Eyuboglu, Yener; Dudas, Francis O.; Santosh, M.; Zhu, Di-Cheng; Yi, Keewook; Chatterjee, Nilanjan; Jeong, Youn-Joong; Akaryali, Enver; Liu, Ze

The Eastern Pontides Orogenic Belt represents one of the best examples of fossil convergent margins in the eastern Mediterranean region. However, the origin and geodynamic setting of the late Mesozoic-Cenozoic magmatism in this belt remain controversial due to lack of systematic geological, geochemical and chronological data. The general consensus is that the late Mesozoic-Cenozoic igneous activity is related to northward subduction of oceanic lithosphere in the late Mesozoic and following collision between Tauride and Pontide blocks in the early Cenozoic Here we present a comprehensive study focusing on the origin and geodynamic setting of gabbro bodies exposed along a narrow zone, parallel to the southeastern coast of the eastern Black Sea basin, in the Northern Zone of the Eastern Pontides Orogenic Belt. The studied gabbro bodies are hosted within late Cretaceous basaltic, andesitic, and dacitic volcanics including pyroclastic rocks and interbedded sedimentary rocks. The gabbro bodies range in size from 0.1 km(2) to 1.5 km(2), and outcrop patterns vary from round or elliptical to markedly elongate with sharp and discordant contact with the host rocks. Their mineral assemblage includes mainly clinopyroxene, plagioclase, minor olivine, amphibole, magnetite and rarely orthopyroxene, biotite, zircon and titanite. The occurrence of sutured grain boundaries on clinopyroxene and plagioclase, and the presence of reverse compositional zoning in clinopyroxene and olivine suggest mixing between magmas of contrasting compositions during mineral growth. Thermobarometric computations indicate that the temperature at the beginning of crystallization was similar to 1250 degrees C and crystallization was polybaric Zircon and titanite U-Pb ages indicate that these small intrusions were emplaced into crustal rocks of the Eastern Pontides Orogenic Belt during Lutetian (45 +/- 2 Ma). The depletion of HFSE is consistent with the involvement of an arc-related source in the petrogenesis of these rocks, and low to moderate enrichment Ce, Rb, Ba, K, Pb, Sr and Th suggests that involvement of subducted oceanic sediment was modest. The low Th content and low Th/Yb indicate that the role of sediment addition was nevertheless limited. The Nd, Sr and Pb isotopic data are consistent with the interpretation that the dominant source component in these gabbros is a depleted, peridotitic mantle, and that crustal contamination is relatively unimportant. We suggest that mafic magmas that produced the gabbroic intrusions were derived from melting of a depleted mantle source under the forearc region of the Eastern Pontides Orogenic Belt during southward subduction of two oceanic plates separated by a mid-ocean ridge, leading to the formation of a slab window. We also infer fractional crystallization and assimilation during both magma storage in the crust-mantle transition zone and transfer into the overlying arc crust. (C) 2015 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

Eyuboglu, Yener; Santosh, M.; Yi, Keewook; Tuysuz, Necati; Korkmaz, Sadettin; Akaryali, Enver; Dudas, Francis O.; Bektas, Osman

The Meso-Cenozoic geodynamic evolution of the eastern Pontides orogenic belt provides a key to evaluate the volcanogenic massive sulfide (VMS) deposits associated with convergent margin tectonics in a Cordilleran-type orogenic belt. Here we present new geological, geochemical and zircon U-Pb geochronological data, and attempt to characterize the metallogeny through a comprehensive overview of the important VMS mineralizations in the belt. The VMS deposits in the northern part of the eastern Pontides orogenic belt occur in two different stratigraphic horizons consisting mainly of felsic volcanic rocks within the late Cretaceous sequence. SHRIMP zircon U-Pb analyses from ore-bearing dacites yield weighted mean Pb-206/U-238 ages ranging between 91.1 +/- 1.3 and 82.6 +/- 1 Ma. The felsic rocks of first and second horizons reveal geochemical characteristics of subduction-related calc-alkaline and shoshonitic magmas, respectively, in continental arcs and represent the immature and mature stages of a late Cretaceous magmatic arc. The nature of the late Cretaceous magmatism in the northern part of the eastern Pontides orogenic belt and the various lithological associations including volcaniclastics, mudstones and sedimentary facies indicate a rift-related environment where dacitic volcanism was predominant. The eastern Pontides VMS deposits are located within the caldera-like depressions and are closely associated with dome-like structures of felsic magmas, with their distribution controlled by fracture systems. Based on a detailed analyses of the geological, geophysical and geodynamic information, we propose that the VMS deposits were generated either in intra arc or near arc region of the eastern Pontides orogenic belt during the southward subduction of the Tethys oceanic lithosphere. (C) 2013 Elsevier B.V. All rights reserved.

Eyuboglu, Yener; Santosh, M.; Dudas, Francis O.; Akaryali, Enver; Chung, Sun-Lin; Akdaǧ, Kemal; Bektaş, Osman

The eastern Pontides orogenic belt provides a window into continental arc magmatism in the Alpine-Himalayan belt. The late Mesozoic-Cenozoic geodynamic evolution of this belt remains controversial. Here we focus on the nature of the transition from the adakitic to non-adakitic magmatism in the Kale area of Gumushane region in NE Turkey where this transition is best preserved. The adakitic lithologies comprise porphyries and hyaloclastites. The porphyries are represented by biotite-rich andesites, hornblende-rich andesite and dacite. The hayaloclastites represent the final stage of adakitic activity and they were generated by eruption/intrusion of adakitic andesitic magma into soft carbonate mud. The non-adakitic lithologies include basaltic-andesitic volcanic and associated pyroclastic rocks. Both rock groups are cutting by basaltic dikes representing the final stage of the Cenozoic magmatism in the study area. We report zircon U-Pb ages of 48.71 ± 0.74 Ma for the adakitic rocks, and 44.68 ± 0.84 Ma for the non-adakitic type, suggesting that there is no significant time gap during the transition from adakitic to non-adakitic magmatism. We evaluate the origin, magma processes and tectonic setting of the magmatism in the southern part of the eastern Pontides orogenic belt. Our results have important bearing on the late Mesozoic-Cenozoic geodynamic evolution of the eastern Mediterranean region.

Eyuboglu, Yener; Dudas, Francis O.; Santosh, M.; Yi, Keewook; Kwon, Sanghoon; Akaryali, Enver

High-silica adakites intrude the Kop ultramafic massif in the eastern Pontide orogenic belt of NE Turkey. These dominantly dacitic to rhyodacitic plagioclase porphyries form clusters of small intrusions. U-Pb dating of zircons from two intrusions yields ages between 52 and 59 Ma. The best-constrained crystallization age is 55.83 +/- 0.04 Ma, indicating that these are the oldest adakitic rocks of the eastern Pontides. The rocks show geochemical signatures typical of adakites. The absence of systematic geochemical variation within the adakite suite indicates a subordinate role for fractional crystallization and binary mixing, and suggests that individual intrusions may represent separate, small batches of magma. Modeling of partial melting suggests that the degree of melting was 5-20%, that the residues after melting likely contained garnet, clinopyroxene, and rutile, and that plagioclase was either absent or totally consumed during partial melting. The most likely source compositions had large-ion lithophile elements >10 times, and heavy rare earth elements <10 times, chondritic concentrations. Modeled bulk distribution coefficients indicate that the adakites were generated as silicic magmas, and are not derived by fractional crystallization from basaltic or andesitic parental melts. Sr, Nd, and Pb isotopic data indicate at least three isotopically distinct sources, one of which is depleted mantle. The other components include upper crust (possibly a mixture of subducted sediment and Hercynian continental basement rocks) and a component that may include both lower crust and sublithospheric mantle. These adakites are unique among the late Cretaceous and Cenozoic igneous rocks of the eastern Pontides in having a large proportion of the depleted mantle component. The prominent role of the depleted mantle in their petrogenesis suggests that the adakites may be related to a slab window in a south-directed subduction zone. (C) 2012 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

Eyuboglu, Yener; Chung, Sun-Lin; Santosh, M.; Dudas, Francis O.; Akaryali, Enver

The formation of the eastern Pontides orogenic belt has been widely assigned to a northward subduction of the Neotethyan oceanic slab during the late Mesozoic-Cenozoic. Here we provide an alternate model based on new geological, geochemical and isotopic data. The magmatic activity in the far south of the belt started in the early Campanian with shoshonitic trachyandesites and associated pyroclastics. This sequence is covered by the late Campanian-early Maastrichtian reefal limestones and another stage of high-K volcanism represented by analcimized leucite-rich ultrapotassic rocks of the Maastrichtian-early Paleocene (?) ages. The shoshonitic and ultrapotassic rocks, with K2O contents ranging from 0.26 to 6.95 wt.%, display broadly similar rare earth and multi-element distribution patterns. Both rock types are enriched in LILE and LREE and depleted in HFSE (Nb,Ta and Ti), suggesting a subduction-enriched mantle source for the magma generation. Subsequently, during the late Paleocene, a stage of acidic magmatism (SiO2 of 53.25-73.61 wt.%) that shows adakitic geochemical characteristics including high Sr/Y (46-416) and La/Yb (11-51) and low Y (2.6-12.2 ppm), is documented characterized by melting of a mafic source such as the MORB crust with garnet in the residue. The adakitic magmatism began at similar to 56 Ma and migrated toward the north through time, culminating with porphyritic andesites (similar to 47 Ma) that were emplaced in the Gumushane-Bayburt line and its vicinity. North of this line, coeval magmas show typical calc-alkaline nature and continued to develop toward further north until the middle to late Eocene. Based on the spatial and temporal variations in the magmas generated in the eastern Pontides orogenic belt, we propose a new geodynamic model to explain the tectonomagmatic evolution of these rocks and correlate the adakitic magmatism to ridge subduction and slab window process within a south-dipping subduction zone. Our model is in contrast to the previous proposals which envisage partial melting or delamination of thickened lower continental crust due to the collision in the south during the Paleocene-Eocene. (C) 2010 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

Eyuboglu, Yener; Santosh, M.; Dudas, Francis O.; Chung, Sun-Lin; Akaryali, Enver

The eastern Pontide orogenic belt in Turkey offers a critical region to evaluate the geodynamic evolution of the eastern Mediterranean region during the late Mesozoic-Cenozoic. Here we synthesize results from our exhaustive database on the geochemical characteristics of adakitic magmatism from 137 bodies covering 7 domains in the southern zone of eastern Pontides. Together with new Pb-Sr-Nd results, we show that the adakitic magmatism in the eastern Pontide belt is clearly linked to slab window processes during ridge subduction, as against the earlier models that assumed partial melting of a delaminated or thickened lower continental crust following collision. The isotopic compositions display a complex pattern and suggest mixing of multiple source materials such as depleted mantle, enriched lithospheric mantle, and upper crust. We also confirm a southward polarity of subduction as against the previous models that assumed a northward subduction. We supplement our model with evidence for a northward propagation of the adakitic magmatism, spatial and temporal variations in arc magmatism, presence of south-dipping reverse faults and a synthesis of the available paleomagnetic data. Our studies also suggest that the Black-Sea is a remnant of the Tethyan ocean that was situated to the north of the arc during the late Mesozoic-Cenozoic as against the correlation to a late Mesozoic-Cenozoic back-arc basin related to a northward subduction of the Paleo- or Neo-tethyan oceanic lithosphere in alternate models. (C) 2010 Elsevier Ltd. All rights reserved.