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- MakaleYalnız künye2017Late Jurassic Magmatism and Stratigraphy in the Eastern Sakarya Zone, Turkey: Evidence for the Slab Breakoff of Paleotethyan Oceanic Lithosphere
Dokuz, Abdurrahman; Aydinçakir, Emre; Kandemir, Raif; Karsli, Orhan; Siebel, Wolfgang; Derman, Ahmet Sami; Turan, Mehmet
Middle-Late Jurassic Cimmerian events in Turkey have been actively discussed in the past three decades, but proposed tectonic models associated with magmatism, metamorphism, and stratigraphic features remain controversial. To address this issue, Upper Jurassic mafic lavas are investigated at three locations (Alucra, Gumushane, and Olur) in the eastern Sakarya Zone, northeastern Turkey. These lavas are submarine and form planar flows parallel with the bedding plane in the Upper Jurassic carbonate sequence near the base or just below in the clastic sedimentary rocks. The basaltic lavas show calc-alkaline features and possess Nb-Ta values and Nb/U, Nb/La, and Ce/Pb ratios that are greater than those of island arc basalts. Multielement patterns are almost hump shaped, similar to ocean island basalts, which experience Pb depletion and weak negative Nb-Ta, Zr-Hf, and Ti anomalies. The low initial (Sr-87/Sr-86) ratios (0.70372-0.70554), positive initial epsilon(Nd) values (+2.7 to +4.4), and initial Pb isotope ratios that plot between mid-ocean-ridge and ocean island basalts are consistent with a melt derived from subcontinental lithospheric mantle, metasomatized by earlier fluids from subducted sediments and plume materials from the asthenosphere. Moderate Dy/Yb ratios with an average value of 1.8 imply partial melting in the spinel-garnet transition zone at depths of similar to 70-100 km. Slab breakoff is suggested as a geodynamic mechanism that accounts for these geochemical signatures. This inference is also favored by stratigraphic and sedimentologic evidence from the Upper Jurassic-Lower Cretaceous sedimentary rocks, which is consistent with short-lived vertical (epirogenic) movements in the region. Lower-Middle Jurassic sequences are transgressive, suggesting that subduction-related extension opened a backarc basin (Neotethys) in the south of the Sakarya Zone. Upper Jurassic-Lower Cretaceous carbonates point tectonically to tranquility during carbonate deposition in the Neotethys Ocean, which seems to have been achieved by complete closure of the Paleotethys in the north. About 15-20 m.yr. later (Kimmeridgian), after first carbonate deposition, intraplate-typemafic lavas ascended up to the shelf surface of the Neotethys. This was followed by formation of disconformity surfaces and then accumulation of coarse clastic sediments. All this points to a short-lived epirogenic movement that we ascribe to the breakoff of the southward-subducting Paleotethyan oceanic lithosphere in the Late Jurassic.
- MakaleYalnız künye2017Cordierite-bearing strongly peraluminous Cebre Rhyolite from the eastern Sakarya Zone, NE Turkey: Constraints on the Variscan Orogeny
Dokuz, Abdurrahman; Külekçi, Elif; Aydinçakir, Emre; Kandemir, Raif; Alçiçek, M. Cihat; Pecha, Mark E.; Sünnetçi, Kadir
The Cebre Rhyolite with an outcropping area up to 12 km(2) is one of the rare extrusions in the Variscan basement of the Sakarya Zone. The unit consists of high-K calc alkaline rhyolites (SiO2 = 74-82 wt.%). Abundant phenocrysts of quartz and K-feldspar are accompanied by subordinate cordierite phenocrysts, rare muscovite microphenocrysts and biotite microcrysts set in a fine-grained groundmass. Three types of rock fragments (xenoliths) have been recognized; (i) porphyritic, (ii) equigranular hypabyssal and (iii) hypocrystalline fragments. Zircon U-Pb dating indicates that the Cebre Rhyolite was extruded at 332.8 +/- 4.38 Ma, which post-dates the Variscan low temperature metamorphism and pre-dates the emplacement of I-type granitic intrusions (325-303 Ma).The samples are strongly peraluminous with A/CNK values ranging from 1.48 to 2.95 and A/NK from 1.49 to 2.99. They have very high K2O (3.72-7.42 wt.%) and Al2O3 (10.77-14.11 wt.%) contents, but very low CaO (0.02-0.21 wt.%), Na2O (0.05-0.78 wt.%) and MgO (0.3-0.21 wt.%) contents. The samples show geochemical affinity with the upper continental crust, e.g., enrichment of large ion lithophile elements (LILEs; K, Rb, U, Th, Pb), depletion of high field strength elements (HFSEs; Nb, Ta, Ti), Sr, P and Eu, but epsilon Nd-(t) values (- 3.06 to - 8.75) and isotope ratios of Sr-(t)((87) Sr/Sr-86 = 0.70499-0.70915) and Pb-(t) (Pb-206/Pb-204 = 16.41-17.570, Pb-207/Pb-204 = 15.54-15.59, Pb-208/Pb-204 = 36.20-37.22) are similar to those of the lower crust. Geochemical and isotope data indicate that the Cebre Rhyolite was generated by melting of metapelitic rocks with some addition of intermediate metaigneous derived magma. As a geodynamic model, we propose that the Variscan Orogeny in Turkey was occurred by collision of Gondwana with an arc/terrane separated from the southern margin of Laurussia. This collision was followed shortly after by splitting of oceanic lithosphere into two pieces and sinking down into asthenosphere. Rapid upwelling of asthenosphere to space emptied by previous oceanic lithosphere provided heat for high temperature metamorphism and anatexis of metasedimentary rocks in the crust. (C) 2017 Elsevier B.V. All rights reserved.
- MakaleYalnız künye2017Zircon Lu-Hf isotope systematics and U–Pb geochronology, whole-rock Sr-Nd isotopes and geochemistry of the early Jurassic Gokcedere pluton, Sakarya Zone-NE Turkey: a magmatic response to roll-back of the Paleo-Tethyan oceanic lithosphere
Karsli, Orhan; Dokuz, Abdurrahman; Kandemir, Raif
The early Mesozoic was a critical era for the geodynamic evolution of the Sakarya Zone as transition from accretion to collision events in the region. However, its complex evolutionary history is still debated. To address this issue, we present new in situ zircon U–Pb ages and Lu-Hf isotope data, whole-rock Sr-Nd isotopes, and mineral chemistry and geochemistry data of plutonic rocks to better understand the magmatic processes. The Gokcedere pluton is mainly composed of gabbro and gabbroic diorite. LA-ICP-MS zircon U–Pb dating reveals that the pluton was emplaced in the early Jurassic (177 Ma). These gabbros and gabbroic diorites are characterized by relatively low SiO2 content of 47.09 to 57.15 wt% and high Mg# values varying from 46 to 75. The samples belong to the calc-alkaline series and exhibit a metaluminous I-type character. Moreover, they are slightly enriched in large ion lithophile elements (Rb, Ba, Th and K) and light rare earth elements and depleted in high field strength elements (Nb and Ti). Gabbroic rocks of the pluton have a depleted Sr-Nd isotopic composition, including low initial 87Sr/86Sr ranging from 0.705124 to 0.705599, relatively high εNd (t) values varying from 0.1 to 3.5 and single-stage Nd model ages (TDM1 = 0.65–0.95 Ga). In situ zircon analyses show that the rocks have variable and positive εHf (t) values (4.6 to 13.5) and single-stage Hf model ages (TDM1 = 0.30 to 0.65 Ga). Both the geochemical signature and Sr-Nd-Hf isotopic composition of the gabbroic rocks reveal that the magma of the studied rocks was formed by the partial melting of a depleted mantle wedge metasomatized by slab-derived fluids. The influence of slab fluids is mirrored by their trace-element characteristics. Trace-element modeling suggests that the primary magma was generated by a low and variable degree of partial melting (~5–15%) of a depleted and young lithospheric mantle wedge consisting of phlogopite- and spinel-bearing lherzolite. Heat to melt the mantle material was supplied by the ascendance of a hot asthenosphere triggered by the roll-back of the Paleo-Tethyan oceanic lithosphere. The rising melts were accompanied by fractional crystallization and encountered no or minor crustal contamination en route to the surface. Taking into account these geochemical data and integrating them with regional geological evidence, we propose a slab roll-back model; this model suggests that the Gokcedere gabbroic pluton originated in a back-arc extensional environment associated with the southward subduction of the Paleo-Tethyan oceanic lithosphere during the early Jurassic period. Such an extensional event led to the opening of the northern branch of the Neotethys as a back-arc basin. Consequently, we conclude that the gabbroic pluton was related to intensive extensional tectonic events, which peaked during the early Jurassic in response to the roll-back of Paleo-Tethyan oceanic slab in the final stage of oceanic closure.
- MakaleYalnız künye2017Petrogenesis of ultramafic rocks from the eastern Orhaneli ophiolite, NW Turkey: Hints on the initiation and evolution of melt-peridotite interaction processes within a heterogeneously depleted mantle section
Uysal, Ibrahim; Dokuz, Abdurrahman; Kapsiotis, Argyrios; Saka, Samet; Karsli, Orhan; Kaliwoda, Melanie; Mueller, Dirk
The eastern Orhaneli ophiolite in NW Anatolia (Turkey) consists of voluminous dunite and minor harzburgite intruded by clinopyroxenite veins. Harzburgite contains spinel of low Cr# [100 x Cr/(Cr + Al) = 40-45] and diopside of low Al2O3 and TiO2 contents, whereas dunite contains spinel of higher Cr# (62-82) and diopside (blebs) (even more) depleted in Al2O3 and TiO2 (than harzburgite). The concentrations of Heavy Rare Earth Elements (HREE) in harzburgite are consistent with derivation of this type of peridotite from 19% dry melting of a fertile mantle protolith at a MOR regime. Dunites have lower concentrations of HREE than harzburgite implying that dunites were generated by higher degrees (> 30%) of (cumulative) melting of the same protolith. Furthermore, the characteristic U-shapes of the chondrite-normalized REE-patterns of dunites indicate the involvement of hydrous melt-peridotite interaction processes in their genesis. Nevertheless, a set of mosaic-in texture dunite samples contain olivine that has lower Fo# [100 x Mg/(Mg + Fe2+)] and NiO contents than olivine in harzburgite. These dunites are probably of cumulate origin as it is also indicated by their enrichment in Pt and Pd (<= 17.92 ppb). Micro-textural and Re-Os isotopic data support that clinopyroxenite intrusions do not have a pure magmatic origin and their formation was partly controlled by metasomatic processes. Overall data indicate that the eastern Orhaneli ultramafic rocks have a complex petrological history including various stages of partial melting, metasomatism and magmatism in an evolving from MOR to SSZ geotectonic setting.
- Proje2016Demirözü (Bayburt) Permo-Karboniferi ReyikOkyanusu Kalıntısı mı? Sakarya Zonu Erken-OrtaPaleozoyik Jeodinamik Evrimi İçin Yeni Bulgular
Alçiçek, Mehmet Cihat; Dokuz, Abdurrahman; Aydınçakır, Emre; Kandemir, Raif
Sakarya Zonu?nun doğusunda (Doğu Pontidler) yüzeyleyen Erken Karbonifer yaşlı Cebre Riyoliti Variskan temele ait az sayıdaki yüzey kayaçlarından biridir. Zirkon U-Pb yaşlandırması Cebre Riyoliti?nin 341.8±0.60 my önce Variskan temele yerleştiğini öngörmektedir. S-tipi granitlere benzer şekilde, Al-silikat fazlarından kordiyerit ve muskovit içerir ve güçlü peralümin bir jeokimyasal karaktere sahiptir. Birime ait kayaçların Sr-Nd-Pb izotopik oranları, alt kabuk ile uyumludur. Jeokimyasal ve izotopik veriler Cebre Riyolitinin çarpışma sırasında kilce zengin bir sedimanter kökenden itibaren kısmi ergime yolu ile oluştuğuna işaret etmektedir. Önerilen jeodinamik modelde, Reyik Okyanusu?nun kapanmasını takiben Lavrazya?dan kopan kıtasal yay ile Gondwana?nın Erken Karbonifer?de çarpışması öngörülmektedir. Çarpışma, kısa bir süre sonra, okyanusal litosfer delaminasyonu tarafından izlenmiştir. Okyanusal litosfer tarafından boşaltılan kısmın yükselen astenosfer tarafından hızlı bir şekilde doldurulması metamorfizma ve kısmi ergime için gerekli ısıyı sağlamıştır. Geç Paleozoyik yaşlı sedimanter istif, Variskan sonrası jeolojik olayların kaydedildiği çok az sayıdaki otokton birimlerden biridir. İstif, Variskan temele ait Erken Karbonifer yaşlı düşük dereceli metamorfitleri ve Cebre Riyoliti?ni uyumsuz olarak örter. Geç Paleozoyik istifi tabanda 610 m kalınlığında kuvars arenit ve kireçtaşlarının sekansiyel ardışmından oluşan Çatalçeşme Formasyonu ile başlar. Üzerine, çakıltaşları ile başlayan ve üste doğru sırasıyla arkozik arentlere ve şeyllere geçiş yapan 880 m kalınlığındaki Hardişi Formasyonu uyumlu olarak gelir. U-Pb kırıntı zirkon yaşları her iki formasyon için ~300 ? 2870 my arasında değişmekle birlikte, ana yaş popülasyonları 300 ? 350 my arasındadır. Bununla birlikte Çatalçeşme kumtaşlarına ait 7 adet zirkon Permiyen (298 ? 258 My) yaşı üretmiştir. Çatalçeşme kumtaşları pasif kenar ortamlarına ait kayaçlara benzer jeokimyasal özelliklere sahiptir. Buna karşılık Hardişi kumtaşları yitimle ilişkili ortamların jeokimyasal özelliklerini taşır. Jeokimyasal ve izotopik veriler, Variskan deformasyonlarını takiben Gondwana?nın kuzey kenarı boyunca açılan kısa ömürlü bir kenar havzada çökeldiklerine işaret etmektedir. Mermer, kireçtaşı ve kumtaşı blokları yanı sıra MORB ve IAT karakterli bazik bloklar da içeren Çamdere Ofiyolitik Karışığı?nın, kenar havzanın kapanması sırasında kıtasal kabuk üzerine yerleştiği öngörülmektedir.
- MakaleYalnız künye2016Subduction-related Late Carboniferous to Early Permian Magmatism in the Eastern Pontides, the Camlik and Casurluk plutons: Insights from geochemistry, whole-rock Sr–Nd and in situ zircon Lu–Hf isotopes, and U–Pb geochronology
Karsli, Orhan; Dokuz, Abdurrahman; Kandemir, Raif
Late Carboniferous to early Permian granitoid rocks represent a volumetrically minor component of the Eastern Pontide lithosphere, but they preserve useful information about the region's tectonomagmatic history. The Casurluk and Camlik plutons primarily consist of gabbro, gabbroic diorite, diorite, monzogabbro, monzodiorite and monzonite, which intrude early to middle Carboniferous granitic basement rocks in the region. In this study, we use in situ zircon U–Pb ages and Lu–Hf isotopic values, whole-rock Sr–Nd isotopic values, and mineral chemistry and geochemistry of these plutons to determine petrogenesis and crustal evolution; we also discuss geodynamic implications. LA–ICP-MS zircon U–Pb dating of magmatic zircons from the rocks suggests that the plutons were emplaced during the late Carboniferous to early Permian (302 Ma). The metaluminous and I-type intrusive rocks belong to the high-K calc-alkaline series. In addition, they are relatively enriched in light rare earth elements (LREEs) and large-ion lithophile elements (LILEs); they are depleted in heavy rare earth elements (HREEs) and high field strength elements (HFSEs), such as Nb and Ti. All of the samples have homogeneous initial ISr values (0.70675 to 0.70792) and low εNd (t) values (− 5.1 to − 3.3). Zircons from the rocks of both plutons have uniform negative to slightly positive εHf (t) values (− 3.5 to 1.4) and old Hf two-stage model ages (1323 to 1548 Ma), implying that they have the same source, as well as suggesting the involvement of old enriched lithospheric mantle materials during their magma genesis. These results, combined with the εHf (t) values and two-stage model ages, demonstrate that the primary magmas were derived from partial melting of old lithospheric mantle material metasomatized by subduction-related fluids. Considering other regional geological data from the Sakarya Zone where these plutons formed, we conclude that late Carboniferous to early Permian magmatism in the area involved the partial melting of a phlogopite- and spinel-bearing lherzolite under an extensional regime in a subduction-related setting. Such an extension event would have caused hot asthenospheric upwelling and was likely triggered by back-arc rifting during subduction of the Paleo-Tethyan oceanic slab. Thus, the magmas are likely subduction-related products, implying that southward subduction of Paleo-Tethys oceanic lithosphere in the region began during the late Carboniferous to early Permian.
- MakaleYalnız künye2015Geochemistry, Re-Os isotopes and highly siderophile element abundances in the Eastern Pontide peridotites (NE Turkey): Multiple episodes of melt extraction-depletion, melt-rock interaction and fertilization of the Rheic Ocean mantle
Dokuz, Abdurrahman; Uysal, Ibrahim; Dilek, Yildirim; Karsli, Orhan; Meisel, Thomas; Kandemir, Raif
We report on the structure, geochemistry, Re-Os isotopes and relative abundances of highly siderophile elements (HSEs) of the Paleozoic peridotite-basalt occurrences in the Eastern Pontide belt of northeastern Turkey. These peridotites and the associated basaltic rocks are the remnants of the Rheic oceanic lithosphere, incorporated into the Eurasian continental margin during the Variscan (Hercynian) orogeny. The peridotites display a complex record of multiple magmatic, metasomatic and metamorphic events in different tectonic settings during the evolution of the Rheic upper mantle. The Beyçam harzburgite (BH) contains low Al2O3 (0.51-1.88 anhydrous wt.%) and high MgO (41.35-42.34 wt.%) contents, and its bulk-rock trace element compositions are less than 10% of the primitive upper mantle (PUM) values. The platinum, Pd and Re contents of the Beyçam harzburgite are highly depleted, whereas its Os, Ir, and Ru contents are slightly enriched relative to the PUM values. Its Pd and Re contents that are higher than those of the fertile Pulur lherzolite (PL) to the south and the absence of an isochronous relationship between its 187Os/188Os and 187Re/188Os show that the trace element distribution and the isotope ratios of the Beyçam harzburgite were significantly modified after the first melt-extraction episode. The first melt extraction occurred beneath the Rheic mid-ocean ridge spreading center, whereas the second melt extraction occurred in a mantle wedge above a Rheic subduction zone. The primary magmatic phases of the Pulur lherzolite show the geochemical characteristics typical for fertile lherzolite, formed in the early stages of oceanic lithosphere generation subsequent to the continental break up. The Pulur lherzolite also contains a secondary magmatic phase in the form of networks of clinopyroxene veins and channels, which are interpreted as an evidence for solid state melt-rock reactions between the lherzolite and a percolating basaltic melt above a subduction zone. This clinopyroxene addition resulted in the formation of variable concentrations of Al2O3 (2.47-4.33wt.%) and MgO (29.76-40.10wt.%) in the lherzolite. The rhenium, Pd and Pt concentrations of the Pulur lherzolite are depleted relative to the PUM values, whereas the Os, Ir and Ru concentrations are in the range of the PUM values as commonly observed in peridotites with a melt depletion history. The high suprachondritic 187Os/188Os is, however, inconsistent with a simple melt depletion history, and can be explained by the addition of radiogenic Os-bearing sulfide phases into the lherzolite as a result of melt-rock reactions. Basaltic rocks with an island arc tholeiitic composition from the Beyçam area represent the partial melting product of the moderately depleted Beyçam harzburgite and the basaltic parental melt from which the clinopyroxene precipitated. The covariation between the 187Re/188Os and 187Os/188Os of these basaltic rocks defines an isochron age of 377±8Ma (late Devonian). The combined structural, geochemical and isotope data indicate a prolonged history of multiple episodes of melt extraction-depletion, and melt-rock interaction and fertilization of the mantle lithosphere of the Rheic Ocean.
- MakaleYalnız künye2014Geochemical fingerprints of Late Triassic calc-alkaline lamprophyres from the Eastern Pontides, NE Turkey: A key to understanding lamprophyre formation in a subduction-related environment
Karsli, Orhan; Dokuz, Abdurrahman; Kaliwoda, Melanie; Uysal, Ibrahim; Aydin, Faruk; Kandemir, Raif; Fehr, Karl-Thomas
The Eastern Pontides in NE Turkey is one of the major orogenic belts in Anatolia. In this paper, we report our new 40Ar/39Ar dating, mineral chemistry, major and trace elements and Sr-Nd-Pb isotopic analyses of the lamprophyre intrusions in this region. The lamprophyres are widely scattered and intrude Late Carboniferous granitoid rocks. The lamprophyres exhibit fine-grained textures and are mineralogically uniform. Hornblende 40Ar/39Ar dating yielded a plateau age of 216.01±10.64Ma. Based on their geochemistry, mineral compositions and paragenesis, the lamprophyres are classified as calc-alkaline lamprophyres in general and spessartites in particular, which are rich in large ion lithophile elements (e.g., Rb, Ba, K) but depleted in Nb and Ti. Our samples exhibit moderate fractionation in LREE patterns approximately 100 times that of chondrite but HREE abundances less than 10 times that of chondrite. These calc-alkaline lamprophyres display a range of ISr (216Ma) values from 0.70619 to 0.71291 and εNd (216Ma) values from -1.4 to 4.1, with TDM=1.11 to 2.20Ga. Their Pb isotopic ratios indicate an enriched mantle source. The enrichment process is related to metasomatism of a subcontinental lithospheric mantle source, which is caused by a large quantity of H2O-rich fluids, rather than sediments released from oceanic crust at depth during the closure of the Paleotethys Ocean in Triassic times. All of the geochemical data and the trace element modeling suggest that the primary magma of the calc-alkaline to high-K calc-alkaline spessartites was generated at depth by a low degree of partial melting (~1-10%) of a previously enriched lithospheric mantle wedge consisting of phlogopite-bearing spinel peridotite. The ascendance of a hot asthenosphere triggered by extensional events caused partial melting of mantle material. The rising melts were accompanied by fractional crystallization and crustal contamination en route to the surface. All of the geochemical features combined with regional data suggest that the Eastern Pontides calc-alkaline lamprophyres originated in an extensional environment along an active continental margin throughout the Late Triassic. Such an extensional event, causing upwelling of hot asthenosphere, led to the opening of the northern branch of the Neotethys as a back-arc basin farther south of the Eastern Pontides.
- MakaleYalnız künye2013Post-collisional adakitic volcanism in the eastern part of the Sakarya Zone, Turkey: Evidence for slab and crustal melting
Dokuz, Abdurrahman; Uysal, Ibrahim; Siebel, Wolfgang; Turan, Mehmet; Duncan, Robert; Akçay, Miǧraç
New geochemical and isotopic data for post-collisional Early Eocene and Late Miocene adakitic rocks from the eastern part of the Sakarya Zone, Turkey, indicate that slab and lower crustal melting, respectively, played key roles in the petrogenesis of these rocks. The Early Eocene Yoncali{dotless}k dacite (54.4 Ma) exhibits high Sr/Y and La/Yb ratios, low Y and HREE concentrations, moderate Mg# (44-65), and relatively high εNd and low ISr values, similar to adakites formed by slab melting associated with subduction. Geochemical composition of the Yoncali{dotless}k dacite cannot be explained by simple crystal fractionation and/or crustal contamination of andesitic parent magma, but is consistent with the participation of different proportions of melts derived from subducted basalt and sediments. Sr/Y correlates horizontally with Rb/Y, and Pb/Nd correlates vertically with Nd isotopic composition, indicating that Sr and Pb budgets are strongly controlled by melt addition from the subducting slab, whereas positive correlations between Th/Nd and Pb/Nd, and Rb/Y and Nb/Y point to some contribution of sediment melt. In addition to low concentrations of heavy rare earth elements (~2-3 times chondrite), a systematic decrease in their concentrations and Nb/Ta ratios with increasing SiO2 contents suggests that slab partial melting occurred in the garnet stability field and that these elements were mobilized by fluid flux. These geochemical and isotopic signatures are best explained by slab breakoff and fusion shortly after the initiation of collision. Although the Late Micone Tavdaǧi{dotless} rhyolite (8.75 Ma) has some geochemical features identical to adakites, such as high Sr/Y and La/Yb ratios, low Y and HREE concentrations, other requirements, such as sodic andesite and/or dacite with relatively high MgO and Mg# (>50), relatively high Ni and Cr, low K2O/Na2O (<0.4), high Sr (>400 ppm), for slab-derived adakites are not provided. It is sodic in composition and shows no traces of fractionation from dacitic parent magma. Low Nd and high Sr isotope ratios suggest derivation by partial fusion of calc-alkaline, juvenile crust with high Sr/Y and La/Yb ratios.
- MakaleYalnız künye2012A-type granitoids from the Eastern Pontides, NE Turkey: Records for generation of hybrid A-type rocks in a subduction-related environment
Karsli, Orhan; Caran, Şemsettin; Dokuz, Abdurrahman; Çoban, Hakan; Chen, Bin; Kandemir, Raif
The Eastern Pontides is characterized by the many of intrusive bodies formed throughout the late Mesozoic-early Cenozoic. Most of these are I-type granitoids, but here, we present for the first time an A-type pluton from the region to assess source characteristics and geodynamic implications. The A-type Pirnalli pluton has a SHRIMP zircon U-Pb age of 81.2±1Ma. It is composed of granite, syenite and quartz monzonite, and its enclaves are monzonitic in composition with elevated Ga/Al ratios and low Mg# (<42). The pluton is dominantly metaluminous (A/CNK=0.88 to 1.00) and belong to shoshonitic and ultra-potassic series. The samples are highly enriched in LREE and show significant negative Eu (Eu/Eu *=0.33 to 0.92), Ba, Nb, Sr and Ti anomalies in the spidergrams. The host rocks and their enclaves posses quite similar Sr-Nd isotopic compositions (I Sr=0.70693 to 0.70736, ε Nd (81Ma)=-2.6 to -2.0, T DM=0.94 to 1.12Ga), pointing to a lower crustal parental magma with a minor contribution of lithospheric mantle. We hypothesize that upwelling of asthenosphere triggered melting of chemically enriched upper mantle and basic magma formed. Melting of lower crust was provided by the underplating of this basic magma. Mixing between lower crust- and mantle-derived melts at depths of lower crust appears to be most reasonable petrogenetic process responsible for generation of the pluton. Sr-Nd isotope modeling suggests mixing of 82-90% of the lower crustal-derived melt with ~10-18% of the mantle-derived melt. Then the hybrid melt ascended to shallower crustal level and underwent a limited fractionation process to generate a variety of rock types. Our data also suggest that the A-type Pirnalli pluton likely formed at an extensional environment of active continental margin throughout the late Cretaceous. Ongoing extension then led to opening of Black Sea as a back-arc basin further north of the Eastern Pontides.
- Proje2011Kalk-alkalen lamprofirlerin petrojenezleri, KD Türkiye: Doğu pontidlerin senozoyik jeodinamik evrimi için yeni bulgular
Uysal, İbrahim; Dokuz, Abdurrahman; Karsli, Orhan; Aydın, Faruk; Kandemir, Raif
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- MakaleYalnız künye2011A slab detachment and delamination model for the generation of Carboniferous high-potassium I-type magmatism in the Eastern Pontides, NE Turkey: The Kose composite pluton
Dokuz, Abdurrahman
The Hercynian Kose composite pluton (KCP) is located in the Eastern Pontides, Turkey, and consists of two units of high-K calc-alkaline, primarily peraluminous granites: (i) the internal body, and (ii) the external body. The internal body, which was emplaced at 322-318 Ma ((40)Ar/(39)Ar ages on biotite and hornblende, respectively), displays a wide compositional range (49-71 wt.% SiO(2)) and contains several lithologies: hybrid equigranular rocks, microgranular magmatic enclaves, mafic dikes, porphyry dikes and mylonites. The external body, which was emplaced at 306.7 Ma ((40)Ar/(39)Ar age on K-feldspar), consists exclusively of monzogranite (> 71 wt.% SiO(2)). Field relationships, mineralogy, major- and trace element geochemistry, and initial Sr-Nd isotope values (I(sr)= 0.70821 to 0.71002, e(Nd)(t) = -6.6 to -8.0) show that the internal body was differentiated and evolved by crystal fractionation and magma mixing processes. The end-members of the mixing process were a mafic rock and a felsic rock. Mafic magma was derived from a relatively deep-seated (25-30 km) crustal storage reservoir, not directly from the mantle, and underwent significant differentiation by fractional crystallization and crustal contamination before mixing. In addition, these magma storages probably supplied the additional heat necessary to initiate crustal melting. Some of the additional heat may have also been released by the radiogenic decay of heat producing elements. Eventually, the existing felsic magma from the melting of K-bearing meta-greywackes was raised to its emplacement level at a depth of similar to 10-16 km. After partial crystallization, it was sporadically intruded by modified mafic magma from the deeper crustal reservoir to generate hybrid rocks. The hybrid rocks were then elevated to a shallower depth by normal faults during the collapse of the orogen and erosion. Mylonites that were later overprinted by pseudotachylites are typically constrained to the internal body and are regarded as markers of this event. The external body is characterized by a significantly less radiogenic and limited range of Sr-Nd isotope values (I(sr)= 0.70639 to 0. 70792, e(Nd)(t) = -4.4 to -6.5) than those of the internal body and a lack of rocks documenting the open system differentiation processes. Fractional crystallization is the exclusive process responsible for the elemental range within the body. The rocks also contain less biotite relative to those of the internal body. All these involve less K-bearing mid-crustal rocks (orthogneisses) in their source, which was probably located at depths near the lower crust. The absence of purely lower crustal-derived melts can be explained by the removal of this type of material during the formation of the parental melt. This melt later ascended to its emplacement level at a depth of around similar to 5-10 km and cut the hybrid rocks of the internal body and regional metamorphic rocks that had been raised previously due to ongoing erosion. The melt that injected into the cracks of the internal body crystallized into porphyries because there was not enough time for the entire crystallization of magma. The data presented here indicate that late Early Carboniferous and Late Carboniferous magmatism occurred in a collisional setting. Slab detachment and subsequent delamination seem to be the most plausible mechanisms for the generation of the Hercynian high-K calc-alkaline magmatism in the Eastern Pontides, Turkey. (C) 2010 International Associatio for Gondwana Research. Published by Elsevier B.V. All rights reserved.
- MakaleAçık Erişim2011Early abyssal- and late SSZ-type vestiges of the Rheic oceanic mantle in the Variscan basement of the Sakarya Zone, NE Turkey: Implications for the sense of subduction and opening of the Paleotethys
Dokuz, Abdurrahman; Uysal, Ibrahim; Kaliwoda, Melanie; Karsli, Orhan; Ottley, Chris J.; Kandemir, Raif
The boundary between the Gondwana and Laurussia, or a terrane separated from it, is traced by a suture recording the closure of the Rheic ocean in the Pulur, Beyçam and Kurtoǧlu areas of the eastern Sakarya Zone of Turkey. Pulur lherzolite has a lower Cr# [100Cr/(Cr+Al)=5-37] of the spinel and slightly enriched flat medium-heavy rare earth element (M-HREE) whole-rock pattern typical for those of the fertile abyssal-type lherzolite. By contrast, the Beyçam harzburgite displays the mineralogical and geochemical features of a moderately depleted residue at a supra subduction zone (SSZ). Some of spinels from the Beyçam peridotites show TiO2 contents (0.15-0.48wt.%) identical to a later interaction with a melt of island-arc tholeiite (IAT) composition. Meta-basalt from the Beyçam area is the representative of such a melt and displays the whole-rock M-HREE pattern of a complementary product of the Beyçam harzburgite. Data presented in this study support the primarily northward directed closure of the Rheic ocean, but suggest that the Variscan basement in the eastern part of the Sakarya Zone consists of mantle peridotites formed in a south-vergent SSZ. This dual polarity of subduction in the Rheic ocean, at least in the second half of its closure, is also favored by the geological structure and lithological diversity of the Variscan rocks. These are: (1) spatial distribution of the peridotites, e.g., SSZ-type harzburgites to the north and abyssal-type lherzolites to the south, (2) lack of high-pressure metamorphic rocks, (3) decrease in the degree of Variscan metamorphism toward the north, and (4) emplacement of Variscan syn-collisional magmatism into the low grade metamorphic rocks to the north. Paleotethys was the other ocean of the region created in the Late Paleozoic. Northward subduction of the Rheic ocean from Early Devonian to Early Carboniferous appears to be the most plausible mechanism responsible for the opening of Paleotethys. In this scenario the Paleotethys is envisaged as a back-arc basin opened to the north in the wake of separated terranes from Laurussia. Onset of trench-Rheic mid-ocean ridge collision in the Early Devonian along the northern margin of Laurussia has been proposed to account for the initiation of SSZ-type subduction to the south. The short duration of this subduction appears insufficient to open a fully developed back-arc basin to the south because the elimination of Rheic mid-ocean ridge to the north accelerated the oceanic closure.
- MakaleYalnız künye2011Adakite-like granitoid porphyries in the Eastern Pontides, NE Turkey: Potential parental melts and geodynamic implications
Karsli, Orhan; Ketenci, Murat; Uysal, Ibrahim; Dokuz, Abdurrahman; Aydin, Faruk; Chen, Bin; Kandemir, Raif; Wijbrans, Jan
The tectonic setting of the Eastern Pontides during the late Mesozoic to early Cenozoic remains a subject of debate. Petrogenesis of adakite-like granitoid porphyries plays a critical role in determining the nature of the lower continental crust and mantle dynamics during orogenic processes in the region. Here we describe, for the first time, the late Paleocene to early Eocene adakite-like granitoid porphyries from the northern part of the Eastern Pontides although their counterparts in the southern part have recently been found. The adakitic porphyries, which emplaced into the subduction-related Turonian-Santonian volcanics from northern part of the region, consist of I-type calc-alkaline quartz monzonite-tonalite (SiO2 = 62.89-65.07 wt.%) and high-k calc-alkaline granodiorite-granite associations (SiO2 = 69.06-70.43 wt.%). The former displays peraluminous to metaluminous signatures, whereas the latter shows peraluminous geochemical character. The granite-granodiorite porphyries have high K2O (3.32-3.84 wt.%), and low Na2O (3.48-4.61 wt.%) and MgO (0.91-1.04 wt.%) relative to the quartz monzonite-tonalite association (K2O = 1.50-1.92 wt.%; Na2O = 4.08-6.45 wt.%; MgO = 1.44-2.07 wt.%). Ar-Ar geochronology studies on the amphibole separates reveal that the adakite-like porphyries have a crystallization ages of 51.34 +/- 0.27 to 53.55 +/- 0.34 Ma. Here, we contend that these rocks were formed by partial fusion of a mafic lower continental crust in a collisional phase but not in a subduction setting. All the samples exhibit the typical geochemical characteristics of adakite, that is, high Sr (250-1141 ppm), high Sr/Y ratios (16-147), low Y (6.8-14.8 ppm) and low HREE concentrations; they are similar to adakites formed by slab melting associated with the subduction zone. However, the rocks exhibit heterogeneity in isotopic composition, with I-Sr ranging from 0.70554 to 0.70986, epsilon(Nd) (51 Ma) from -8.5 to -0.9 and Nd model ages from 0.72 to 1.26 Ga. The samples are characterized by relatively high Th, Th/U and no significant Eu anomalies, implying that garnet was stable in their source during partial melting. The compositional diversity between rock groups is probably related to partial melting of heterogeneous lower crustal source. All of the features are inconsistent with a slab-melting origin and slab-related petrogenetic model, but instead, they favor an origin by melting of heterogeneous lower continental crust due to a thermal anomaly that was induced by the upwelling of the asthenosphere through a slab break-off in a collisional setting. These interpretations argue against the evidence for the presence of an early Cenozoic arc setting in the Eastern Pontides. Instead, the early Cenozoic time in the region may be attributed to a geodynamic response to a post-collisional uplift phase that occurred along the continent-continent collision between the Pontide and the Anatolide-Tauride blocks. (C) 2011 Elsevier B.V. All rights reserved.
- MakaleYalnız künye2010Generation of the Early Cenozoic adakitic volcanism by partial melting of mafic lower crust, Eastern Turkey: Implications for crustal thickening to delamination
Karsli, Orhan; Dokuz, Abdurrahman; Uysal, Ibrahim; Aydin, Faruk; Kandemir, Raif; Wijbrans, Jan
Early Cenozoic (48-50 Ma) adakitic volcanic rocks from the Eastern Pontides, NE Turkey, consist of calc-alkaline and high-K calc-alkaline andesite and dacite, with SiO2 contents ranging from 56.01 to 65.44 wt.%. This is the first time that Early Eocene volcanism and adakites have been reported from the region. The rocks are composed of plagioclase, amphibole, quartz, and Mg-rich biotite. They have high and low-Mg# values ranging from 55 to 62 and 13 to 42, respectively. High-Mg# rocks have higher Ni and Co contents than low-Mg# samples. The rocks exhibit enrichments in large ion lithophile elements including the light rare earth elements, depletions in Nb, Ta and Ti and have high La/Yb and Sr/Y ratios. Their relative high ISr (0.70474-0.70640) and low εNd (50 Ma) values (- 2.3 to 0.8) are inconsistent with an origin as partial melts of a subducted oceanic slab. Combined major- and trace element and Sr-Nd isotope data suggest that the adakitic magmas are related to the unique tectonic setting of this region, where a transition from a collision to an extension stage has created thickening and delamination of the Pontide mafic lower crust at 50 Ma. The high-Mg adakitic magmas resulted from partial melting of the delaminated eclogitic mafic lower crust that sank into the relatively hot subcrustal mantle, and its subsequent interaction with the mantle peridotite during upward transport, leaving garnet as the residual phase, elevates the MgO content and Mg# of the magmas, whereas low-Mg# magmas formed by the melting of newly exposed lower crustal rocks caused by asthenospheric upwelling, which supplies heat flux to the lower crust. The data also suggest that the mafic lower continental crust beneath the region was thickened between the Late Cretaceous and the Late Paleocene and delaminated during Late Paleocene to Early Eocene time, which coincides with the initial stage of crustal thinning caused by crustal extensional events in the Eastern Pontides and rules out the possibility of an extensional regime before ~ 50 Ma in the region during the Late Mesozoic to Early Cenozoic.
- MakaleAçık Erişim2010Relative contributions of crust and mantle to generation of Campanian high-K calc-alkaline I-type granitoids in a subduction setting, with special reference to the Harşit Pluton, Eastern Turkey
Karsli, Orhan; Dokuz, Abdurrahman; Uysal, Ibrahim; Aydin, Faruk; Chen, Bin; Kandemir, Raif; Wijbrans, Jan
We present elemental and Sr-Nd-Pb isotopic data for the magmatic suite (~79 Ma) of the Harşit pluton, from the Eastern Pontides (NE Turkey), with the aim of determining its magma source and geodynamic evolution. The pluton comprises granite, granodiorite, tonalite and minor diorite (SiO2 = 59.43-76.95 wt%), with only minor gabbroic diorite mafic microgranular enclaves in composition (SiO2 = 54.95-56.32 wt%), and exhibits low Mg# (<46). All samples show a high-K calc-alkaline differentiation trend and I-type features. The chondrite-normalized REE patterns are fractionated [(La/Yb)n = 2.40-12.44] and display weak Eu anomalies (Eu/Eu* = 0.30-0.76). The rocks are characterized by enrichment of LILE and depletion of HFSE. The Harşit host rocks have weak concave-upward REE patterns, suggesting that amphibole and garnet played a significant role in their generation during magma segregation. The host rocks and their enclaves are isotopically indistinguishable. Sr-Nd isotopic data for all of the samples display ISr = 0.70676-0.70708, εNd(79 Ma) = -4.4 to -3.3, with TDM = 1.09-1.36 Ga. The lead isotopic ratios are (206Pb/204Pb) = 18.79-18.87, (207Pb/204Pb) = 15.59-15.61 and (208Pb/204Pb) = 38.71-38.83. These geochemical data rule out pure crustal-derived magma genesis in a post-collision extensional stage and suggest mixed-origin magma generation in a subduction setting. The melting that generated these high-K granitoidic rocks may have resulted from the upper Cretaceous subduction of the Izmir-Ankara-Erzincan oceanic slab beneath the Eurasian block in the region. The back-arc extensional events would have caused melting of the enriched subcontinental lithospheric mantle and formed mafic magma. The underplating of the lower crust by mafic magmas would have played a significant role in the generation of high-K magma. Thus, a thermal anomaly induced by underplated basic magma into a hot crust would have caused partial melting in the lower part of the crust. In this scenario, the lithospheric mantle-derived basaltic melt first mixed with granitic magma of crustal origin at depth. Then, the melts, which subsequently underwent a fractional crystallization and crustal assimilation processes, could ascend to shallower crustal levels to generate a variety of rock types ranging from diorite to granite. Sr-Nd isotope modeling shows that the generation of these magmas involved ~65-75% of the lower crustal-derived melt and ~25-35% of subcontinental lithospheric mantle. Further, geochemical data and the Ar-Ar plateau age on hornblende, combined with regional studies, imply that the Harşit pluton formed in a subduction setting and that the back-arc extensional period started by least ~79 Ma in the Eastern Pontides.
- MakaleYalnız künye2010Sources and petrogenesis of Jurassic granitoids in the Yusufeli area, Northeastern Turkey: Implications for pre- and post-collisional lithospheric thinning of the eastern Pontides
Dokuz, Abdurrahman; Karsli, Orhan; Chen, Bin; Uysal, Ibrahim
Small granitoids emplaced into the early Jurassic volcani-clastic succession in the Yusufeli area, northeastern Turkey, can be temporally and geochemically classified into two groups: early Jurassic low-K and late Jurassic high-K. 40Ar-39Ar hornblende analyses yielded 188.0 ± 4.3 Ma for the Dutlupi{dotless}nar intrusion, dating the subduction related rifting in the region. It comprises metaluminous to weakly peraluminous (ASI = 0.94-1.11) granodiorite and, to a lesser extent, tonalite whose K2O-poor (< 2.04 wt.%) nature and weak negative Eu anomalies (Eu/Eu* = 0.9-0.7) preclude derivation by fractional crystallization from a K-rich melt. Sr, Nd and Pb isotopic data reveal derivation by partial melting from an already cooled tholeiitic basic rocks which had mantle-like isotope signature. The Sumbated intrusion formed in the late Jurassic (153.0 ± 3.4 Ma) and consists chiefly of metaluminous (ASI = 0.84-0.99) quartz monzodiorite. Medium to high-K2O, relatively high MgO and Sr contents, flat HREE patterns without prominent Eu anomalies, slightly positive εNd(t) values (+ 1.5 to + 2.5) and low ISr ratios (0.7046-0.7056) are consistent with an origin by dehydration melting of a juvenile source, above the garnet stability field, dominated by likely K-amphibole bearing calc-alkaline mafic rocks. Geochemical data show that fractional crystallization from a Sumbated-like quartz monzodioritic magma is the fundamental process responsible for the evolved compositional range of the Keçikaya intrusion. The geochemical and geochronological data presented here indicate that the late Jurassic magmatism occurred in a post-collisional setting. Slab-breakoff, which was followed by shortly after collision, seems to be the most plausible mechanism for the generation of medium to high-K calc-alkaline rocks of the Sumbated and the Keçikaya intrusions, indicating a switch in the geodynamic setting, e.g., from pre-collision to post-collision in the middle Jurassic in the eastern Pontides.
- MakaleYalnız künye2006Geochemical constraints on the provenance, mineral sorting and subaerial weathering of Lower Jurassic and Upper Cretaceous clastic rocks of the eastern Pontides, Yusufeli (Artvin), NE Turkey
Dokuz, Abdurrahman; Tanyolu, Erkan
An integrated petrographic and geochemical study of the shales and sandstones of Early Jurassic and Late Cretaceous age in the Yusufeli area (Turkey) was carried out to obtain more information on their provenance, sedimentological history and tectonic setting, as well as to evaluate the influence of weathering, hydraulic sorting and recycling processes upon source-rock signature. Depending on their matrix and mineralogical content the Lower Jurassic sandstones are identified as arkosic arenite and wacke, while the Upper Cretaceous sandstones are defined as lithic arenite and wacke. Elemental ratios such as La/Sc, Cr /Ni, Co/Th, Th/Sc and Cr/Th indicate a mafic source for the marjority of each of the sandstone sequences. However, in several samples, high Th/Sc and low Cr/Th ratios suggest a contribution from a felsic source. Using element-ratio diagrams, all of the samples plot along a curve consistent with a two-component mixing model, consisting of a dominant mafic and a subordinate felsic source. The relatively low Cr/Ni ratios provide no support for significant amounts of ultramafic lithologies in the source area. The lower trace-element contents of the studied sandstones in relation to post-Archean Australian shales (PAAS), which are representative of the composition of the upper continental crust, indicate that recycling processes in their source areas were probably less intense than those of the PAAS. The more LREE-depleted patterns of the shales relative to sandstones likely do not reflect changes in source-area composition but, rather, variations in mineral sorting, chemical weathering and/or sediment recycling. Chemical index of alteration (CIA) indices observed in the Lower Jurassic and Upper Cretaceous sediments (average of 53 & 46, respectively) suggest that their source area underwent moderate degrees of chemical weathering processes. However, an upward increase in the values of the CIA indices in the Lower Jurassic profiles may indicate that the source area gradually underwent more intense chemical weathering, possibly due to climatic and/or tectonic variations. On the other hand, an upward decrease in the CIA indices of the Upper Cretaceous profiles generally demonstrate that the sediments were derived from a relatively less-weathered source terrain, reflecting an increased erosion rate likely due to increasing tectonic activity. The compositional immaturity of the analysed sandstone samples is typical of subduction-related environments, and their SiO2/Al2O3 and K2O/Na2O ratios and Co, Sc, Th and Zr contents reflect their oceanic and continental-arc settings.
- MakaleAçık Erişim2006A mantle- and a lower crust-derived bimodal suite in the Yusufeli (Artvin) area, NE Turkey: Trace element and REE evidence for subduction-related rift origin of Early Jurassic Demirkent intrusive complex
Dokuz, Abdurrahman; Tanyolu, Erkan; Genç, Salim
The Yusufeli area, in the Eastern Black Sea Region of Turkey, contains a crystalline complex that intruded into the Carboniferous metamorphic basement and is composed of two intrusive bodies: A gabbro-diorite and a tonalite-trondhjemite. The mafic body (45-57 wt% SiO2) displays a broad lithological spectrum ranging from plagioclase-cumulate to quartz diorite. Primitive varieties of the body have Mg-number, MgO and Cr contents that are close to those expected for partial melts from mantle peridotite. Data are consistent with the magma generation in an underlying mantle wedge that was depleted in Zr, Nb and Ti, and enriched in large ion lithophile elements (K, Rb, Ba, Th). However, high Al2O3, CaO and generally low Ni (<65 ppm) contents are not in agreement with the unfractionated mantle-derived primitive magmas and require some Al2O3- and CaO-poor mafic phases, in particular, olivine and orthopyroxene. Absence of orthopyroxene in crystallization sequence, uralitization, and a common appearance of clinopyroxene surrounded by hornblende imply an anhydrous phase fractionated from highly hydrous (5-6%) parent. Geochemical modelling suggests derivation by 15-20% melting of a depleted-lherzolitic mantle. The tonalite-trondhjemite body (58-76 wt% SiO2) ranges in composition from quartz diorite to granodiorite with a low-K calc-alkaline trend. Although LILE- and LREE- enriched characteristics of the primitive samples imply a metasomatic sub-arc mantle for their source region, low MgO, Ni and Cr concentrations rule out direct derivation from the mantle wedge. Also, lack of negative Eu anomalies suggests an unfractionated magma and precludes a differentiation from the diorites of mafic body, which show negative Eu anomalies. Their Na enrichments relative to Ca and K are similar to those of Archean tonalites, trondhjemites and granodiorites and Cenozoic adakites. However, they exhibit important geochemical differences from them, including low-Al (<15 wt%) contents, unfractionated HREE patterns and evolution towards the higher Y concentrations and lower Sr/Y ratios within the body. All these features are obtained in experimentally produced melts from mafic rocks at low pressures (≤5 kbar) and also widespread in the rocks of arc where old (Upper Cretaceous or older) oceanic crust is being subducted. Major and REE modelling supports formation of the quartz dioritic parent to the felsic intrusive rocks by 70% partial melting of a primitive gabbroic sample (G694). Therefore, once taking into account the extensional conditions prevailing in the Pontian arc crust in Early Jurassic time, former basic products (gabbros) seem to be the most appropriate source for the tonalite-trondhjemite body. Magmatic emplacement of stratigraphically similar lithologies in the Pulur Massif, just southwest of the Yusufeli, was dated to be 184 Ma by the 40Ar/ 39Ar method on amphibole, and is compatible with the initiation of Early Jurassic rifting in the region.