U-Pb geochronology, bulk-rock geochemistry and petrology of Late Cretaceous syenitic plutons in the Golkoy (Ordu) area (NE Turkey): Implications for magma generation in a continental arc extension triggered by slab roll-back

Abstract

Late Cretaceous felsic plutons in the Eastern Pontides orogenic belt of Turkey are found in varying ages, sizes and compositions. The Late Cretaceous syenitic bodies, namely Direkli and Yenikoy Tepe plutons in the Ordu area, intruded into the Late Cretaceous volcanics and volcanoclastics, and compositionally consist of syenite and minor quartz-syenite with mainly granular and rarely porphyric, poikilitic and perthitic textures. The studied plutons contain K-feldspar, plagioclase, quartz, clinopyroxene, biotite and minor hornblende, and Fe-Ti oxide, apatite, and zircon. Mineral thermobarometer estimations imply that the syenitic plutons were crystallized in P-T conditions representing mid to shallow crustal levels. Petrochemically, these syenitic plutons show I-type, metaluminous-peraluminous transitional (A/CNK = 0.85-1.04) and shoshonitic features. LA-ICP-MS U-Pb zircon dating on the syenitic plutons yielded weighted mean ages of 78.5-72 Ma. Primitive mantle-normalized trace element patterns of the studied plutons are characterized by negative anomalies of Ba, Sr, Nb, Ta and Ti, and enrichment in Rb, Th, K, Ce and La. The chondrite-normalized rare earth element (REE) plots of the studied plutons show moderately enriched concave-shaped patterns (La-N/Lu-N = 10.3-10.7 for the Direkli pluton; La-N/Lu-N = 6.8-11.7 for the Yenikoy Tepe pluton) with negative Eu anomalies. The plutons have Sr-87/Sr-86 ratios (0.705368-0.706954), Nd-143/Nd-144 ratios (0.512600-0.512672), Pb-206/Pb-204 ratios (19.29-19.52) and Pb-207/Pb-204 ratios (15.65-15.67). The geochemical data, as well as regional geology, suggest that the Late Cretaceous syenitic plutons have formed in an extensional continental arc setting triggered by slab roll-back. The parental magma of syenitic plutons was derived from melts of lower crustal materials (metabasalt/metaandesite) and lithospheric mantle component.