lithospheric segment and that mantle diapirism beneath spreading centers is unsteady.

This reconstitution of a frozen ridge segment provides a logical structural framework for the interpretation of melt extraction structures from the mantle. The upwelling zone is made of homogeneous harzburgites (20–30% opx) which are interpreted as the residue left after partial melting and melt extraction al greater (>10 km) but unknown depth in the diapir. Basaltic melt relics trapped there are particularly scarce pointing to the efficiency of melt extraction processes at great depth. The occurrence in the upwelling of solid/melt reaction structures reflects intense circulation of a hot basaltic melt equilibrated at depth. Abundant melt extraction structures are observed in the dunitic corridor at the very top of the upwelling. They point to the formation of a very low viscosity crystal mush horizon. Most of the basaltic melt delivered into the overlying magma chamber is produced by the cornpaction of this horizon. Focusing the magmatic activity at ridge axis might be accounted for by the coupling between plastic flow and melt migration leading to the formation of a narrow crystal mush horizon at the top of mantle upwellings. The zone of diverging flow is devoid of melt extraction structures but abundantly intruded by gabbro dykes. This zonation points to significant temperature decrease away from the upwelling. The progressive deformation of the crystal mush horizon away from the axial zone is correlated to the increase of the curvature radius of the mantle flow lines at the top of the upwelling. This evolution likely reflects a progressive increase in the effective viscosity of the uppermost mantle away from the ridge axis.