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 U[sbnd]Pb, biotite Ar/Ar and apatite (U[sbnd]Th)/He geochronology

This paper presents new field, petrographic, geochemical, Sr[sbnd]Nd isotopic and geochronological data from Kestanbol Magmatic Complex (KMC) in the western Anatolia. Zircon U[sbnd]Pb 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.