Constraints on the formation and nature of the Hellenic Triassic rift-related lavas

Triassic volcanism developed during the rifting stage of Gondwana, with subsequent formation and development of the Tethyan oceanic basin as the Pangaea (Apulia promontory) and Pelagonia continents spread apart. Volcanic rocks formed from this activity outcrop over all mainland Greece, comprising of trachybasalts and basaltic trachyandesites. Relatively immobile to the effects of alteration processes major and trace element abundances classify the volcanics into OIB and E-MORB lavas. They have been distinguished based upon their: i) LREE contents, ii) silica-saturation index, iii) Zr/Nb and Nb/Y ratio values; iv) Th, U, and Ta contents v) geotectonic discrimination diagrams. Their geochemistry indicates that most rocks were affected by moderate to extensive differentiation processes, mostly expressed by clinopyroxene fractionation. Some of the OIB and E-MORB volcanics are considered as being primitive undersaturated, displaying relatively low SiO₂ and S.I. index values and also high Mg# and CaO/Al₂O₃ ratios.

Calculated average mantle potential temperatures are comparable (1410 ˚C OIB; 1370 ˚C E-MORB), with melt fractions estimated at 3-5% for primary OIB magmas and 6-8%for primary E-MORB magmas. An asthenospheric origin is inferred for the OIB lavas, with melting in the garnet stability field (75-95 km; 2.5-3.0 GPa), whereas E-MORB parent magmas were generated with shallower melting processes within the garnet/spinel (transitional) stability field (55-70 km; 1.8-2.2 GPa). Lithospheric attenuation and extension, followed by subsequent asthenospheric upwelling of the mantle was enhanced due to lithospheric thinning as rifting progressed. The rather high calculated partial melting degrees and the observed relatively thick lava formations account for fast-spreading rift settings, consistent with the opening of the Tethys during the Triassic. Temperature results indicate that the Hellenic Triassic rift-related magmas were generated from mantle at ambient temperature, precluding a mantle plume-based scenario or of thermal anomalies.