Temizel, İrfan; Arslan, Mehmet; Abdioğlu Yazar, Emel; Aslan, Zafer; Kaygusuz, Abdullah; Baki Eraydın, Tuğba

Mafic intrusions of the Eastern Pontides Orogenic Belt (EPOB) in NE Turkey are found rarely in varying ages, sizes and compositions. Of these, tholeiitic gabbro stocks, namely Kovanlık gabbro, consist of pyroxene gabbro and minor hornblende gabbro with a mainly granular texture. The pyroxene gabbro contains plagioclase, clino- and ortho-pyroxene, olivine, and Fe[sbnd]Ti oxides, whereas the hornblende gabbro includes mainly plagioclase, hornblende, rarely biotite and Fe[sbnd]Ti oxides. The clinopyroxene compositions of the pyroxene gabbro exhibit arc-related tholeiitic magmas. The Kovanlık gabbroic rocks have high U/Yb zircon ratios, implying continental arc-related tectonic settings. Zircon U[sbnd]Pb dating on the Kovanlık gabbro yielded a concordia age of ~92 Ma (Turonian). Petrochemically, the gabbroic rocks have a tholeiitic character and show arc- and MORB-like signatures. The gabbroic rocks show positive Rb, Ba, K, Pb, Sr, Ti and negative Nb, Ce, Nd, Hf, Zr anomalies in N-MORB normalized diagrams. The chondrite-normalized REE plots of the rocks show flat to feeble right-dipping REE patterns (LaN/LuN = 0.94–2.56) with positive Eu anomalies (EuN/Eu* = 1.15–2.14; except for two pyroxene gabbro samples). The gabbroic rocks have 87Sr/86Sr(i) ratios (0.704533 to 0.706265), εNd(i) values (4.04–4.85), 206Pb/204Pb (18.56–18.68) and 207Pb/204Pb (15.61–15.64) ratios. Low Nb/La and Nb/Y ratios, as well as high Sr/Yb and Ba/Rb ratios, indicate that source regions were modified by slab-derived fluids and/or melts. The obtained zircon dating, isotopic and geochemical data, as well as regional geology, revealed that the tholeiitic mafic magma derived from a spinel-lherzolite mantle at a relatively shallow depth, and the Kovanlık gabbro in the EPOB represents one of the first mafic intrusions of Late Cretaceous bimodal arc magmatism related to the roll-back of the northwards subducting Tethys oceanic plate.

Temizel, Irfan; Arslan, Mehmet; Yücel, Cem; Abdioğlu Yazar, Emel; Kaygusuz, Abdullah; Aslan, Zafer

Eocene intermediate to felsic plutons of different sizes and compositions are widespread in the Eastern Pontides Orogenic Belt in northern Turkey. Of these, the Taşlık Tepe pluton in the Havza (Samsun) area is fine-to-medium-grained, with granular, porphyritic, and micrographic textures, and include mafic microgranular enclaves (MMEs). LA-ICP-MS U-Pb zircon dating yielded emplacement ages of 42.9 (± 1.4) and 40.5 (± 1.3) Ma for the host granodioritic pluton and the dioritic MMEs, respectively. Petrochemically, the host pluton has I-type, high-K calc-alkaline, and metaluminous-to-slightly peraluminous features (A/CNK = 0.95–1.06). The host pluton also shows geochemical features of adakite-like rocks with high SiO2 (67–68 wt%) and Al2O3 (15.5–16.0 wt%) content and Ba/La (17–23), Sr/Y (40.7–61.6), and LaN/YbN (14.4–23.7) ratios and low Y (8.2–9.9 ppm) and YbN (3.1–4.4) contents. Whole-rock major and trace element variations suggest that fractional crystallisation played a significant role in the pluton evolution. The N-MORB normalised trace element patterns of the pluton are similar to those of MMEs with enrichment in large-ion lithophile elements, Th and Ce, and negative Nb and Ti anomalies. Chondrite-normalised rare earth element plots show moderate-to-highly enriched concave patterns (LaN/LuN = 14.2–21.6) with insignificant negative Eu anomalies (EuN/Eu* = 0.86–1.14), all of which imply hornblende fractionation during magmatic evolution. The pluton samples have 87Sr/86Sr ratios of 0.704767 to 0.704927, 143Nd/144Nd ratios of 0.512767–0.512774, εNd values of (+2.52)–(+2.65), and δ18O values of 7.9–9.7‰. The isotopic compositions of the host pluton and MMEs are similar to I-type granitoids derived from mantle sources. The MMEs show incomplete magma mixing/mingling, representing small bodies of mafic parental magma. Combined with regional studies, these new data suggest that the parental magma of the studied adakite-like pluton was generated from the lithospheric mantle and then modified by fractional crystallisation and assimilation in a post-collisional setting.

Yücel, Cem; Arslan, Mehmet; Temizel, İrfan; Abdioğlu Yazar, Emel; Ruffet, Gilles

The Eocene and Miocene volcanic rocks between the cities of Trabzon and Giresun in the Eastern Pontides (NE Turkey) erupted as mildly and moderately alkaline magmas ranging from silica-saturated to silica-undersaturated types. 40Ar-39Ar dating and petrochemical data reveal that the studied volcanic rocks are discriminated in two: Lutetian (Middle Eocene) mildly alkaline, (basaltic rocks: 45.31 ± 0.18 to 43.86 ± 0.19 Ma; trachytic rocks: 44.87 ± 0.22 to 41.32 ± 0.12 Ma), and Messinian (Late Miocene) moderately alkaline volcanic rocks (tephrytic rocks: 6.05 ± 0.06 and 5.65 ± 0.06 Ma). The trace and the rare earth element systematic, characterised by moderate light earth element (LREE)/heavy rare earth element (HREE) ratios in the Eocene basaltic and trachytic rocks, high LREE/HREE ratios in the Miocene tephrytic rocks, and different degrees of depletion in Nb, Ta, Ti coupled with high Th/Yb ratios, show that the parental magmas of the volcanic rocks were derived from mantle sources previously enriched by slab-derived fluids and subducted sediments. The Sr, Nd and Pb isotopic composition of the Eocene and Miocene volcanic rocks support the presence of subduction-modified subcontinental lithospheric mantle. During the magma ascent in the crust, parental magmas of both the Eocene and Miocene volcanic rocks were mostly affected by fractional crystallisation rather than assimilation coupled with fractional crystallisation and mixing. The silica-undersaturated character of the Miocene tephrytic rocks could be attributed to assimilation of carbonate rocks within shallow-level magma chambers. The parental magmas of the Eocene volcanic rocks resulted from a relatively high melting degree of a net veined mantle and surrounding peridotites in the spinel stability field due to an increase in temperature, resulting from asthenospheric upwelling related to the extension of lithosphere subsequent to delamination. The parental magmas for the Miocene volcanic rocks resulted from a relatively low melting degree of a net veined mantle domain previously modified by metasomatic melts derived from a garnet peridotite source after decompression due to extensional tectonics, combined with strike-slip movement at a regional scale related to ongoing delamination.