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 rocks from the strongly dismembered Othris Ophiolite. Part of the mantle section was impregnated with melt, crystallizing plagioclase and clinopyroxene as cumulate phases and refertilizing previously depleted peridotites. Melt impregnation occured late in the deformation history of the host peridotites. The deformation took place at stresses of 13-26 MPa and at temperatures around 1000-1200°C, at the base of the thermal lithosphere. The melt therefore impregnated relatively cold mantle rocks, implying that the thermal lithosphere reached into the mantle during magmatic activity. We conclude that the Othris Ophiolite represents a spreading environment with a relatively thick lithosphere, such as that near an axial discontinuity or or transform fault of a slow-spreading ridge. The proposed magmatic and deformation history of the periodotites is in agreement with episodic magmatism at slow-spreading ridges. We thus conclude that the heterogeneous character of the mantle sectiion of the Othris Ophiolite results from melt impregnation processes. We suggest that the presence of therzolitic ophiolite types among harzburgitic ophiolite types in the Hellenic-Dinaric chain reflects variable degrees of melt impregnation and refertilization rather than partial melting and melt extraction.