Evidence for the effects of subduction in Triassic lavas from the Northern Peloponnese (Greece): A mineralogical, geochemical and isotopic (Sr-Nd) approach

Triassic volcanism in Greece is mainly associated with the rift phase of the Neotethys that resulted in the formation of E-MORB and OIB alkali basalts, which are widespread throughout the Hellenic mainland[1]. In most of the outcrop localities (e.g. Pindos, Koziakas, Othris, Argolis) these basalts are closely related in the field with other more differentiated volcanics that display a clear subduction signature [1,2]. In the Northern Peloponnese and specifically from the localities of Drakovouni, Palaiohouni and Perachora, three types of lavas were identified: basaltic andesites, andesites and rhyodakites, which are fine to medium grained and displaying either porphyritic or even equigranular textures in the more felsic varieties. These lavas were classified based on their Si, Na and K contents, as well as their Nb/Y vs. Zr/Ti ratios, which were subjected to rather restricted metasomatic processes (LOI:1.1-3.7, partial albitization and uratilization). Based on their potassium contents, as well as upon the AFM geochemical ternary plot and their FeO/MgO ratios, they are geochemically classified as calc-alkaline volcanics, clearly being affected by subduction-related processes. The latter is confirmed by: presence of magmatic magnesiohornblende in all types of lavas at variable amounts, enhanced Th/Yb contents (2.4-4.1), LREE enrichments [(La/Yb)_CN=6.2-10.0], lower normalized values of Th and U compared to Nb and Ta, positive K and Pb anomalies, negative Ti anomalies in the PM-normalized diagrams, noticeable LILE enrichments (e.g. Cs, Rb, Ba).

Fractional crystallization played a significant role in the differentiation processes. This is confirmed by: presence of primary clinopyroxene and amphibole in the basaltic andesites whose modal composition significantly decreases in the andesites and rhyodakites (only accessory amphibole), relatively strong correlation between Sc/Y with CaO/Al₂O₃ (R² = 0.91), positive correlation between P₂O₅/TiO₂ and (La/Yb)_N (R² = 0.87), higher Cr and Ni contents in the least differentiated lavas, increase of Nb/Yb in the highly fractionated lavas, increasing Eu negative anomalies from the compositionally basic to the felsic varieties (basaltic andesites Eu_CN/Eu*= 0.73-0.80; andesites Eu_CN/Eu* = 0.63-0.74, rhyodakites Eu_CN/Eu* = 0.51-0.61). Apart from fractional crystallization, crustal assimilation (AFC processes) likely played an additional role during differentiation, shown by the strongly positive correlation between SiO₂ and Nb/Yb (R² = 0.92). The Sr-Nd isotopic data further confirm the effect of crustal contamination and AFC processes, with lower ¹⁴³Nd/¹⁴⁴Nd and higher ⁸⁷Sr/⁸⁶Sr ratios for the rhyodakites compared to the andesites and basaltic andesites.

References: [1]Koutsovitis, P., Magganas, A., Ntaflos, T., Koukouzas, N., Rassios, A.E., Soukis, K., 2020. Petrogenetic constraints on the origin and formation of the Hellenic Triassic rift-related lavas. Lithos 368-369, 105604, [2] Pe-Piper, G., Piper, D.J.W., 2002. The Igneous Rocks of Greece. Borntraeger, Stuttgart, pp. 1–645.