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 studyof mantle rocks from the strongly dismembered Othris Ophiolite.Part of the mantle section was impregnated with melt, crystallizingplagioclase and clinopyroxene as cumulate phases and refertilizingpreviously depleted peridotites. Melt impregnation occurredlate in the deformation history of the host peridotites. Thedeformation took place at stresses of 13–26 MPa and attemperatures around 1000–1200°C, at the base of thethermal lithosphere. The melt therefore impregnated relativelycold mantle rocks, implying that the thermal lithosphere reachedinto the mantle during magmatic activity. We conclude that theOthris Ophiolite represents a spreading environment with a relativelythick lithosphere, such as that near an axial discontinuityor transform fault of a slow-spreading ridge. The proposed magmaticand deformation history of the peridotites is in agreement withepisodic magmatism at slow-spreading ridges. We thus concludethat the heterogeneous character of the mantle section of theOthris Ophiolite results from melt impregnation processes. Wesuggest that the presence of lherzolitic ophiolite types amongharzburgitic ophiolite types in the Hellenic–Dinaric chainreflects variable degrees of melt impregnation and refertilizationrather than partial melting and melt extraction. KEY WORDS: lithospheric mantle deformation; melt impregnation; microstructures; Othris Ophiolite; plagioclase peridotites