Yücel, Cem; Aydınçakır, Emre; Kaygusuz, Abdullah; Arslan, Mehmet; Yi, Keewook; Jeong, Youn-Joong; Cicerali, Derya

The Late Cretaceous magmatic history of the Eastern Pontides Orogenic Belt (NE Turkey) was generally characterized by common I-type and lesser A-type plutons in varying size and composition. Of these, A-type plutons from the Şebinkarahisar (Giresun) area yield SHRIMP zircon U-Pb age of 77.36 ± 0.96 Ma. The rocks of these plutons display a wide range of SiO2 contents (53.63 to 70.96 wt.%) and Mg-numbers (6.2–32.19) with A-type affinities characterized by enrichment in K2O and Na2O, Ga, Zr, and strong negative Eu anomalies. They also show arc-type petrochemical features including enrichment of large-ion lithophile elements (e.g. Cs, K, Rb) and light rare earth elements (LaN/LuN = 6.27–15.61), but depletion in high strength field elements (e.g. Nb, Ta, Ti, and P). The (87Sr/86Sr)i ratios and ɛNd(i) values of the studied rocks are in the range of 0.705129 to 0.0705347 and −3.64 to −3.12, respectively, while bulk rock ɛHf(i) values are between 1.86 and 4.36. They are characterized by positive zircon ɛHf(i) (1.74 to 10.88) plotting between the chondrite and depleted mantle evolutionary line values. Our data combined with the previous studies suggest that the magmas, that formed the Late Cretaceous plutons, were generated in an extensional tectonic environment due to slap roll-back and trench ward migration of the subduction zone to the south. The generation of hybrid magmas forming the studied plutons is in response to the melts derived from a metasomatized lithospheric mantle source, a relatively deeper mantle source, and the juvenile crust along with the heat transfer and decompression. Multi-sourced magmas rise through the continental crust and reach the final composition by predominantly fractional crystallization processes.

Aydınçakır, Emre; Yücel, Cem; Kaygusuz, Abdullah; Bilici, Özgür; Yi, Keewook; Jeong, Youn-Joong; Güloğlu, Z. Samet

The Eastern Pontides in NE Turkey hosts a very limited number of plutonic rocks from the Jurassic period. Here we submit new geochemical, Sr-Nd-Pb and zircon Lu-Hf isotope data, and zircon U-Pb ages for plutonic rocks in this region. Zircon U-Pb dating for the Alemdar and Işıkdere plutons yielded an age of 175–170 Ma (Middle Jurassic). The Alemdar and Işıkdere plutons have I-type, medium to high-K calc-alkaline series, and metaluminous to slightly peraluminous characteristics. On PM-normalized diagrams, the plutons are enriched in large-ion lithophile elements (LILEs) with pronounced depletion of high field strength elements (HFSEs, Ta, Nb, Ti). Chondrite-normalized plots for the plutons show variable enrichment in light rare earth elements (LREEs) with depletion of heavy rare earth elements (HREEs). The rocks of the Alemdar Pluton have homogeneous ISr values (0.70585 to 0.70673), low εNd(i) values (−3.9 to −1.6), pozitif εHf(i) values (+3.3 to +9.1), and Hf single-stage model ages (563–712 Ma), whereas those of Işıkdere have high ISr values (0.70666 to 0.70673), low εNd(i) values (−3.9 to −3.8), εHf(i) values (−3.1 to +1.1), and Hf single-stage model ages (791–969 Ma). These values imply that they have different sources. These results suggest that the magma of the Alemdar was formed by the partial melting of a relatively depleted lithospheric mantle wedge interaction of the OIB-like melts, while those of Işıkdere have derived from the partial melting of an enriched mantle metasomatized by slab-derived sediments and then modified by fractional crystallization and crustal contamination in crustal magma chambers. Considering the available data, we conclude that the studied plutons were related to extensional tectonic events during the Middle Jurassic in response to the roll-back of the Paleotethys oceanic slab in the final stage of oceanic closure. These rocks are an example of the formation of deep subduction products following Early Jurassic subduction.

Sipahi, Ferkan; Gücer, Mehmet Ali; Dokuz, Abdurrahman; Yi, Keewook; Kaygusuz, Abdullah; Akaryall, Enver; Saydam Eker, Çiǧdem; Doruk, Cüneyt

Magmatic activity in the Sakarya Zone, an important segment of the Alpine orogenic belt, continues intermittently from the middle Carboniferous to Miocene. In this study, we provide geochronological and geochemical data from the Daǧdibi Pluton in the eastern Sakarya Zone to present some inferences on the source region and petrogenesis of the middle Eocene magmatism. U-Pb zircon geochronology from two granodiorite samples gives middle Eocene ages of 44.75 ± 0.92 and 45.01 ± 0.59 Ma. The pluton is mainly composed of K-feldspar, plagioclase, quartz, Mg-hornblende/actinolite, Fe-Ti oxides and small amounts of biotite, and secondary chlorite and epidote. Parental magma of the intrusive rocks has a high-K calc-alkaline affinity with metaluminous character. The oxygen fugacity values vary between -18 and -17. The rocks show slightly radiogenic 87Sr/86Sr(i) (0.704845-0.705726) ratios and μNdi values between -0.96 and +0.52. Pb-Pb isotope ratios are typical for those of the lower continental crust. μHf(i) values of the zircons range from 0.14 to 10.26. The geochemical and isotopic features of the pluton point to a parental magma derived from a depleted mantle that was metasomatized by fluids during previous subduction events. The volumetric abundances of the rock types are decreased as the silica content increase, implying that the fractional crystallization is the most important process during the formation of the present compositional range of the pluton. Amphibole, plagioclase and Fe-Ti oxides are the fractionated phases while K-feldspar is largely accumulated. In the light of the data presented above, slab breakoff is regarded as the geodynamic process responsible for the formation of the Daǧdibi Pluton in the middle Eocene.

Dokuz, Abdurrahman; Gücer, Mehmet Ali; Karsli, Orhan; Yi, Keewook

New U-Pb zircon data from the Kurtoglu Massif (eastern Sakarya Zone, Turkey) reveal a Cadomian back-arc basin, evolving to closure of the Rheic Ocean. A host metapsammite from the lower tectonic slice of the massif yielded detrital zircon ages ranging from Neoarchean (2.73 Ga) to late Neoproterozoic with a major population between 610 and 553 Ma and a peak at 592 Ma, implying that the basement of the Sakarya Zone has a Cadomian affinity. This metasedimentary sequence was intruded by an orthogneiss in the early Ordovician (481 Ma) probably during the opening of the Rheic Ocean. Zircon grains from the overlying metasedimentary sample yielded concordant ages spreading throughout the Mesoproterozoic (1608-1032 Ma) and early Paleozoic (516-351 Ma), suggesting that they were deposited in the Rheic Ocean likely prior to its final closure. The muscovite metagranite, emplaced into the host rocks of the Kurtoglu Massif at 388 Ma, formed probably during northward closure of the Rheic Ocean. Zircon grains of a metasiltstone from the upper tectonic slice of the massif, represented mainly by phyllites, yielded similar age spectra to that of the host metapsammite of the lower tectonic slice. Such phyllites in the Pulur Massif similar to 35 km to the southeast display a close association with the ophiolitic complexes of the Rheic Ocean. This relationship together with the age spectra mentioned above suggests that sedimentation was on a place away from the Paleozoic igneous sources, in an analogous position to the southern passive margin of the Rheic Ocean.

Kaygusuz, Abdullah; Yücel, Cem; Arslan, Mehmet; Temizel, İrfan; Yi, Keewook; Jeong, Youn-Joong; Siebel, Wolfgang; Sipahi, Ferkan

The Eastern Pontides orogenic belt in NE Turkey hosts numerous I-type plutons of Eocene epoch. Here, we report new U–Pb SHRIMP zircon ages and in situ zircon Lu-Hf isotopes along with bulk-rock geochemical and Sr-Nd-Pb-O isotope data from the Kemerlikdağı, Aydıntepe and Pelitli plutons and mafic microgranular enclaves (MMEs) to constrain their parental melt source(s) and evolutionary processes. U-Pb SHRIMP zircon dating yielded crystallization ages between 45 and 44 Ma for the studied plutons and their MMEs. The plutons range from gabbro to granite and have I-type, medium to high-K calc-alkaline, and metaluminous to slightly peraluminous characteristics. On the primitive mantle-normalized multi-trace-element variations, the plutons and their MMEs are characterized by significant enrichment in LILE/HFSE. Chondrite-normalized REE patterns of the plutons and their MMEs are close to each other and show moderate enrichment with variable negative Eu anomalies. The studied plutons have fairly homogeneous isotope composition (87Sr/86Sr(i) = 0.70502 to 0.70560; εNd(i) = +0.9 to–1.4; δ18O = +5.0 to +8.7‰, εHf(i) =–2.2 to +13.5). The MMEs show medium to high-K calc-alkaline and metaluminous character. Although the isotope signatures of the MMEs (87Sr/86Sr(i) = 0.70508 to 0.70542; εNd(i) = +0.9 to −1.1; δ18O = +5.8 to +8.0, εHf(i) = +4.3 to +10.4) are very similar to those of the host rocks. Fractionation of plagioclase, amphibole, pyroxene and Fe-Ti oxides played an important role in the evolution of the plutons. The isotopic composition of the studied plutons and MMEs are similar to I-type plutons derived from mantle sources. The MMEs show incomplete magma mixing/mingling, representing small bodies of mafic parental magma. The parental magma(s) of the studied plutons were generated from the enriched lithospheric mantle and then modified by fractional crystallisation, and lesser assimilation and mixing/mingling in the crustal magma chambers.

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; 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.