The origin of alkali granites and Th-U ± REE enrichments in Kestanbol Magmatic complex (NW Anatolia) revisited: Evidences from bulk-rock geochemistry and isotopic data, zircon UPb, biotite Ar/Ar and apatite (UTh)/He geochronology

Abstract

This paper presents new field, petrographic, geochemical, SrNd isotopic and geochronological data from Kestanbol Magmatic Complex (KMC) in the western Anatolia. Zircon UPb ages from the KMC were in the range 21.91 Ma and 21.52 Ma, indicating Miocene emplacement. ⁴⁰Ar/³⁹Ar dating results of biotites from the same samples show a narrow range of ages between 20.0 and 22.7 and a weighted mean of 21.41 ± 0.40 Ma, and those of hornblende analysis yield ages between 21.52 and 31.19 Ma with a weighted mean of 22.70 ± 0.99 Ma, are interpreted as the cooling age of the KMC. The average (U-Th/He) ages from the KMC yielded an average of 21.5 Ma and 19.8 Ma. These new age data indicate rapid cooling following the emplacement of the KMC at ∼21 Ma. We suggest that the cooling was due to rapid uplift in the western Anatolia. The studied monzonitic, syenitic and alkaline subvolcanic rocks of the northern KMC are characterized by high K₂O (4.34–10.7 wt%), low to moderate SiO₂ (50.0–69.9 wt%), and P₂O₅ (0.03–1.07 wt%). They have moderate initial ⁸⁷Sr/⁸⁶Sr (0.707245–0.707875) and high initial ¹⁴³Nd/¹⁴⁴Nd (0.512441–0.512508) ratios, consistent with some crustal contamination. The studied rocks are enriched in Th (up to 204 ppm), U (up to 54.9 ppm), REE (up to 565.9 ppm) and, some LILE's including K (up to 8.85%), Rb (up to 447.1 ppm), Sr (up to 2053 ppm) and Ba (up to 2578 ppm). The geochemical and isotopic data suggest that the magmatic evolution of KMC is dominated by events including post-collisional tectonics, flux induced partial melting, fractional crystallization. The enrichments of incompatible elements are mostly caused by the fractional crystallization and K-metasomatism that affected the earlier magmatic phases during the cooling of the complex.