Petrochemistry and age of garnet-bearing xenoliths and their host volcanic rocks of Central Anatolia, Turkey: Implications for the evolution of magma-hydrothermal systems

Abstract

The Hisarlıkaya volcanic rocks (HVR) are located southwest of Ankara, and presents a complex magmatic and metamorphic evolution. Understanding its petrogenesis provides crucial insights into the geological processes and magma evolution in Central Anatolia. This paper presents mineral chemistry, whole-rock major, trace element and Sr-Nd-O isotope, ⁴⁰Ar-³⁹Ar age of host volcanic rocks for magmatism and U-Pb dating of garnet for crystallization of contact metamorphosed rocks to constrain the magma source and the metasomatic processes in the contact zone of magma intrusion. The HVR samples have SiO₂ from 60 to 64 wt%, Al₂O₃ (16–18 wt%), MgO (1.1–2.4 wt%), high concentrations of Large Ion Lithophile Elements (LILEs) and Light Rare Earth Elements (LREEs), and low concentrations of High Field Strength Elements (HFSEs) and Heavy Rare Earth Elements (HREEs), and high Sr/Y (70–90) and La/Yb (43–55) ratios. The HVR consists of trachytes and trachyandesites, displaying calc-alkaline characteristics with adakitic features with these geochemical properties. Initial ¹⁴³Nd/¹⁴⁴Nd, ⁸⁷Sr/⁸⁶Sr ratios, and δ¹⁸O values of HVR rocks are between 0.51263–0.51271, 0.70537–0.70542, and 8.60–9.20 ‰, respectively. These features together with geothermobarometric estimates suggest that the crystallization depth of the HVR's plagioclases and amphiboles was ∼25–30 km. Magmatism was initiated by partial melting of hydrated lithospheric mantle wedge in the Early Cretaceous followed by magmatic underplating and eruption of more felsic magma derived partly from the thickened lower crust.

These volcanic rocks contain xenoliths of quartzite, hornfels, and garnet-bearing skarn presumed to be from the contact zone. These were metamorphosed during earlier subduction and were subsequently assimilated and extruded by HVR magma. The ⁴⁰Ar-³⁹Ar plateau age of the HVR is 20.25 ± 0.10 Ma and the U-Pb dating of two garnet crystals from the skarn xenoliths are determined to be 22.08 ± 0.18 Ma to 21.30 ± 0.17 Ma, indicating that mineralization in the hydrothermal-metasomatic alteration zone is older and/or approximately contemporaneous with magmatism.