Geochemistry and tectonic significance of metamorphosed mafic ophiolitic rocks in the upper high-grade basement unit of the eastern Rhodope Massif (Bulgaria–Greece)

Abstract

: Metamorphosed mafic ophiolitic rocks in the metamorphic section of the eastern Rhodope Massif in Bulgaria and Greece are important for understanding the oceanic lithosphere fragments, which have been involved in Alpine tectonic–metamorphic processes. Petrography and mineral compositions of the meta-mafic rocks (mostly gabbro–basalt to minor diorite–andesite) point to main amphibolite-facies overprint, which strongly obliterated the primary textures, and the original igneous grain-sizes are partly preserved only of the plagioclase. The meta-mafic rocks are classified as low-K and low- to high-Ti tholeiitic affinity igneous protoliths of basaltic to andesitic compositions, in which high-Ti and low-Ti groups are identified on the basis of Ti concentrations. They also differ with respect to trace element and REE characteristics. A complex chemistry of high-Ti group indicates an origin primarily from MORB mantle source, subsequently modified by subduction-zone derived LILE- and REE-enriched melts and contribution of HFSE-enriched component that produce the oceanic island tholeiites. The low-Ti group displays IAT affinity, with clearly defined subduction-related component demonstrated by LILE enrichment, HFSE and HREE depletion relative to N-MORB and negative Nb and Ti anomalies, all indicative for an island arc petrogenesis. A single dunite sample studied also displays geochemical characteristics of the low-Ti group meta-mafic rocks. Geochemical diversity of the meta-basic rocks with MORB, transitional MORB/IAT and IAT affinities hints their supra-subduction zone (SSZ) origin in an island arc/ back-arc system, with identifiable arc-related and rifting/sea-floor spreading magmatic products represented by the low-Ti and high-Ti groups, respectively. At present, the available Middle-Late Paleozoic/Early Triassic radiometric ages of the meta-mafic rocks protoliths predominate over the Early Paleozoic ages, which suggests that the development of the inferred arc/back-arc system relates mostly to the ocean-floor magmatic evolution of the Paleotethyan realm.