PETROGENETIC EVOLUTION OF AN OPHIOLITE FRAGMENT IN AN ENSIALIC MARGINAL BASIN, NORTHERN AEGEAN (SAMOTHRAKI ISLAND, GREECE)

Ophiolitic reeks on Samothraki Island comprise gabbros, diorite with a ssociated plagiogranite pods and veins, massive dolerite and basalt; d oleritic dykes are rooted in the gabbro and have acted as feeders for the basalt. The mineralogy and chemistry of clinopyroxenes from the ga bbro and basalt suggest that both were crystallized under low pressure . Minimum equilibrium temperatures were similar to 950 degrees C for t he clinopyloxenes of gabbros and 1050-1250 degrees C for those of basa lt. Hornblende-plagioclase pairs in the diorite yield equilibrium temp eratures of 690-740 degrees C. The petrogenetic evolution of the ophio lite suite was controlled by plagioclase and clinopyroxene fractionati on under increasing f(O2); olivine was probably eliminated as an early stage of fractionation. Whole-rock chemistry indicates that the diori tes have bimodal or at least transitional affinities between MORE and IAT. Two generations of dolerite dykes are recognized: a younger set w ith a strong signature of a subduction component and an older set, mor e differentiated and chemically similar to MORE. The chemistry of the younger set of dykes is compatible with mixing of an evolved magma wit h a more primitive one, derived from the melting of a subducted slab. Both basalts and massive dolerites have MORE affinities with a weak ar e imprint. The chemical variability of the ophiolitic lithologies is a lso consistent with magma mixing. Field evidence coupled with mineralo gical and petrochemical data suggest that the Samothraki ophiolite ori ginated in an ensialic back-are basin. This basin was probably develop ed in the broad area of the northern Aegean Sea region; the Guevgueli, Chalkidiki, Evros and Lesvos ophiolites, which are located in this ar ea, are thought to have been formed during the evolution of this basin .