Rift and intra-oceanic subduction signatures in the Western Tethys during the Triassic: The case of ultramafic lavas as part of an unusual ultramafic–mafic–felsic suite in Othris, Greece

Abstract

The Triassic igneous rocks of Othris, Greece, exhibit unusual and extreme compositional variations. Abundant E-MORB and rare OIB alkaline basalts appear to be linked to NW Gondwana (Apulia) margin rifting, Pelagonian microcontinent detachment and opening of the Pindos oceanic strand at Western Tethys. They are spatially associated with wehrlites, ultramafic lavas, picrites, transitional boninites, tholeiitic basaltic andesites and calc-alkaline andesitic to rhyodacitic rocks, whose new geochemical and petrological data suggest that they were formed in a short-lived subduction system, developed shortly after rifting/opening within the Pindos ocean.OIB alkaline basalts seem to have been formed from an enriched, possibly garnet bearing mantle source, while formation of E-MORB basalts may represent partial melts (~20%) of an enriched mantle source. The liquidus temperature for the primary rift-related magma is estimated at ~1330°C, while mantle potential temperature reached ~1435°C, with ~14wt.% MgO. Subduction-related rocks were produced after differentiation of primary picritic magma, generated after ~32% partial melting of a fertile mantle source within the newly formed mantle wedge. Temperature conditions were similar to those calculated for the rift-related primary magma, but with higher MgO contents (~16wt.%). Olivine and clinopyroxene accumulation of a batch of the subduction-related primary magma led to the formation of ultramafic magma under hydrous conditions. Wehrlites represent ultramafic magma stacked at the bottom of a magma chamber. Ultramafic lavas were formed when ultramafic magma was brought to the surface, most likely with the aid of an upwelling asthenospheric E-MORB flow passing through a slab break-off or by the bottom edge of the infant slab. The remainder of the primary picritic magma underwent variable degrees of fractional crystallization forming transitional boninites and tholeiitic basaltic andesites in a front-arc setting and calc-alkaline intermediate and felsic rocks in places closer to the passive margin of the Pelagonian microcontinent.