Carboniferous high-potassium I-type granitoid magmatism in the Eastern Pontides: The Gümüşhane pluton (NE Turkey)

The Gümüşhane pluton, a high-K calc-alkaline I-type granodiorite/granite complex, forms an important component of the pre-Liassic basement of the Eastern Pontides (NE Turkey). In its eastern part, the pluton shows a compositional zonation ranging from biotite-hornblende granodiorite in the NW through biotite-hornblende granite to leucogranite/granophyre in the SE. Numerous mafic microgranular enclaves (up to ∼ 40 cm in diameter) suggest the former presence of globules of mafic melt during crystallization. Emplacement of the pluton occurred during the latest Early Carboniferous, as shown by the 320 ± 4 Ma ⁴⁰Ar-³⁹Ar biotite/hornblende and 324 ± 6 Ma LA-ICP-MS U-Pb zircon ages. In Harker diagrams, samples of the different rock types exhibit well-defined data trends. With increasing SiO₂, the abundances of TiO₂, Al₂O₃, Fe₂O₃^tot, MnO, MgO, CaO, P₂O₅ and Sc decrease, but those of K₂O and Rb increase. However, the variations of Sr, Ba, (La/Yb)_cn, Sr/Y and ∑ REEs vs. SiO₂ form distinctive groupings, which cannot be explained by a simple fractional crystallization. Chondrite-normalized (cn) REE patterns of granodiorite/granite samples show concave-upward shapes with (La/Yb)_cn ranging from 5.2 to 12.4 and Eu/Eu* from 0.84 to 0.47, while there is almost no fractionation of the middle REE relative to the heavy REE. In primitive mantle-normalized element concentration diagrams, all rocks display marked negative anomalies in Ba, Nb/Ta, Sr, P and Ti, but positive anomalies in K and Pb. These geochemical features imply a fractionating mineral assemblage of clinopyroxene, amphibole and plagioclase without significant involvement of garnet. The granophyres are, on the other hand, characterized by higher K₂O/Na₂O and Rb/Sr ratios, lower (La/Yb)_cn ratios (1.3 to 4.8) and more pronounced negative anomalies in Ba, Nb/Ta, Sr, Eu, P and Ti. Initial ε_Nd values range from - 3.78 to - 5.30 and Nd model ages from 1.38 to 163 Ga. The magmas of the granite/granodiorite portion were probably generated by partial melting of high-potassic amphibolitic rocks, and those of the granophyres by a relatively felsic micaceous crustal source. The Gümüşhane pluton was emplaced at the wake of the low-pressure-high-temperature metamorphism, and is regarded as a late phase of Hercynian orogeny in the Eastern Pontides.