Structure and geochemistry of the adakitic Horoz granitoid, Bolkar Mountains, south-central Turkey, and its tectonomagmatic evolution

Abstract

High-Al granitic and granodioritic rocks of the 55 Ma Horoz pluton in the Bolkar Mountains of southern Turkey provide important new constraints on the early Cenozoic evolution of the eastern Mediterranean region. The ENE–WSW-trending, sill-like pluton is intrusive into late Palaeozoic–early Mesozoic metamorphic rocks of the Central Tauride block, and is unconformably overlain by Plio-Pleistocene alluvial deposits. The metaluminous to peraluminous granitoids have high-K calc-alkaline to high-K shoshonitic compositions, and show enrichment in large ion lithophile and depletion in high-field strength elements relative to ocean ridge granite. Their high Al2O3 contents (15.9–20.06 wt%) and low SiO2, MgO, and Mg numbers are consistent with adakitic compositions. These geochemical features, coupled with low Sr/Y and La/Yb ratios and trace-element patterns, suggest that the Horoz magmas were produced in part by partial melting of a subduction-metasomatized mantle. The high-Al adakitic and calc-alkaline compositions are consistent with partial melting of a hydrated lithospheric mantle and an amphibolitic–eclogitic mafic lower crust that was triggered by delamination-induced asthenospheric upwelling. We propose that, following Palaeocene continental collision between the Tauride and Central Anatolian Crystalline Complex, the inferred lithospheric delamination was a result of foundering of the overthickened orogenic root. Asthenospheric upwelling beneath the young orogenic belt thermally weakened the crust, and caused uplift and tectonic extension leading to core complex formation (Nigde massif), development of an extensional volcanic province (Cappadocia), and tectonic collapse of the Central Tauride block (Bolkar Mountains). The shallow-level Horoz pluton was unroofed by ∼23 Ma as a combined result of crustal uplift and erosion throughout the Palaeogene.