Late Cretaceous high-K volcanism in the eastern Pontide orogenic belt: implications for the geodynamic evolution of NE Turkey

Abstract

High-K volcanics were produced in two different Late Cretaceous cycles during the infilling of back-arc basins of the eastern Pontides. The first cycle, represented by shoshonitic trachyandesites and associated pyroclastics, containing high K2O (2.74-4.81 wt-%) and Na2O (3.60-5.51 wt-%), overlies a mid-Cretaceous ophiolitic-olistostromal melange formed during the rifting stage of a back-arc basin (Neotethys). Ar-40/Ar-39 dating of biotite from trachyandesite at the base of the first cyle indicates that shoshonitic volcanism in the far south of the eastern Pontides started in the early Campanian. Volcanic rocks of this cycle are overlain by upper Campanian-Maastrichtian rudist-bearing reefal limestones. The second cycle of high-K volcanism is represented by analcime-bearing volcanic rocks erupted in a Neotethyan lagoonal environment. These volcanic rocks, intercalated with continental detritus, are characterized by high Na2O (3.22-7.16 wt-%), now concentrated in secondary analcime crystals. Their K2O contents also range between 0.83 and 6.05 (wt-%). All of these units are disconformably overlain by Eocene turbidites with a basal conglomerate. Volcanic rocks belonging to both cycles show various enrichment degrees in LILE, HFSE, LREE, and HREE with respect to primitive mantle and chondrite [(La/Lu)(CN)=11.10-25.89]. Negative Nb, Ta, Zr, Hf, and Ti anomalies are typical of these subduction-related arc magmas. Geochemical similarities between the two volcanic cycles suggest that trachyandesites and analcime-bearing volcanics were derived from similar enriched mantle sources, and that they formed in the same geotectonic setting of a back-arc basin environment of the eastern Pontide magmatic arc during the Late Cretaceous. In addition, Nd-Sr and Pb isotope ratios of the investigated volcanic units indicate that their mantle melt sources were similar. The new geological, geochemical and isotopic data suggest that southward subduction of the Palaeotethyan oceanic crust can be separated into two main stages for the late Mesozoic Pontide orogenic belt. Early southward subduction ended by slab break-off during the mid-Cretaceous, resulting in upwelling and injection of asthenospheric mantle that caused the opening of the eastern Pontide back-arc basin (Neotethys). Resumption of southward subduction began with intense TH-CA bimodal volcanism in the northern front of the arc during the Turonian-Coniacian. The intensity of this Late Cretaceous magmatism in the north decreased towards the south, and a transition to monogenic calcalkaline-alkaline (CA-A) Campanian andesitic volcanism developed in the southern zone. Yet farther south, andesitic igneous activity graded into the Campanian-Maastrichtian shoshonitic and ultrapotassic volcanism in the Bayburt-Maden, Tokat, Amasya, and Gumushacikoy areas. This zoning of the Late Cretaceous arc magmatism (TH-CA, CA-A and shoshonitic-ultrapotassic, from north to south) also supports the concept of southward subduction during late Mesozoic evolution of the eastern Mediterranean region.