Petrogenesis of a Late Cretaceous composite pluton from the eastern Pontides: the Dagbasi pluton, NE Turkey

Abstract

Zircon U-Pb ages, major and trace elements, and Sr-Nd isotope data are reported for the Late Cretaceous Dagbasi pluton from the eastern Pontides, NE Turkey. This pluton has calc-alkaline affinity and displays typical features of volcanic arc granitoids. It is a composite pluton consisting of tonalite, granodiorite, and monzogranite, all containing mafic microgranular enclaves (MMEs). Based on U-Pb zircon SHRIMP dating, magma emplacement ages of the tonalite, granodiorite, and monzogranite are 88.1 +/- 1.7 Ma, 86.0 +/- 2.0 Ma, and 82.9 +/- 1.3 Ma, respectively. The tonalites have low SiO2(60-64 wt%) and display metaluminous to peraluminous characteristics. They have similar La-cn/Lu-cn (1.5-7.3) and Eu/Eu* (0.65-0.88) values compared to granodiorites and monzogranites. The isotopic signatures of these rocks (epsilon(Nd(i)) = -1.4 to 1.7; Sr-87/Sr-86((i)) = 0.7056-0.7060) are largely similar to granodiorites and monzogranites. Samples from the granodiorites and monzogranites have SiO2 between 69 and 76 wt% and display peraluminous to slightly metaluminous characteristics. Chondrite-normalized REE patterns are characterized by moderate La-cn/Lu-cn ratios (1.6-7.1), with pronounced negative or positive Eu anomalies (Eu/Eu* = 0.46-1.11). Radiogenic isotope signatures of granodiorites and monzogranites show overlapping Sr-87/Sr-86((i)), (0.7057-0.7067) and epsilon(Nd(i)), (-3.2 to + 1.5) values. The MMEs have similar geochemical and isotopic features as the host rocks. All plutonic rocks are enriched in large ion lithophile elements (LILE) and light rare earth elements (LREE) relative to high field strength elements (HFSE). Fractionation of plagioclase, hornblende, and Fe-Ti oxides played an important role in the evolution of Dagbasi granitoids. All these geochemical and isotopic characteristics suggest an origin by dehydration melting of an amphibolitic lower crustal source.