Gabbroic rocks in ophiolitic occurrences from East Othris, Greece: petrogenetic processes and geotectonic environment implications

Abstract

East Othris area consists of scattered ophiolitic units, as well as ophiolitic melange occurrences, which encompass gabbroic rocks. These rocks have been affected by low-grade ocean floor metamorphism (T < 350°C and P < 8 kbar). Based on their petrography, mineral chemistry and geochemistry gabbroic rocks have been distinguished into gabbros and diorites, with the latter being divided into two groups. Gabbros seem to have been formed from moderate to high partial melting degrees (~8–25%) of a highly depleted mantle source, while group (1) diorites have been differentiated after variable fractionation processes (up to 30%). Group (2) diorites seem to have been derived from low partial melting degrees (~3%) of a fertile or moderately depleted mantle source and with extensive fractionation processes (~50%). Geochemical results suggest that partial melting processes occurred at relatively shallow depths, in the plagioclase-spinel stability field, while amphibole chemistry data indicate shallow level crystallization. Chondrite and PM-normalized patterns, Th/Yb, and Nb/Th ratios as well as mineral chemistry analyses show that gabbros and group (1) diorites (with relatively low PM-normalized Nb and Ta values and negative Ti anomalies) suggest subduction processes, while group (2) diorites are MORB or BAB related. Some gabbros have been characterised as high-Mg, being compositionally similar to picrites or boninites. Variability in extent of partial melting of the mantle source and the different geotectonic environment affinities are consistent with a supra-subduction zone (SSZ) origin of the east Othris ophiolites. The fact that IAT related rocks are more abundant in east rather than in west Othris may possibly be explained by a slab rollback model retreating to the east within the Pindos oceanic basin.