Geochronological evidence and tectonic significance of Carboniferous magmatism in the southwest Trabzon area, eastern Pontides, Turkey

Abstract

The northern and southern zones of the eastern Pontides (northeast Turkey) contain numerous plutons of varying ages and compositions. Geochemical and isotopic results on two Hercynian granitoid bodies located in the northern zone of the eastern Pontides allow a proper reconstruction of their origin for the first time. The intrusive rocks comprise four distinct bodies, two of which we investigated in detail. Based on LA-ICP-MS U-Pb zircon dating, the Derinoba and Kayadibi granites have similar Pb-206/U-238 versus Pb-207/U-235 Concordia ages of 311.1 +/- 2.0 and 317.2 +/- 3.5 million years for the former and 303.8 +/- 1.5 million years for the latter. Aluminium saturation index values of both granites are between 0.95 and 1.35, indicating dominant peraluminous melt compositions. Both intrusions have high SiO2 (74-77 wt.%) contents and show high-K calc-alkaline and I- to S-type characteristics. Primitive mantle-normalized element diagrams display enrichment in K, Rb, Th, and U, and depletion in Ba, Nb, Ta, Sr, P, and Ti. Chondrite-normalized rare earth element patterns are characterized by concave-upward shapes and pronounced negative Eu anomalies with La-cn/Yb-cn = 4.6-9.7 and Eu-cn/Eu* = 0.11-0.59 (Derinoba), and La-cn/Yb-cn = 2.7-5.5 and Eu-cn/Eu* = 0.31-0.37 (Kayadibi). These features imply crystal-melt fractionation of plagioclase and K-feldspar without significant involvement of garnet. The Derinoba samples have initial epsilon Nd values between -6.1 and -7.1 with Nd model ages and T-DM between 1.56 and 2.15 thousand million years. The Kayadibi samples show higher initial epsilon Nd-(I) values, -4.5 to -6.2, with Nd model ages between 1.50 and 1.72 thousand million years. This study demonstrates that the Sr isotope ratios generally display negative correlation with Nd isotopes; Sr isotope ratios were lowered in some samples by hydrothermal interaction or alteration. Isotopic and petrological data suggest that both granites were produced by the partial melting of early Palaeozoic lower crustal rocks, with minor contribution from the mantle. Collectively, these rocks represent a late stage of Hercynian magmatism in the eastern Pontides.