High Temperature Dikes in Peridotites: Origin by Hydraulic Fracturing

Abstract

The orientation of high temperature dikes has been studied with respect to the plastic flow foliation and lineation in peridotites from several massifs. Layering and dikes are composed of various types of pyroxenites and gabbros; dunites, thought to represent residua along high temperature dikes, have been also studied. The layering is composed of dunites, websterites and/or orthopyroxenites, usually with the same minerals as in the host peridotite. The same rock types can be observed in the dikes emplaced early during plastic deformation although the sequence tends to evolve toward more ariegitic or gabbroic facies. Dikes emplaced during plastic flow display two dominant preferred orientations, one is at a high angle to the stretching lineation, the other is parallel to the shear planes deduced from the flow regime analysis in the peridotite (a unique shear plane oblique to the foliation in the case of rotational flow, two shear planes conjugate with respect to the foliation in the case of irrotational flow). Fracturing in all these orientations is ascribed to the magma pressure created by partial melting. Comparatively low magma pressure and high applied deviatoric stress result in shear fractures; high magma pressure and moderate applied stress result in tension fractures (at high angles to the lineation). Dikes emplaced before or early during plastic flow are strongly deformed and tectonically rotated toward the foliation and lineation directions. For a large deformation they become parallel to the foliation orientation. The ubiquitous mafic and ultramafic layering present in most mantle peridotites may originate in this way.