Contemporaneous Triassic Calc-Alkaline Volcanism and Fast-Spreading Magmatism in the Western Tethys: Implications for the Eastern Mediterranean Palaeogeographic Evolution

Triassic calc-alkaline volcanics that are mainly exposed in the Hellenides—Dinarides orogen, as well as eastwards in the Afyan Zone, are considered to constrain the petrogenetic mechanisms that led to their formation, and to unravel the geodynamic processes that occurred in the western Tethys, related to the NW Gondwana (Apulia) promontory and the northwards transposing Pelagonian microcontinent. Lavas from Triassic key localities were examined from the broader northern Peloponnese, which include basaltic andesites, andesites and rhyodacites, and are classified as medium-K calc-alkaline volcanics. Partial melting of ∼10%–12% of a fertile peridotitic mantle source contributed to primitive magma formation. Whole-rock geochemical data and Sr-Nd isotopic compositions point to upper crustal assimilation processes (AFC) for the formation of the andesites and rhyodacites, with variable fractionation degrees (∼20%–50%), coupled by mixing with ∼2%–4% of an upper crustal component. To elucidate the co-existence of forearc volcanics with the Triassic rift-related E-MORB and OIB basalts, we propose two scenarios for the geotectonic setting of the associated magmatic suites. Scenario-1 considers intra-oceanic induced subduction initiation with southwestern dipping of the oceanic slab beneath the Pelagonian margin, whereas scenario-2 considers fast-spreading rift oceanic accretion, leading to northeastern subduction of the oceanic lithosphere towards the Pelagonian microcontinent. We envisage the potential for the two proposed scenarios to have acted in conjunction with slab break-off, facilitating the subduction polarity-reversal process. This can successfully account for the occurrence of calc-alkaline volcanics as adjacent outcrops along with IAT-type volcanics or even solely as calc-alkaline pyroclastic tuff formations.