Lithospheric stretching and hydrothermal processes in oceanic gabbros from slow-spreading ridges

Abstract

Proposes a model to explain the metamorphic and deformational characteristics of oceanic gabbros. It is suggested that early lithospheric stretching beneath the ridge allows seawater penetration in the lower crust when it is still very hot, through permeability created by shear zones and associated synkinematic cracks. Therefore, hydration of the lower crust starts at high temperature (750°C), in contrast with a simple cracking front model in which hydration starts at temperature below 500°C. The amount of stretching may be related to the spreading rate through the magma budget. Gabbroic series from ophiolite complexes may show either this early stretching and high temperature metamorphism associated with ductile shear zones (Western Alps ophiolites) or a crack network related to the cracking front and moderate temperature metamorphism (Haylayn maffic, Oman ophiolite). -from Authors