From isotropic to layered gabbro: evolution record in the Oman ophiolite

The origin of gabbro layering in ophiolites is widely debated because it is linked to the processes of melt circulation beneath spreading ridges. The Moho Transition Zone (MTZ) of the Oman ophiolite contains layered gabbro lenses that are tens of meters wide. At meter scale, these rocks are not distinguishable from crustal layered gabbros. We describe the first known occurrence of an outcrop of isotropic gabbro in the Oman ophiolite MTZ; the outcrop extends over three hundreds meters and grades into poorly to nicely layered gabbros towards the periphery of the outcrop. When they are present, layering, and magmatic lineation are parallel to the host peridotite plastic foliation and lineation respectively, with microstructures indicative of simple shear deformation. Dunite heterogeneities within the isotropic gabbro, and diffuse limits, suggest that the isotropic gabbro results from melt impregnation of the host dunite, and that olivine within the gabbro is of mantelic origin. Crystallographic preferred orientations (CPO) measurements of olivine and plagioclase show a progressive evolution from a random fabric in the isotropic gabbro, to a well defined fabric in the roughly to nicely layered gabbros. Olivine show [001] (010) B-TYPE fabrics that we interpret as a magmatic flow fabric. In the most deformed samples, the plagioclase texture remains magmatic but olivine CPO is indicative of plastic deformation with a classic A-TYPE pattern. We argue that as a critical threshold of olivine connectivity is reached in the well defined olivine-rich layers, olivine deformation rapidly switches from magmatic to plastic. This last stage of deformation probably contributes to the layering sharpening, as deformation in olivine and in plagioclase rich layers must tend to be decoupled at this stage. We conclude that our observations illustrate rarely preserved transitional stages for the layered gabbros formation, showing that layering can result from the progressive tectonic transposition of a dispersed and initially isotropic melt intrusion in the host rock. To build the isotropic stage, melt must accumulate more rapidly than the deformation of the host rock, which suggests that melt is extracted from beneath through hydrofracturing dikes, not by porous flow.