Petrogenetic evolution of ultramafic rocks from Lesvos Island (NE Aegean, Greece)

The ophiolitic unit in the SE Lesvos constitutes the uppermost tectonic nap pe, consisting of ultramafic rocks with minor gabbroic dykes and an amphibo lite sole. The most abundant lithologies comprise Iherzolite and harzburgit e with minor dunite. The harzburgite member and the dunite are intensively serpentinized. The harzburgite has chemical affinity analogous to Alpine pe ridotites while the Iherzolite is more fertile and resembles some abyssal p eridotites. Both have undergone partial melting and depletion as indicated by geochemical and mineralogical results. The exsolution of clinopyroxene a nd spinel from orthopyroxene indicates elemental redistribution in the pyro xenes and hence geothermometric calculations based on pyroxenes cannot give the original temperatures of formation. Therefore, our geothermobarometric calculations indicate a later cooling episode, which occurred after the fo rmation of porphyroclasts in a temperature range of 1100-650 degrees C and at pressures of about 6-7 kbars. Metasomatic phenomena that affected the Le svos peridotites resulted in low MgO/FeOt ratios and enrichment in some inc ompatible trace elements together with the formation of amphibole. Chemical affinities and mineralogical compositions suggest that the Lesvos peridoti tes originated in a marginal basin regime.