Geochronology, geochemistry, and petrogenesis of the Eocene Bayburt intrusions, Eastern Pontide, NE Turkey: implications for lithospheric mantle and lower crustal sources in the high-K calc-alkaline magmatism

Abstract

Eocene I-type plutons are widespread in the Eastern Pontides, Northeastern Turkey. Geochemical and isotopic results on two Eocene intrusive bodies located in the southern zone of the Eastern Pontides were utilized to achieve a proper reconstruction of their origin. Based on laser ablation ICP-MS U–Pb zircon dating, Kılıçkaya and Kozluk intrusions present similar 206Pb/238U versus 207Pb/235U concordia ages of 46.75 ± 0.79 and 46.9 ± 0.68 Ma, respectively. Both intrusions exhibit similar petrographical, geochemical, and isotopical features, and contain abundant mafic microgranular enclaves (MMEs). The intrusive rocks are composed of diorite, tonalite, and granodiorite, whereas MMEs are composed of diorite. Both the intrusive rocks and the MMEs are I-type, high-K calc-alkaline in composition and display a metaluminous character. They are enriched in large-ion lithophile and light rare-earth elements and are depleted in high-field strength elements. The intrusive rocks show moderate variation in 87Sr/86Sr(i) (0.70531–0.70576), εNd(i) (+0.3 to −0.5), and δ18O values (+6.6 to +6.9), as well as Nd model ages (0.73–0.82 Ga). Their Pb isotopic ratios are 206Pb/204Pb = 18.67–18.86, 207Pb/204Pb = 15.61–15.62, and 208Pb/204Pb = 38.62–39.03. Although the isotope signatures of MMEs (87Sr/86Sr(i) = 0.70528–0.70561; εNd(i) = +0.7 to −0.3; δ18O = +6.7 to +6.8) are significantly similar to those of the host rocks, the MMEs are characterized by relatively low contents of SiO2 (57–59 wt.%), low aluminum saturation index (0.8–0.9), and high Mg numbers (31–36). All of these data suggest magma generation by mixing of enriched lithospheric mantle- and lower crustal-derived melts. Moreover, Sr–Nd isotopic modeling reveals ∼17% to 23% lower crustal magma contribution to the mantle component.