Constraints on the petrogenesis of Evros ophiolite extrusives, NE Greece

Abstract

The incomplete and dismembered Evros ophiolite is situated in Thrace, NE Greece, and belongs to the Circum-Rhodope Belt (CRB). Its age is considered to be Jurassic–Lower Cretaceous. The geotectonic setting of the ophiolite was a volcanic arc-marginal basin system in the Vardar oceanic realm. In the uppermost level of the ophiolitic sequence, volcanic and pyroclastic rocks of tholeiitic composition are underlain massive and pillow lavas with a few tuffaceous rocks and lava breccia. All these rocks were involved in intense deformation and low-grade metamorphic events during and after their formation. The underlying tholeiites were regionally metamorphosed to greenschist facies ( lower metavolcanics (LMV)), whereas the upper lavas were extensively recrystallized mainly by ocean floor metamorphism of prehnite–pumpellyite facies ( upper metavolcanics). REE data and wide ranges in Zr/Y, Ti/Zr and Al/Ti ratios suggest that the parental magma of the upper metavolcanics underwent extensive fractional crystallization. Tholeiitic basalts and mainly basaltic andesites to andesites were formed at the initial stages of the differentiation by crystal fractionation, which involved olivine, Cr-spinel, clinopyroxene and occasionally plagioclase. Subordinate dacites to rhyodacites were created when plagioclase became the main fractionating phase. The oxygen fugacity remained nearly stable during the formation of the main volume of the rock association, but it changed when rhyodacitic rocks started to crystallize. The protoliths of the upper metavolcanics was boninitic, produced in the forearc area by about 30% partial melting of an already depleted mantle source. This partial melting was induced by aqueous fluids and/or melts from the subducting oceanic crust and its sedimentary cover. The depleted mantle source of the upper metavolcanics was the residue of an earlier partial melting event that had generated the magma of the lower metavolcanic protoliths in an extensional regime.