Geochemistry of mafic lavas from Sivas, Turkey and the evolution of Anatolian lithosphere

Abstract

Near-contemporaneous suites of mafic lavas from Sivas, Central Anatolia record different petrogenetic histories on the eastern and western sides of a major regional suture marked by the Kızılırmak River. The Sivas basaltic suite has major and trace element compositions suggesting derivation from an anhydrous peridotitic mantle source region. Basaltic trachyandesites in this group are related by up to ~65% fractional crystallization of the observed anhydrous mineral phases from a frequently-erupted basaltic parent with ~9wt.% MgO. Chondrite-normalized Tb/Yb values indicate that the basaltic magmas likely originate from melting peridotite at pressures near the spinel-garnet transition. Clinopyroxene with a range of Alvi/Aliv values suggest multiple stages of mineral growth, consistent with a mantle origin followed by fractional crystallization at shallow crustal levels. In contrast, Sivas basanites have higher abundances of key incompatible elements that suggest a source area in the subcontinental lithosphere with hydrous metasomatic mineral phases. Clinopyroxene in Sivas basanites have Alvi/Aliv values that cluster around 0.25, suggesting the magmas stalled at mid-crustal depths prior to rapid ascent and eruption. High abundances of incompatible major and trace elements in the most magnesian basanites show that the basanite lavas are not related to one another or to the basalt suite by fractionation. The thermal environment that led to formation of parental magmas for both the basalt suite and the basanites likely reflects ascent of asthenospheric material around and through torn slabs beneath Central Anatolia, providing heat to melt both ascending mantle and regionally metasomatized subcontinental lithosphere. This process is enhanced by recent trans-tensional deformation in the Anatolian plate that allows mafic melts to rise to shallow depths on both sides of the Kızılırmak River.