Structural petrology of plagioclase peridotites in the west Othris mountains (Greece): Melt impregnation in mantle lithosphere

We present the results of a structural and petrological study of mantle roc ks from the strongly dismembered Othris Ophiolite. Part of the mantle secti on was impregnated with melt, crystallizing plagioclase and clinopyroxene a s cumulate phases and refertilizing previously depleted peridotites. Melt i mpregnation occurred late in the deformation history of the host peridotite s. The deformation took place at stresses of 13-26 MPa and at temperatures around 1000 1200 degreesC, at the base of the thermal lithosphere. The melt therefore impregnated relatively cold mantle rocks, implying that the ther mal lithosphere reached into the mantle during magmatic activity. We conclu de that the Othris Ophiolite represents a spreading environment with a rela tively thick lithosphere, such as that near an axial discontinuity or trans form fault of a slow-spreading ridge. The proposed magmatic and deformation history of the peridotites is in agreement with episodic magmatism at slow -spreading ridges. We thus conclude that the heterogenous character of the mantle section of the Othris Ophiolite results from melt impregnation proce sses. We suggest that the presence of lherzolitic ophiolite types among har zburgitic ophiolite types in the Hellenic-Dinaric chain reflects variable d egrees of melt impregnation and refertilization rather than partial melting and melt extraction.