Eastern Pontide Orogenic Belt Research Articles

Current status and potentials of enhanced geothermal system in the Eastern Pontide Orogenic Belt, Turkey

Current status and potentials of enhanced geothermal system in the Eastern Pontide Orogenic Belt, Turkey

U–Pb zircon dating, Sr-Nd whole-rock and Lu-Hf zircon isotope analyses of the Eocene Arslandede pluton, Eastern Pontides, NE Turkey: Implications for mantle source and magma evolution

The Eastern Pontide Orogenic Belt (EPOB) hosts numerous plutonic bodies with different dimensions, compositions and ages ranging from Paleozoic to Late Eocene in NE Turkey. U-Pb zircon dating suggests that the Arslandede pluton crystallized at 44.50 ± 0.29 Ma, corresponding to the Lutetian (Middle Eocene) period. Rocks of this pluton have monzonitic character, with compositions ranging from monzodiorite to granite (SiO2 = 49˗71 wt%). The studied monzonitic rocks have I-type, metaluminous and shoshonitic character and are enriched in large-ion lithophile elements (LILEs). The rare earth elements (REEs) have concave up shape (LaN/YbN = 6.64–11.57) and show negative to slightly positive Eu anomalies (EuN/Eu* = 0.37–1.24). 87Sr/86Sr(i) values of 0.704801–0.705102 and εNd(i) values of 1.01–1.34 correspond to the mantle series on isotope ratio diagrams. Positive εHf(i) values (5.01–14.91) plot between the depleted mantle and the chondritic evolution lines. The petrological features of the rocks from the Arslandede pluton show that fractional crystallization with low rates of assimilation and/or magma mixing were effective during crystallization. All data show that the magma source of the pluton derived from an enriched lithospheric mantle and emplaced into the crust after differentiation in a deep seated magma chamber contaminated by relatively small proportions of crustal rocks.

Petrology, Geochronology and Tectonic Setting of Early Triassic Alkaline Metagabbros From the Eastern Pontide Orogenic Belt (NE Turkey): Implications for the Geodynamic Evolution of Gondwana's Early Mesozoic Northern Margin

AbstractThe Triassic geodynamic evolution of the Pontides Belt, which geographically corresponds to northern Anatolia, is still a matter of debate. We present and interpret geological, petrographical, geochemical, and geochronological data from small, discordant, metagabbroic rocks exposed in the Tokat Massif. Field and petrographic characteristics distinguish four subgroups: (i) medium‐grained, slightly lineated, clinopyroxene‐rich metagabbros; (ii) fine‐grained, nonlineated, clinopyroxene‐rich metagabbros; (iii) medium‐grained, nonlineated, brown amphibole‐rich metagabbros; and (iv) medium‐grained, slightly lineated, brown and blue amphibole‐rich metagabbros. Magmatic kaersutites give 39Ar/40Ar plateau ages of 244.6 ± 0.5 and 243.16 ± 0.92 Ma and overlap the U‐Pb age of titanite from a clinopyroxene‐rich sample. Inherited zircons (~460, ~880, and ~2600 Ma) in fine‐grained clinopyroxene‐rich metagabbros indicate interaction with a range of crustal rocks during the intrusion. The blue amphiboles are magnesioriebeckites that formed during metamorphism or late‐stage crystallization. Geochemical data, including whole rock major and trace element concentrations, and Sr‐Nd‐Pb isotope compositions reveal that both clinopyroxene‐ and amphibole‐rich metagabbros are alkaline in composition but derived from distinct mantle sources. They are compositionally distinct from mafic igneous rocks in the contemporaneous Permo‐Triassic Karakaya Complex that is well exposed in the western part of the Pontides Belt. Considering all data, we suggest that the Pontides Belt was shaped above a south dipping subduction zone during Permo‐Triassic and the alkaline gabbros that were emplaced into crustal rocks during Early Triassic back‐arc rifting of the northern margin of Gondwana.

Gamma Ray Spectrometry Method In Detecting The Low Sulfidation Type Epithermal Au+Ag Mineralization (Gümüşhane, Ne Turkey)

Summary The gamma ray spectrometry has been widely used in mineral exploration, geological mapping and environmental radiation monitoring. The Eastern Pontide orogenic belt, which constitutes an important part of the Alpine-Himalayan system, hosts various types of economic mineral deposits of massive sulfide, porphyry copper, skarn, and epithermal vein-type ore deposits. Potassium is generally the most useful pathfinder element for gold mineralization zones. High level K/eTh, K/eU and Efimov F-Parameter (KeU/eTh) anomalies showed the alteration zones transmitting hydrothermal ore solutions to the surface. As a result, the gamma ray spectrometry, which is fast and economics method, might be used for estimation of prospective Au ore zones associated with the K alteration..

Geology, U-Pb geochronology, and stable isotope geochemistry of the Tunca semi-massive sulfide mineralization, Black Sea region, NE Turkey: Implications for ore genesis

Upper Cretaceous volcano-sedimentary sequences of the Eastern Pontide orogenic belt, NE Turkey, are host to significant VMS mineralization, including near Tunca. The initial stages of felsic volcanism within the mineralized area are marked by the eruption of dacitic lavas and breccias of the Kızılkaya Formation. This was accompanied by the emplacement of domelike hematitic dacites. Autobrecciated and volcaniclastic rocks, both in situ and resedimented, were likely generated from extrusive portions of these dacite bodies. Basaltic volcanism is marked by the eruption of the lava flows and pillow lavas of the Çağlayan Formation. Hiatuses in basaltic activity are marked by thin horizons of volcaniclastics and mudstones. The uppermost felsic volcanic units were accompanied by resedimentation of autoclastic facies from previous volcanism and represent the latest phase of Upper Cretaceous volcanism in the area. The semi-massive sulfide mineralization is associated with a late stage of the initial felsic volcanism. U-Pb LA-ICP-MS zircon dating of a dacitic tuff breccia yielded an age of 88.1±1.2Ma (Coniacian-Upper Cretaceous), which is interpreted to be the age of the sulfide occurrences.A concentric zoned alteration pattern is observed in the footwall rocks. The alteration pattern is considered to have formed by lateral migration of hydrothermal fluids which had ascended along the discharge conduit. Fluid inclusion data indicate precipitation or mobilization processes within a relatively narrow temperature range of 152–255°C (avg. 200°C). The low-salinity fluids in the fluid inclusions, less than 5.9wt% NaCl equivalent, are consistent with typical modified seawater-dominant hydrothermal vent fluids. Sulfur isotope analysis of the Tunca sulfides yields a narrow range of 1.5–4.1per mil. These δ34S values are also typical of many VMS deposits. Most of the recorded δ18O values (+7.1 to +14.0permil) are greater than 9per mil. The most intensely hydrothermally altered rocks tend to have lower δ18O values relative to the less altered rocks. Collectively, the geologic relationships, mineralization style, and the lack of seafloor ore facies suggest that mineralization is principally of sub-seafloor origin. The most geologically reasonable interpretation of the genesis of the Tunca mineralization is the continuous interaction between the host rocks and seawater-derived fluids, without significant involvement of a magmatic fluid.

Petrology of the shoshonitic Çambaşı pluton in NE Turkey and implications for the closure of the Neo-Tethys Ocean: Insights from geochemistry, geochronology and Sr–Nd isotopes

The Eastern Pontide Orogenic Belt (EPOB) is one of the well-preserved continental magmatic arcs in the Alpine–Himalayan Belt and consists of Paleozoic to Miocene plutonic bodies. This paper presents new geological, bulk-rock major and trace element analyses, biotite Ar–Ar ages and Sr–Nd isotope data on representative samples from the Çambaşı granitic body exposed in the northwestern part of the EPOB. Although the majority of the plutons in the EPOB show calc-alkaline affinity and I-type character, the Çambaşı pluton has a shoshonitic affinity and an I-type, metaluminous signature. The pluton is composed of mainly monzonite, quartz monzonite and granite bodies including monzo-dioritic mafic microgranular enclaves (MMEs). Excluding the MMEs, these rocks have 57.7–70.2wt.% SiO2, 13.1–17.5wt.% Al2O3, 3.06–3.64wt.% Na2O, 3.66–5.92wt.% K2O, 0.33–3.3wt.% MgO and Mg# <30, indicating intermediate to evolved compositions. They are characterized by light rare earth element (LREE)-enrichment, relatively flat heavy rare earth element (HREE) patterns with small negative Eu anomalies and moderately fractioned REE patterns [average (La/Yb)N=11.32]. Decreasing Fe2O3T, MgO, CaO, TiO2, Ba, Eu, and Sr with increasing SiO2 contents are consistent with fractional crystallization and can be related to fractionation of plagioclase, clinopyroxene, hornblende, apatite and Fe–Ti oxides. Initial 87Sr/86Sr ratios vary between 0.70470 and 0.70478, while those of 143Nd/144Nd lie between 0.51262 and 0.51263. Major, trace element and initial Sr–Nd homogeneity show that fractional crystallization played a key role in the evolution of the pluton. 40Ar/39Ar dating of four biotite samples from the pluton gave plateau ages of between 44.62±0.1 Ma and 44.10±0.0 Ma, which are interpreted as cooling ages of the plutonic rocks. We suggest that the Çambaşı pluton formed in a post-collisional extensional setting where partial melting of chemically enriched lithospheric mantle was controlled by lithospheric thinning and upwelling of asthenosphere that followed the closure of the northern branch of Neo-Tethys Ocean in Lutetian time on the northern margin of Gondwanaland.

MINERALOGY AND GEOCHEMISTRY OF THE KOCADAL (TORUL, GÜMÜŞHANE, EASTERN BLACK SEA REGION, TURKEY) Zn-Pb-Ag, Au and Cu MINERALIZATIONS

The Kocadal base and precious metal mineralizations are located in the southwest of Gümüşhane province of the eastern Pontide orogenic belt. In the vicinity of the Kocadal mineralization, Gümüşhane granite, lithologies of the Hamurkesen, Berdiga, and Mescitli formations, dacite porphyry and andesite porphyry are present with abundant alluvium. Based on geological, mineralogical, and geochemical features, three mineralization styles have been recognized at the Kocadal area: (i) Mineralizations around Batarya tepe include (ia) Zn mineralization within the conglomerates and sandstones of the Hamurkesen formation and (ib) Zn-Pb-Ag-(Au) mineralization associated with dacite prophyry, (ii) Au mineralization, which occurs to the southwestern of Batarya tepe, and (iii) Cu mineralizations related to quartz veins and veinlets at Gözelerin Dere. Mineralized gravels within the conglomerates contains mainly sphalerite and pyrite, whereas hydrothermal mineralizations associated with porphyritic dacite comprise pyrite and sphalerite, with minor galena, chalcopyrite, pyrrhotite, arsenopyrite, marcasite, fahlerz, pyrargyrite, and proustite. Alteration patterns of hydrothermal mineralization in the field, from older to younger, are classified as: (i) tremolite-actinolite±garnet, (ii) quartz-sericite-chlorite, and (iii) carbonate-quartz. Mineralized gravels within the conglomerates contains mainly sphalerite and pyrite, whereas pyrite, chalcopyrite, and galena are common in quartz veins at Gözelerin Dere. Geostatistical studies based on the results of geochemical analysis of core samples reveal the presence of the distinct element associations for the different styles of mineralizations.

Geochemistry and geochronology of late Mesozoic volcanic rocks in the northern part of the Eastern Pontide Orogenic Belt (NE Turkey): Implications for the closure of the Neo-Tethys Ocean

This paper presents 40Ar/39Ar and U-Pb age data, Sr-Nd isotopes, whole-rock and mineral compositions of Upper Cretaceous volcanic rocks from the Ordu area of the Eastern Pontide Orogenic Belt (EPOB) in northeastern Turkey. The volcanic rocks exhibit a wide compositional range: basalt, basaltic-andesites, andesites and a rhyodacite suite; they are characterized by subparallel light rare earth element (LREE)-enrichment, relatively flat heavy rare earth element (HREE) patterns with Eu anomalies and moderate fractionation [average (La/Yb)N=8.55]. The geochemical results show that the volcanic rocks have calc-alkaline affinity consistent with arc volcanic rocks erupted in an active continental margin. Initial 87Sr/86Sr values vary between 0.70569 and 0.70606, while initial 143Nd/144Nd values lie between 0.51244 and 0.51249. Crustal contamination affected the mantle-originated primary magma, as indicated by increased 87Sr/86Sr and decreased 143Nd/144Nd ratios with increasing SiO2. New precise laser ablation inductively coupled plasma mass spectrometer (LA-ICP-MS) 206Pb–238U age analyses of zircon and 40Ar/39Ar age data of plagioclase from the volcanics enable a more precise reconstruction of the EBOP. The ages provide insight into the timing of arc formation in this region, constrain the volcanic activity between 86 My (Coniacian) and 75 My (Campanian) and constrain the timing of closure of the Neo-Tethys.

Constraints of C–O–S isotope compositions and the origin of the Ünlüpınar volcanic-hosted epithermal Pb–Zn ± Au deposit, Gümüşhane, NE Turkey

The Eastern Pontide Orogenic Belt (EPOB) constitutes one of the best examples of the metallogenic provinces in on the Alpine-Himalayan belt. This study focuses on the genesis of the Ünlüpınar Pb–Zn±Au deposit in the southern part of the Eastern Pontide Orogenic Belt. The main lithological units in the study area are the Early Carboniferous Kurtoğlu Metamorphic Complex the Late Carboniferous Köse Granitoid and the Early-Middle Jurassic Şenköy Formation. The studied deposit is hosted by the Şenköy Formation, which consists predominantly of basaltic-andesitic rocks and associated pyroclastic rocks that are calc-alkaline in composition. Silicic, sulfidic, argillic, chloritic, hematitic, carbonate and limonite are the most obvious alteration types observed in the deposit site. Ore microscopy studies exhibit that the mineral paragenesis in deposits includes pyrite, chalcopyrite, sphalerite, galena, gold, quartz and calcite. Electron microprobe analyses conducted on sphalerite indicate that the Zn/Cd ratio varies between 84 and 204, and these ratios point at a hydrothermal deposit related to granitic magmas. Fluid inclusion studies in calcite and quartz show that the homogenization temperature of the studied deposit ranges between 90–160°C and 120–330°C respectively. The values of sulfur isotope analysis of pyrite, sphalerite and galena minerals vary between 1.6‰ and 5.7‰, and the results of oxygen and carbon isotope analysis range between 8.4‰ and 18‰ and −5‰ and −3.6‰, respectively. The average formation temperature of the ore was calculated as 264°C with a sulfur isotope geothermometer. All of the data indicate that the Ünlüpınar deposit is an epithermal vein-type mineralization that was formed depending on the granitic magmatism.

Seismic b-Values, Bouguer Gravity and Heat Flow Data Beneath Eastern Anatolia, Turkey: Tectonic Implications

In this paper, we analyze the relationships between the seismic b-values, Bouguer gravity and heat flow data in the Eastern Anatolia region of Turkey. For this purpose, spatial distributions of b-value, Bouguer gravity and heat flow have been presented for different depths and locations. In distinction to previous studies which have used only two parameters (gravity and seismic b-value or heat flow and seismic b-value), we have combined seismic b-values, Bouguer gravity and heat flow data to determine the new results on the active tectonics of the Eastern Anatolia region. Our analysis shows that there are significant and robust correlations amidst the heat flow data, Bouguer gravity anomaly and seismic b-values. The crustal structure is thick in areas where the large negative gravity anomalies and low b-values are observed. On the contrary, the regions with positive gravity anomalies and high b-values are likely to be associated with magma chambers or crustal low-velocity zones. We also provide some evidence suggesting that high b-values and high heat flow values can be related to the magmatic activities beneath the volcanic chain in the Eastern Pontide orogenic belt. Consequently, we have reached some conclusions for the Eastern Anatolia region: (1) The Moho to surface is rather thick and earthquakes are relatively smaller beneath the volcanic chain where the high heat flow values are observed, (2) a southward subduction model could have existed for the development of the Pontides during the late Mesozoic–Cenozoic era, (3) hot and unstable mantle lid zones or a lithosphere deprived of mantle under the study region is much more plausible, (4) a southward movement of the subduction plate and a northward extension of the Black Sea increase the state of stress along the trench axis and decrease the b-value, and (5) these movements may load the stress energy to the fault zones, thereby causing the catastrophic earthquakes in the Eastern Anatolia region.

Mineralogy and trace-element geochemistry of sulfide minerals in hydrothermal chimneys from the Upper-Cretaceous VMS deposits of the eastern Pontide orogenic belt (NE Turkey)

Numerous active hydrothermal chimneys have been discovered on the modern ocean floor, and the structural, textural, and mineralogical features of these chimneys have been described in detail. However, fossil chimneys have not been widely found or studied in detail. Paleo-hydrothermal chimneys were first discovered in Late Cretaceous massive sulfide deposits in the eastern Black Sea district. The well-preserved chimney fragments typically contain distinct concentric sulfide zones. The outer zones of the chimneys contain mainly pyrite and sphalerite and lesser amounts of chalcopyrite. The sulfides within the inner zone consist predominantly of chalcopyrite, with lesser amounts of pyrite and sphalerite. The axial conduits are commonly dominated by barite gangue, with variable amounts of fahlore, sphalerite, chalcopyrite, and galena. The sulfide-rich chimney zones are also characterized by significantly higher metal contents (e.g., Cu, Zn, Fe, Se, Te, Au, Ag). Laser-ablation ICP-MS studies indicate that there is a distinct systematic trace element distribution throughout the horizontal sections of the chimneys. In general, Mn, Co, Ni, Tl, U, and V are enriched in the outermost zones of the chimneys and decrease toward the interior zones. Elements indicative of higher temperature conditions, including Mo, Se, Sn, and Cu, are generally enriched in the inner zones of the chimneys. The highest contents of most of the trace elements are present within the outer walls of the chimneys, suggesting rapid precipitation in high-gradient conditions.These chimney fragments are important evidence of hydrothermal vents in the Cretaceous ocean floor in the eastern Pontides. The presence of mineralized chimney fragments in the massive sulfide deposits of the eastern Pontides and their trace element contents also provides significant data for understanding the physicochemical conditions and depositional histories of massive sulfide mineralizations on the seafloor.

Petrogenesis of the post-collisional volcanic rocks from the Borçka (Artvin) area: Implications for the evolution of the Eocene magmatism in the Eastern Pontides (NE Turkey)

Whole-rock geochemistry, 40Ar–39Ar data, and Sr–Nd isotopes are presented for the Eocene Borçka volcanic rocks on the eastern corner of the eastern Pontide orogenic belt (NE Turkey). Borçka (Artvin) volcanic rocks are divided into three suites—basic dyke, Borçka basalt, and Civanköy suite. These rocks contain plagioclase (An49–88), clinopyroxene (Wo38–49En38–54Fs8–25), hornblende (Mg#=0.57–0.74) phenocrysts, and magnetite/titanomagnetite and apatite microphenocrysts. 40Ar–39Ar ages on hornblendes, ranging from 46.9±0.1 to 39.9±0.5Ma, within the middle Eocene. The volcanic rocks show tholeiitic-calc-alkaline affinities and have low-to-medium K contents. They are enriched in large ion lithophile (LILE) and light rare earth elements (LREE), with pronounced depleted high field strength elements (HFSE). The chondrite-normalized REE patterns (Lacn/Lucn=1–19) show low-to-medium enrichment, indicating similar sources for the rock suite. Initial 87Sr/86Sr values [(87Sr/86Sr)46Ma] vary between 0.70423 and 0.70495, while initial 143Nd/144Nd values [(143Nd/144Nd)46Ma] lie between 0.51263 and 0.51285. The main solidification processes involved in the evolution of the volcanics consist of fractional crystallization, with minor amounts of crustal contamination. All our evidences support the conclusion that the parental magma of the rocks probably derived from an enriched mantle, previously metasomatized by fluids derived from subducted slab, in a post-collisional, extension-related geodynamic setting.

Petrogenesis and U–Pb zircon chronology of adakitic porphyries within the Kop ultramafic massif (Eastern Pontides Orogenic Belt, NE Turkey)

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 59Ma. The best-constrained crystallization age is 55.83±0.04Ma, 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.

Deciphering the shoshonitic monzonites with I-type characteristic, the Sisdaği pluton, NE Turkey: Magmatic response to continental lithospheric thinning

Large-scale late Mesozoic to early Cenozoic plutons in the Eastern Pontide orogenic belt mostly show calc-alkaline and I-type geochemical features. However, we identify the Sisdaği pluton that has shoshonitic affinity and I-type character in the region. The pluton was emplaced at shallow depths (<5km), revealed by Al-in hornblende barometry, in the northern margin of the orogenic belt, with SHRIMP zircon U–Pb age of 41.55±0.31Ma, interpreted as dating magma crystallization. It is composed of monzonite, monzodiorite and monzogabbro. The samples show low zircon saturation temperature (TZr) ranging from 603 to 769°C compared to A-type rocks. They exhibit SiO2 contents of 47.6–61.4wt.%, and high K2O+Na2O (5.0–8.8wt.%) and K2O/Na2O (0.8–1.8). All the samples are characterized by low Mg# (<47) and relatively high but variable Al2O3 content (16.9–22.2wt.%). They are also enriched in light rare earth elements (LREE), large ion lithophile elements (LILE) and depleted in high field strength elements (HFSE), with a weak negative Eu anomaly (Eu/Eu*=0.58–1.26) in mantle-normalized trace element patterns.The samples possess homogeneous initial Sr and Nd isotopic compositions, marked with low ISr=0.70376–0.70408 and εNd (42Ma)=+1.3 to +2.4. TDM ages of the samples range from 0.70 to 0.85Ga. The Pb isotopic ratios are (206Pb/204Pb)=18.64–18.72, (207Pb/204Pb)=15.51–15.58 and (208Pb/204Pb)=38.31–38.65. These geochemical features imply that the parental magma resulted from melting of chemically enriched lithospheric mantle source. In such a case, a hot upwelling asthenosphere is necessary to partially melt the lithospheric mantle in order to form the parental magma. The pluton is considered to be a post-orogenic intrusion that was emplaced in an environment of lithospheric extension, triggering asthenospheric upwelling. The thermal anomaly induced by asthenospheric upwelling resulted in partial fusion of chemically enriched subcontinental lithospheric mantle beneath the region. Then, the shoshonitic melt, which subsequently underwent fractional crystallization with minor crustal contamination, ascent to shallower crustal levels to generate a monzonitic rock series ranging from monzogabbro to monzonite. All these data combined with the regional geology suggest that the crustal thickening as a consequence of regional compression during the Paleocene changed into a lithospheric extension and thinning throughout the early Cenozoic (at ∼42Ma) in the Eastern Pontides. Hence, the middle Eocene shoshonitic I-type magmatism is a unique pluton, signifying initiation of lithospheric thinning and thus of hot asthenospheric upwelling in the region. These interpretations also argue against the presence of an early Cenozoic subduction of oceanic slab in the Eastern Pontides.

Petrochemistry and U-Pb Zircon Ages of Adakitic Intrusions from the Pulur Massif (Eastern Pontides, NE Turkey): Implications for Slab Rollback and Ridge Subduction Associated with Cenozoic Convergent Tectonics in the Eastern Mediterranean

The well-preserved magmatic arc in the eastern Pontides orogenic belt of northeastern Turkey offers critical clues on convergent margin tectonics associated with the late Mesozoic–early Cenozoic geodynamic evolution of the eastern Mediterranean region. Here we investigate the petrology, geochemistry, and U-Pb zircon chronology of adakitic intrusions from the Pulur region in the southern zone of the eastern Pontides belt. The intermediate to felsic intrusives in the Pulur region are characterized by the presence of abundant plagioclase, amphibole, and biotite phenocrysts. The rocks display extreme light rare earth element enrichment ; high Al2O3 (15.46–17.47 wt%), Na2O (3.57–8.43 wt%), Sr (324.8–1468 ppm), and La (13.9–55.8 ppm) concentrations; a high Sr/Y ratio (35–473); and low Y (3–12.6 ppm) and heavy rare earth element concentrations, closely comparing with the typical features of adakites. Zircons extracted from the Pulur adakites show euhedral crystal shapes, oscillatory zoning patterns, and high Th/U ratios (up to 3.33) consistent with their magmatic origin. Their 206Pb/238U ages show a range of 55.21–53.07 Ma, correlating the magmatism to the early Eocene. We propose a tectonic model involving the initiation of a slab rollback and simultaneous opening of a slab window during ridge subduction in a south-dipping subduction zone for the origin of the Tertiary adakitic and nonadakitic magmatism in the eastern Pontides orogenic belt. We also identify a close relation between slab rollback processes and backarc extension, which led to the opening of the Tertiary basins in the southern part of the eastern Pontide orogenic belt. The eastern Pontide orogenic belt is one of the world’s largest metallogenic provinces, and it hosts various economic mineralizations such as volcanogenic massive sulfide, epithermal gold-silver, porphyry copper-molybdenum, skarn-type, and chromite. However, the adakitic intrusions exposed in the southern part of the eastern Pontide orogenic belt are devoid of ore deposits, which leads us to propose that the Lutetian or later nonadakitic granitic intrusions exposed at the north of the Gumushane-Bayburt-Ispir line were probably responsible for the epithermal gold mineralization in this belt.

Geochemical and Sr-Nd Isotopic Characteristics of Post-Collisional Calc-Alkaline Volcanics in the Eastern Pontides (NE Turkey)

Major, trace element, K-Ar age and Sr-Nd isotopic data are presented for the Eocene Torul volcanics in the eastern Pontide orogenic belt (NE Turkey). The studied rocks are composed of basaltic andesitic, andesitic, trachyandesitic, and minor trachydacitic lavas associated with their pyroclastics. These rocks contain plagioclase (An2-44), hornblende (Mg#= 0.78-0.98), clinopyroxene (Wo43-46 En41-43 Fs10-15), biotite, quartz, and minor sanidine phenocrysts. K-Ar ages on hornblendes ages range from 43.99 (±2.59) to 33.45 (±2.32) Ma, within the Middle to Late Eocene. The volcanic rocks show calc-alkaline affinities and have medium to high K contents. They are enriched in large ion lithophile (LILE) and light rare earth elements (LREE), with pronounced depletion of high field strength elements (HFSE). The chondrite-normalized REE patterns (Lacn/Lucn= 4.0-9.8) show low to medium enrichment, indicating similar sources for the rock suite. Initial 87Sr/86Sr values vary between 0.70457 and 0.70511 and initial 143Nd/144Nd values between 0.51264 and 0.51278. The main solidification processes involved in the evolution of the volcanics consist of fractional crystallization with minor amounts of crustal contamination ± magma mixing. All evidence supports the conclusion that the parental magma(s) of the rocks probably derived from an enriched upper mantle, previously modified by subduction- induced metasomatism in a post-collisional geodynamic setting.

Migrating magmatism in a continental arc: Geodynamics of the Eastern Mediterranean revisited

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

Determination of Tectonic and Crustal Structure of the Eastern Pontide Orogenic Belt (NE Turkey) Using Gravity and Magnetic Data

The Eastern Pontide Orogenic Belt is one of the most complex geodynamic settings within the Alpine belt. Subduction polarity, which is responsible for the formation of the Eastern Pontide Magmatic Arc, is still under debate because of limited geological, geophysical and geochemical data. This orogenic belt is mainly divided into three subzones depending on lithological characteristics and facies changes as in Northern, Southern and Axial Zones from North to South. These zones are separated from each other by near-vertical faults that display the block-faulting tectonic style of this belt. In this study, the tectonic and crustal structure of the Eastern Pontides, which as yet have not been prospected by using geophysical data, has been investigated with potential field data. The horizontal gradient map obtained from gravity data shows a number of steep and gentle lineaments. It seems that these lineaments E-W, NE and NW-trending correspond to major structural zones of continental crust. Additionally, The Moho depth and Curie point depth variation maps of the Eastern Pontide Orogenic Belt have been computed with the power spectral method of the radial wavenumber carried out by using the fast Fourier transform method. As a result of this method, we estimated that the depths of the Moho and Curie point varied between 29.0 ± 1.2-47.2 ± 1.9 km and 14.3 ± 0.7-27.9 ± 1.4 km, respectively. Our findings indicate that the Moho depth generally increases from north to south in the region. However, the Curie point depth level within the crust has an undulating surface, not a horizontal one.

Late Cretaceous high‐K volcanism in the eastern Pontide orogenic belt: implications for the geodynamic evolution of NE Turkey

High‐K volcanics were produced in two different Late Cretaceous cycles during the infilling of back‐arc basins of the eastern Pontides. The first cycle, represented by shoshonitic trachyandesites and associated pyroclastics, containing high K2O (2.74–4.81 wt‐%) and Na2O (3.60–5.51 wt‐%), overlies a mid‐Cretaceous ophiolitic‐olistostromal melange formed during the rifting stage of a back‐arc basin (Neotethys). 40Ar/39Ar dating of biotite from trachyandesite at the base of the first cyle indicates that shoshonitic volcanism in the far south of the eastern Pontides started in the early Campanian. Volcanic rocks of this cycle are overlain by upper Campanian‐Maastrichtian rudist‐bearing reefal limestones. The second cycle of high‐K volcanism is represented by analcime‐bearing volcanic rocks erupted in a Neotethyan lagoonal environment. These volcanic rocks, intercalated with continental detritus, are characterized by high Na2O (3.22–7.16 wt‐%), now concentrated in secondary analcime crystals. Their K2O contents also range between 0.83 and 6.05 (wt‐%). All of these units are disconformably overlain by Eocene turbidites with a basal conglomerate. Volcanic rocks belonging to both cycles show various enrichment degrees in LILE, HFSE, LREE, and HREE with respect to primitive mantle and chondrite [(La/Lu)CN = 11.10–25.89]. Negative Nb, Ta, Zr, Hf, and Ti anomalies are typical of these subduction‐related arc magmas. Geochemical similarities between the two volcanic cycles suggest that trachyandesites and analcime‐bearing volcanics were derived from similar enriched mantle sources, and that they formed in the same geotectonic setting of a back‐arc basin environment of the eastern Pontide magmatic arc during the Late Cretaceous. In addition, Nd‐Sr and Pb isotope ratios of the investigated volcanic units indicate that their mantle melt sources were similar. The new geological, geochemical and isotopic data suggest that southward subduction of the Palaeotethyan oceanic crust can be separated into two main stages for the late Mesozoic Pontide orogenic belt. Early southward subduction ended by slab break‐off during the mid‐Cretaceous, resulting in upwelling and injection of asthenospheric mantle that caused the opening of the eastern Pontide back‐arc basin (Neotethys). Resumption of southward subduction began with intense TH–CA bimodal volcanism in the northern front of the arc during the Turonian–Coniacian. The intensity of this Late Cretaceous magmatism in the north decreased towards the south, and a transition to monogenic calcalkaline–alkaline (CA–A) Campanian andesitic volcanism developed in the southern zone. Yet farther south, andesitic igneous activity graded into the Campanian–Maastrichtian shoshonitic and ultrapotassic volcanism in the Bayburt–Maden, Tokat, Amasya, and Gümüshaciköy areas. This zoning of the Late Cretaceous arc magmatism (TH–CA, CA–A and shoshonitic–ultrapotassic, from north to south) also supports the concept of southward subduction during late Mesozoic evolution of the eastern Mediterranean region.

Two-and-Three-Dimensional Crustal Thickness of the Eastern Pontides (NE Turkey)

The Eastern Pontide orogenic belt is divided into three subzones (northern, southern and axial zones) based on present lithologies and facies associations. NE-SW-, NW-SE- and E-W-trending fault systems, which play an important role in palaeotectonics and neotectonics of the Eastern Pontides, separate these zones. Three different methods were used to estimate the Moho depth from observed gravity value, namely (i) empirical relationship between Moho depth and Bouguer anomaly; (ii) spectral analysis of the radial wave number and; (iii) by the gravity inversion method. Power spectrum, a statistical approach, is a widely used technique to determine the depth of geological sources successfully. The empirical linear relations between the Bouguer anomalies and seismically determined crustal thicknesses have been used to compute depths to the Conrad and Moho discontinuities, which are consistent with the average depths obtained from the power spectrum method. The crustal structure was also determined from the inversion of a Bouguer anomaly profile along longitude 39°. Gravity inversion results are consistent with those obtained from the empirical relations and power spectrum methods. We calculated the maximum crustal thickness of 43.8 km in the studied region by using the gravity inversion method, which also showed that crustal thickness increases from north to south.