High Temperature Dikes in Peridotites: Origin by Hydraulic Fracturing

The orientation of high temperature dikes has been studied withrespect to the plastic flow foliation and lineation in peridotitesfrom several massifs. Layering and dikes are composed of varioustypes of pyroxenites and gabbros; dunites, thought to representresidua 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. Thesame rock types can be observed in the dikes emplaced earlyduring plastic deformation although the sequence tends to evolvetoward more ariegitic or gabbroic facies. Dikes emplaced during plastic flow display two dominant preferredorientations, one is at a high angle to the stretching lineation,the other is parallel to the shear planes deduced from the flowregime analysis in the peridotite (a unique shear plane obliqueto the foliation in the case of rotational flow, two shear planesconjugate with respect to the foliation in the case of irrotationalflow). Fracturing in all these orientations is ascribed to themagma pressure created by partial melting. Comparatively lowmagma pressure and high applied deviatoric stress result inshear fractures; high magma pressure and moderate applied stressresult in tension fractures (at high angles to the lineation).Dikes emplaced before or early during plastic flow are stronglydeformed and tectonically rotated toward the foliation and lineationdirections. For a large deformation they become parallel tothe foliation orientation. The ubiquitous mafic and ultramaficlayering present in most mantle peridotites may originate inthis way.