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

摘要

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, Kilickaya and Kozluk intrusions present similar Pb-206/U-238 versus Pb-207/U-235 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 Sr-87/Sr-86(I) (0.70531-0.70576), epsilon Nd-(i) (+03 to -0.5), and delta O-18 values (+6.6 to +6.9), as well as Nd model ages (0.73-0.82 Ga). Their Pb isotopic ratios are Pb-206/Pb-204 = 18.67-18.86, (207)pb/(204)pb = 15.61-15.62, and (208)pb/(204)pb = 38.62-39.03. Although the isotope signatures of MMEs (Sr-87/Sr-86((i)) = 0.70528-0.70561; epsilon Nd-(i)= +0.7 to -0.3; delta O-18 = +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 similar to 17% to 23% lower crustal magma contribution to the mantle component. (C) 2015 Elsevier Ltd. All rights reserved.