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

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 Kizihrmak 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 similar to 65% fractional crystallization of the observed anhydrous mineral phases from a frequently-erupted basaltic parent with similar to 9 wt.% 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 Al-vi/Al-iv 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 Al-vi/Al-iv 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 Kizilirmak River

Post-delamination magmatism in south-central Anatolia

Quaternary mafic lavas in Central Anatolia provide geochemical insights into melt generation processes following regional delamination of the subducted Tethyan slabs. New geochemical data from the Pleistocene Hasandağ Cinder Cone Province (HCCP) and Karapınar Volcanic Field (KVF) record contributions from subduction-modified lithospheric and sub-lithospheric source domains that are distinct from those sampled elsewhere across Anatolia (e.g., Pearce et al., 1990; Aldanmaz et al., 2006; Keskin, 2007; Chakrabarti et al., 2012; Reid et al., 2017). Hasandağ cinder cones are primarily basaltic, with subordinate trachybasalt and basaltic trachyandesite; Karapınar lavas comprise basalts and andesites with subordinate basaltic trachyandesite and basaltic andesite. Anomalously high Li, Na, Ti and Zr abundances and Ba/Rb values in these South-Central Anatolian primitive mafic lavas suggest significant contribution from metasomatic phases (i.e., amphibole, zircon and rutile); Dy/Ybn values measured in the mafic volcanics indicate melting in the spinel stability field. Correlation between 87Sr/86Sr and εNd in HCCP and KVF lavas indicate pseudo-binary mixing between enriched and depleted endmembers, while εNd-εHf isotopic values that plot above the terrestrial array extend toward sediment compositions sampled from the eastern Mediterranean Sea (Klaver et al., 2015). Together, trace element chemistry and Sr-Nd-Hf-Pb isotope compositions suggest input from a spatially heterogeneous mantle with contributions from depleted MORB-like and recycled sediment sources. The apparent presence of pyroxenite and hydrous metasomes within the HCCP and KVF source regions, and abundant dense mafic cumulates associated with the adjacent Hasandağ stratovolcano, support an unstable small-scale lithospheric density structure in South-Central Anatolia. We suggest regional delamination of the Neotethyan slab on a regional scale caused upwelling asthenosphere to destabilize the remaining Central Anatolian lithosphere and resulted in localized drip melting to produce the mafic volcanism at the HCCP and KVF. Geochemical similarities to Miocene alkali basalts from Galatia and Sivas (Wilson et al., 1997; Varol et al., 2014; Kürkcüoğlu et al., 2015) suggest that this mechanism occurred across Central Anatolia subsequent to Tethys subduction, and thus the ages of monogenetic volcanic activity record the descent of the slab beneath Anatolia. We infer that slab foundering played a more important role in this process than lithospheric loss beneath a tectonically shortened orogen (Göğüs et al., 2017) although further mapping of sedimentary basins and structures is required to resolve this interpretation

Evolution of mafic lavas in Central Anatolia: Mantle source domains

We present new Sr-Nd-Pb-Hf isotopic data on mafic lavas from the Sivas, Develidag, Erciyes, and Erkilet volcanic complexes in central Turkey and Tenchirek in eastern Turkey to evaluate the mantle sources for volcanism in the context of the geodynamic evolution of the Anatolian microplate. Early Miocene through Quaternary volcanism in Western Anatolia and latest Miocene through Quaternary activity in Central Anatolia were dominated by contributions from two distinct source regions: heterogeneous metasomatized or subduction-modified lithosphere, and roughly homogeneous sublithospheric ambient upper mantle; we model the source contributions through mixing between three end members. The sublithospheric mantle source plots close to the Northern Hemisphere reference line (NHRL) with radiogenic Pb-206/Pb-204 of similar to 19.15, while the other contributions plot substantially above the NHRL in Pb isotope space. The lithospheric source is heterogeneous, resulting from variable pollution by subduction-related processes likely including direct incorporation of sediment and/or melange; its range in radiogenic isotopes is defined by regional oceanic sediment and ultrapotassic melts of the subcontinental lithospheric mantle. The geochemical impact of this contribution is disproportionately large, given that subduction-modified lithosphere and/or ocean sediment dominates the Pb isotope signatures of mafic Anatolian lavas. Subduction of the Aegean or Tethyan seafloor, associated with marked crustal shortening, took place throughout the region until ca. 16-17 Ma, after which time broad delamination of the thickened lower crust and/or the Tethyan slab beneath Central Anatolia allowed for sediment and/or melange and slab-derived fluids to be released into the overlying evolving modified mantle. Aggregation of melts derived from both mantle and lithospheric domains was made possible by upwelling of warm asthenospheric material moving around and through the complexly torn younger Aegean-Cyprean slab that dips steeply to the north beneath southern Anatolia