Aydin, Faruk; Oguz-Saka, Simge; Karsli, Orhan; Dokuz, Abdurrahman; Moghadam, Hadi Shafaii; Zhao, Junxing; Schmitt, Axel K.

Understanding melt source evolution during arc-back-arc magmatism is essential for resolving crustal growth processes and the development of felsic-hosted volcanogenic massive sulfide (VMS) systems. Here we integrate new zircon HfO isotope data with published geochronological and whole-rock geochemical datasets to reassess the petrogenesis of Late Cretaceous arc and back-arc magmatic flare-ups in the eastern Sakarya Zone (ESZ), NE T & uuml;rkiye. Two high-flux magmatic episodes are recognized: a Turonian-Santonian pulse (similar to 92-86 Ma) forming the lower volcanic sequence (LVS), and a younger Campanian pulse (similar to 84-75 Ma) forming the upper volcanic sequence (UVS), accompanied by contemporaneous arc-related granitoids. Zircons from both LVS and UVS display predominantly positive epsilon Hf(t) values (+2 to +12) coupled with mantle-like delta O-18 (5.0-5.6 parts per thousand), but contain abundant inherited Mesoproterozoic-Paleozoic cores and show variable two-stage model ages (T-DM (c) = 0.4-1.4 Ga). Together with the peraluminous affinity of many felsic units, these features indicate that the dacite-rhyolite suites hosting VMS deposits were generated by melting of compositionally heterogeneous lower crust variably hybridized by juvenile basaltic inputs. Arc granitoids likewise record a transition from dominantly juvenile, I-type magmatism during the Coniacian-Santonian to more evolved, crust-influenced signatures during the Campanian. We propose that both flare-ups reflect repeated episodes of basaltic underplating and deep-crustal heating, producing voluminous felsic magmas capable of sustaining long-lived hydrothermal systems. The spatial and temporal coincidence between felsic-dominated flare-ups and VMS mineralization suggests that hybridized felsic magmas, rather than purely juvenile mantle melts, played a central role in generating the physiochemical conditions required for metal-bearing hydrothermal fluids in the ESZ.

Karsli, Orhan; Sengun, Firat; Aydin, Faruk; Moghadam, Hadi S.; Dokuz, Abdurrahman; Griffin, William L.; Saka, Simge Oguz; Zack, Thomas

Turkey's Early Paleozoic tectonic evolution has received much attention in recent years. Early Paleozoic intrusive rocks are mainly distributed across Europe, Turkey (Istanbul-Zonguldak Zone, Strandja Zone, and Tauride Block), and Iran. However, the Early Paleozoic rocks have not yet been identified in north Turkey's Sakarya Zone, and thus the Early Paleozoic paleogeography of the Sakarya Zone remains enigmatic. Here, we provide new field observations, petrography, geochemistry, and geochronology of the western Sakarya Zone to identify the nature, origin, and tectonic setting of the Ordovician magmatic rocks cropping out in the So center dot gut area (Bilecik, northwest Turkey). Our Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) U-Pb analyses on zircons suggest that the So center dot gut metagranites were emplaced at-484-461 Ma (except two zircons with ages of 490 and 494 Ma). There are abundant inherited zircon cores in the So center dot gut metagranites, with ages of >500 Ma to-2.2 Ga. The inherited Cadomian cores give concordant ages of 588 and 572 Ma. The So center dot gut metagranites are felsic and have high SiO2 (72-76 wt%) and K2O (4.1-5.3 wt%) contents, with high K2O/Na2O ratios (1.2-1.9), and low Mg# (17-35) values. The samples are Ca-poor (CaO = 0.52-0.68 wt%) and show peraluminous [A/CNK = molar Al2O3/(K2O + CaO + Na2O) = 1.15-1.28] and high-K calc-alkaline geochemical characters, with high modal content of muscovite (-10%), consistent with S-type compositions. The samples have low REE concentrations, with negative Eu anomalies. Ordovician zircon rims show epsilon Hf(t) values of +3.6 to-35.4, whereas inherited zircon cores have epsilon Hf(t) values of +33.9 to-26.1. Our data, in conjunction with previous studies, suggest that the So center dot gut metagranites were generated by fluid-absent muscovite-dehydration melting of metasediments accumulated in a back-arc setting. This back-arc basin was characterized by a denuded continental crust with abundant pelite-dominated sediments and minor input of juvenile mafic melts. Our results, combined with previously published data and paleogeographic reconstructions, allow us to conclude that the Ordovician S-type granites were formed by progressive back-arc extension and crustal thinning prompted by the rollback of the southward subducting oceanic-lithosphere slab. This led to the opening of the Rheic Ocean on the northern side of Gondwana in response to the final closure and termination of the Tornquist Ocean as a branch of the Iapetus Ocean. We argue that the Ordovician S-type granites from the northern side of Gondwana could signal significant crustal reworking away from any clear continental collision zone.

Oguz-Saka, Simge; Aydin, Faruk; Karsli, Orhan; Dokuz, Abdurrahman; Aiglsperger, Thomas; Miggins, Daniel P.; Sen, Cuneyt; Kandemir, Raif; Sari, Bilal; Koppers, Anthony A. P.

The eastern Blacksea magmatic arc (EBMA) in the eastern Sakarya Zone (ESZ) provides an excellent opportunity to investigate birth of an extensional intra-arc and back-arc settings in the Late Cretaceous over the Early Cretaceous northern passive margin of the Neotethys Ocean. Volcano-stratigraphy clearly shows that the Late Cretaceous volcanic activity of the EBMA occurred in two major phases. Bimodality, characterized by mafic/ basaltic rocks at the base and felsic/silicic types on top of it, is a typical feature of the lower (LVS) and upper (UVS) volcanic successions in the Giresun region of the ESZ. U-Pb and Ar-Ar ages support the volcanic suc-cession as two-stage (LVS: ca. 92-85 and UVS: ca. 83-67 Ma) bimodal volcanism. Both the volcanic successions are represented by similar rock types consisting of tholeiitic to calc-alkaline basalt-basaltic andesites and calc-alkaline to shoshonitic dacite-rhyolites. Basaltic (M1-and M2-series) and felsic/silicic (F1-and F2-series) sam-ples of the LVS and UVS have an arc-like signature with enriched large ion lithophile elements (LILEs) and light rare earth elements (LREEs) and depleted high field strength elements (HFSEs). Also, the felsic/silicic samples of the F1-and F2-series show prominent negative Sr and Eu anomalies (Eu/Eu* = 0.4 to 0.9), suggesting that plagioclase fractionation played a key role on the evolution of both felsic series. Bimodal rock series in two phases have a wide range of 87Sr/86Sr(i) (0.7048-0.7075) and 143Nd/144Nd(i) (0.5123-0.5127) ratios with var-iable epsilon Nd(i) values of-3.8 to +3.0. 206Pb/204Pb(i), 207Pb/204Pb(i) and 208Pb/204Pb(i) isotope ratios of the Giresun volcanic rocks vary in the range of 17.97-18.52, 15.55-15.65 and 37.53-38.56, respectively. Geochemical and isotopic data suggest that the parental magma of the M1-basaltic rocks were probably derived from a shallow (spinel-bearing) mantle metasomatized by slab/sediment-derived fluids. In contrast, the M2-basalts seem to have been originated from a deeper mantle source (spinel-garnet transition zone) enriched by slab/sediment-derived fluids and hydrous melts (bulk sediment) metasomatism with some contributions of lower/upper crustal materials. The least evolved basaltic samples in two phases are consistent with moderate (-10-15%) to high degree (-20-30%) partial melting of the metasomatized mantle. The silicic melts of the F1 -and F2-rocks series, on the other hand, were likely derived from melting of lower crustal materials consisting of meta-basalts/andesites and lesser amount of meta-sediments. Subsequently, these melts experienced FC +/- AFC and mixing processes during their ascent and emplacement to generate high-silica (rhyolitic) melts. Our data, combined with previous studies, suggest that two-stage bimodal volcanic rocks of the Late Cretaceous in the ESZ were formed in the transition from an extensional continental intra-arc to a back-arc setting during the north-ward subduction of the northern branch of Neotethys Ocean.

Karsli, Orhan; Sengün, Firat; Dokuz, Abdurrahman; Aydin, Faruk; Kandemir, Raif; Kristoffersen, Magnus; Santos, Jose Francisco; Hofmann, Mandy; Duygu, Levent

Cadomian realms have recently received much attention in the Alpine-Mediterranean orogenic belts, with discontinuous outcrops extending from the Alps to the Istanbul Zone, Menderes Massif, Taurides in Turkey and Iran as well. However, they have not been identified in the Sakarya Zone, northern Turkey to date. Here, we present, for the first time, zircon U-Pb geochronology, zircon Hf-isotopes and trace elements as well as whole-rock geochemistry and Nd-isotopes from the Karamese metagranitoid in the Yusufeli (Artvin) area, NE Turkey. The zircon U-Pb age data demonstrate that the Karamese metagranitoid intruded into the Precambrian basement units in the Early Cambrian (ca. 534-530 Ma). Zircon overgrowths in the Karamese metagranitoid interpreted as a result of metamorphic overprint yielded ages of 328 Ma. This age indicates that the Sakarya Zone experienced the Variscan metamorphic events at 328 Ma. The Early Cambrian metagranitoid shows that some rocks in the basement of the Sakarya Zone date back to Precambrian in age. The Karamese metagranitoid is an S-type granite, with high modal content of muscovite (similar to 10%). The samples show highly peraluminous [A/CNK = molar Al2O3/(CaO + K2O + Na2O) = 1.34-1.58] and medium-K calc-alkaline geochemical affinities. The Karamese metagranitoid displays low REE concentrations and a slight positive Eu anomaly and show slightly light REEs and LILEs enrichments, and HFSEs depletions. The samples have negative epsilon(Nd)(t) of -4.76 to -2.90 and epsilon(Hf) (t) of -4.75 to -1.08 values. These geochemical-isotopic characteristics reveal that the Karamase metagranitoid originated through fluid-absent muscovite dehydration melting of heterogeneous metasedimentary source rocks (mostly greywacke), with a minor input of juvenile mafic melt. We argue that the Early Paleozoic metagranitoids along the eastern Sakarya Zone is an expression of crustal thinning in the Cadomian orogenic belt in northern Gondwana, with the tectonic turnover from convergent margin to crustal extension possibly induced by break-off of Tornquist oceanic lithosphere which is a branch of Iapetus Ocean during Early Cambrian.

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.

Karsli, Orhan; Şengün, Fırat; Santos, José Francisco; Uysal, İbrahim; Dokuz, Abdurrahman; Aydin, Faruk; Kandemir, Raif

The Mesozoic geodynamic evolution of the Paleotethys Ocean is critical but a poorly known piece of the geological history of the Sakarya Zone, north Turkey. Here, we describe new integrated research of zircon U-Pb geochronology, bulk-rock major-trace element and Sr–Nd isotopes as well as Hf isotope of zircons from the granitic rocks of the Demirköy and Kızıldamlar plutons located in Bilecik area along the western Sakarya Zone, northwestern Turkey. Zircon U–Pb ages obtained by LA-ICP-MS reveal that the granitic magma intruded into the basement rocks during the Late Jurassic (ca. 162–157 Ma). The granites are tholeiitic and show slightly metaluminous to peraluminous geochemical affinities. The granite samples are adakitic, having relatively high Na2O (6.35–7.58 wt%) and Sr (172–580 ppm), and low K2O (0.39–1.13 wt%) and Y (3.2–5.3 ppm), and thus have high Sr/Y (70–250). The samples exhibit enrichment of light rare earth elements (REE) and large ion lithophile elements (LILE) and depletion in heavy REE and the high field strength elements (HFSE; e.g., Nb, Ti, Ta). The samples possess low 87Sr/86Sr(t) values of 0.70313 to 0.70372, and εNd(t) of −0.66 to +9.24. Zircon grains exhibit positive εHf (t) values of 5.0 to 12.8 and depleted mantle model ages of Hf (TDM1 = 0.24 to 0.68 Ga). These geochemical features and Sr–Nd–Hf radiogenic isotope systematics reveal that the adakitic rocks were likely formed by low degree melting (<10%) of the eclogitic part of an oceanic slab and subsequent melt fractionation and crustal contamination. An ongoing subduction scenario cannot explain the formation of the investigated adakitic rocks. We suggest that the adakitic plutons were generated through slab break-off after the final closure of the Paleotethys Ocean throughout the Late Jurassic (ca. 162–157 Ma). Considering the previous data, the continental back-arc basin system, leading to the opening of Neotethys Ocean at the southern margin of the continent, has been put forward to decipher the Late Jurassic tectonic evolution of the Paleotethys Ocean and the western Sakarya Zone.

Turlin, Francois; Moritz, Robert; Sönmez, Safak Utku; Keskin, Serdar; Karsli, Orhan; Aydin, Faruk; Ulyanov, Alexey; Dokuz, Abdurrahman

Even though mechanisms of crustal growth and differentiation are well known, the spatial and temporal evolution of the composition of the continental crust remains poorly constrained, even more when considering the metal budget in mountain belts. In this study, we use the Eastern Pontides, NE Turkey, as a natural laboratory to decipher the role of successive tectono-magmatic events during the buildup of the continental crust fertility. This area is a Gondwana-derived terrane that accreted to the Eurasian margin. It underwent successive orogenic phases associated with the closure of the Paleo- and Neotethys oceans, and lastly the Alpine orogeny. It hosts abundant volcanogenic massive sulfide, porphyry and epithermal deposits, mostly formed in the latest stages of arc construction. These features relative to the buildup of metal fertility in this continental crust raise questions such as were the physico-chemical conditions during the early sterile history of the Eastern Pontides unfavorable to form ore deposits or were the metals progressively concentrated in the crust during successive orogenic events? We will use whole-rock geochemistry, U-Pb zircon dating, and Hf-O isotopic and trace element compositions of magmatic zircons to constrain the construction of the continental crust in the Eastern Pontides, and its metal fertility through time.

Karsli, Orhan; İlhan, Mehdi; Kandemir, Raif; Dokuz, Abdurrahman; Aydin, Faruk; Uysal, İbrahim; Duygu, Levent

Late Mesozoic evolution of the Neotethys Ocean is critical and still remains unclear along the entire Sakarya Zone (SZ), Turkey, because previous interpretations of the Middle to Late Jurassic rifting of the SZ have not been consensually accepted. We identified, for the first time, the Early Cretaceous dykes intruded into the Late Jurassic and Late Carboniferous bodies in the Çamlıkaya (Erzurum) area in the SZ. Here, we present a data set of new field observations, 40Ar/39Ar geochronology, whole-rock geochemical and Sr–Nd isotopes to find out the tectonic setting of previously undated basic to intermediate igneous rocks. 40Ar/39Ar technique on the groundmass separations yielded the plateau ages of 128.78 ± 1.02 to 122.89 ± 0.82 Ma. The investigated dykes are compositionally divided into two sub-groups: calc-alkaline lamprophyres (CAL) and high-Nb alkaline basalts (HNB). They show enrichment in large ion lithophile elements (LILE: e.g., Rb, Ba, K) and depletion in high field strength elements (HFSE: Nb and Ti) as well as moderate fractionation of light rare earth elements relative to heavy rare earth elements. The HNB have high Mg# (47–66), and relatively unradiogenic Sr [87Sr/86Sr(t) = 0.70385 to 0.70510] and Nd [εNd(t) = +0.5 to +5.3] isotopic composition, whereas the CAL possess moderate to high Mg# (32–72), and relatively high Sr [87Sr/86Sr(t) = 0.70586 to 0.71379] and Nd [εNd(t) = +1.5 to −9.9] isotopic composition. Early Cretaceous high-angle subduction triggered upwelling of the asthenosphere at 126 Ma, which likely generated a heat anomaly. This anomaly has probably induced melting of the oceanic slab and the metasomatized mantle to form the CAL and HNB. The HNB were generated by low degrees of melting of mantle domain that was previously reacted by oceanic slab-related adakitic melts at the depths of the garnet stability field (≥70 km). However, the CAL were probably formed at shallower mantle levels within the garnet-amphibole field by a relatively higher degree of melting of the mantle, also modified by subducted slab sediments. A new field, age, and petrochemical data from the Early Cretaceous dykes, combined with the presence of Aptian deep-water sediments to the north and the Late Jurassic-Early Cretaceous platform carbonates deposited in the passive margin setting along the entire SZ, imply the initiation of active-margin magmatism in the SZ. We suggest that the coexistence of the Early Cretaceous CAL and HNB points to the formation of an Andean-type active margin above a north-dipping Neotethyan oceanic slab. Their emplacement ages are associated with the inception of the magmatic arc structure and refer to a significant geodynamic turnover from passive-margin to active-margin tectonics along the SZ.

Karsli, Orhan; Sengün, Firat; Dokuz, Abdurrahman; Kandemir, Raif; Aydin, Faruk; Andersen, Tom

The Rheic Ocean is the most significant Paleozoic ocean that detached peri-Gondwana terranes from the northern Gondwana margin throughout the closure of the Iapetus Ocean. The suture of the Rheic Ocean spreads from Mexico to the Middle East, and the timing of its final closure is well-documented by the rocks formed in the Variscan-Alleghanian-Ouachita orogeny which led to the formation of the supercontinent Pangaea. However, as robust paleomagnetic and quantitative data are mostly lacking, the onset and evolution of the subduction of the Rheic Ocean are highly speculative, and they require further confirmation. Recently, the well-preserved metagranitoids along the western Sakarya Zone (SZ) in Anatolia have been identified, and they provide new data that improve our knowledge on the evolution of the Early Paleozoic Rheic Ocean along the northern Gondwana. Here, we present new geochronological, in situ zircon Hf isotope, and whole-rock geochemical analyses of these metagranitoids from the western SZ to enhance our understanding of the subduction processes of the Rheic Ocean. IA-ICP-MS zircon U-Pb dating demonstrated that the Boziiyuk and Borcak metagranitoids from the western SZ were emplaced during the Silurian to Early Devonian (431 +/- 2.7 to 403 +/- 3.5 Ma). The both granitoids have medium- to high-K. calc-alkaline magmatic character, and exhibit peraluminous to slightly metaluminous geochemical signature. They show a typical arc pattern in terms of trace elements and have a uniform, moderate negative epsilon(Hf) (t) of -3.2 to -9.7, with Mesoproterozoic Hf depleted mantle model ages (T-DM1 = 12 to 1.4 Ga). The geochemical and isotopic characteristics are not consistent with those of depleted mantle melts and melts derived from the crustal rocks in an intracontinental environment. Instead, the parental magma is likely generated from the partial melting of a homogeneous and enriched mantle wedge source. We propose that the Silurian to Early Devonian arc-related magmatism is associated with a northward subduction episode of the Rheic Oceanic lithosphere beneath the peri-Gondwana terranes. Hence, we consider that the opening of Paleotethys Ocean formed in a back-arc basin of subduction in Andean style to the north rather than a continental rift to the south in response to south-directed and short-lived supra-subduction zone (SSZ)-type subduction during the Silurian to Early Devonian. (C) 2020 Elsevier B.V. All rights reserved.

Karsli, Orhan; Dokuz, Abdurrahman; Aydin, Faruk; Uysal, Ibrahim; Sengün, Firat; Kandemir, Raif; Santos, Jose Francisco; Andersen, Tom

The initiation of the break-off of the northern branch of the Neotethyan oceanic lithosphere is an important but poorly understood event in the geology of the Sakarya Zone (SZ) in northeastern Turkey. Although it is wellknown that Latest Cretaceous intrusives (-70 Ma) and early Eocene adakitic magmatic rocks are present in the eastern SZ, the outcrops of the early Eocene non-adakitic rocks are very limited, and their tectono-magmatic evolution has not been studied. We describe a small outcrop of non-adakitic quartz diorite porphyry in the Kov area of the Gumushane region in northeastern Turkey. The genesis of these porphyries is significant in evaluating the syn-to post-collision-related magmatism. The LA-ICP-MS zircon U-Pb dating revealed that the Kov quartz diorite porphyries (KQDP) formed at ca. 50 Ma, coeval with adakitic rocks, and-20 Myr later than the slab rollback-related intrusive rocks. The KQDPs are calc-alkaline in composition and enriched in large ion lithophile elements (LILEs) and light rare earth elements (LREEs) and depleted in high field strength elements (HFSEs; e.g., Nb, Ta, Ti), with significant negative anomalies of Nb, Ta, and Ti but positive anomalies of Th, U, and Pb. Isotopic compositions of the samples show limited range of variation and slight enrichment of 87Sr/86Sr(t) (0.70489 to 0.70555), epsilon Nd(t) (-1.4 to -1.2) with TDM of 1.11 to 1.61 Ga. Pb isotopic ratios of the samples point to an enriched mantle source. They were likely crystallized from the melt that originated from an EM2-type spinel-facies subcontinental lithospheric mantle (SCLM), followed by the fractionation with insignificant crustal assimilation. The SCLM was metasomatically enriched, and the metasomatic agent was likely H2O-rich fluids rather than sediments released from subducting oceanic crust during the Late Cretaceous closure of the Neotethyan oceanic lithosphere. In conjunction with the geological background and previous data, we propose that the generation of the KQDPs resulted from a slab break-off event that caused ascending or infiltration of hot asthenosphere, triggering mantle melting. Such sporadic occurrences of the KQDPs, with coeval adakitic rocks in the SZ, are likely associated with the onset of extensional tectonics due to the earlier stage of slab break-off along the region during the early Eocene period.