ALT ORDOVİSİYEN ÖNCESİ YAŞLI YAY MAGMATİZMASINA KUZEY TÜRKİYE'DEN BİR ÖRNEK: ÇAŞURTEPE FORMASYONUNUN JEOKİMYASAL İNCELENMESİ (BOLU, B PONTİDLER).

Batı Pontidler'de, Bolu-Yedigöller arasındaki sınırlandırılmış alanda, bölgedeki Paleozoyik yaşlı kayaçlar için temel oluşturan üç farklı birim yüzeylenir. Bunlar tabandan tavana : i) yüksek dereceli metamorfik kayaçlar (Sünnice grubu), ü) granitoyidler ve iii) granitoyidlerin tavan kesimlerinde sıcak dokanaklar ile kestikleri bir volkanik istifden (Çaşurtepe formasyonu) oluşmaktadır. Granitoyidler, Bolu Granitoyid Kompleksi (BGK) olarak adlandırılan bir grup intrüzyonunun bölgedeki uzantılarıdır ve Çaşurtepe formasyonu ile birlikte Sünnice grubunun üzerinde KD-GB gidişli, KB'ya eğimli bir tektonik hat boyunca yüzeylenirler. Granitleri kesen bol oranda lamprofir daykları mevcuttur. Bu makalenin esas konusunu oluşturan Çaşurtepe formasyonu, taban düzeylerinde genellikle andezit bileşimindeki lavlardan, üst kesimlerinde ise riyolit bileşimindeki volkaniklastiklerin egemen olduğu bir ignimbrit serisinden kuruludur. Volkanik kayaçlar ve granitoyidler yeşil şist fasiyesinde metamorfizmaya uğramış ve bunun etkisiyle, korunmuş birincil mineralerinin yanında albit+epidot+klorit+aktinolit mineral parajenezi gelişmiştir. Volkanik serinin masif lav düzeyleri, yüksek SiO2 içerikli (>%54), kalk-alkali andezit ve yerel olarak da dasit-riyodasit bileşimindedir. N-tipi MORB'a göre LIL element zenginleşmeleri ve LREE (La, Ce, Nd) oranla Nb fakirleşmeleri ile karakteristiktirler. Granit içi dayklar da benzer kimyasal özellikler sunarlar. Çaşurtepe formasyonundan izotop analizi yapılan bir adet örneğin 87Sr/86Sr 550 milyon yıl model değeri 0.706482, 143Nd/144Nd model değeri 0.512450, eNd değeri ise 10.2'dir. Volkanik kayaçların majör- ve iz-element karakteristikleri yitim zonu üzerinde gelişmiş kalk-alkali karakterli bir yay volkanizmasının ürünü olduklarına işaret ederken, 143Nd/144Nd-87Sr/86Sr izotop değerleri ile de MORB'ı karakterize eden değerlerden ayrılarak okyanus içi ada yayları ile uyumlu görünmektedir. Sünnice grubu, Çaşurtepe formasyonu ve BGK'dan oluşan yüksek ve düşük dereceli metamorfik kayaçlar inceleme bölgesi içinde "istanbul Paleozoyik lstifi"nin tabanında yer alan Alt Ordovisiyen yaşlı kırıntılı birimler ile açısal uyumsuzlukla örtülmektedir. Bu nedenle de bu çalışma ile Batı Pontidler'de Alt Ordovisiyen öncesinde yitime bağlı bir magmatizmanın gelişmiş olduğu sonucuna varılmıştır

Tectonic significance of Late Ordovician granitic magmatism and clastic sedimentation on the northern margin of Gondwana (Tavşanlı Zone, NW Turkey)

<p>Metasedimentary schists and a small crosscutting metagranite underlie a Mesozoic metamorphosed carbonate platform within the regional Anatolide tectonic belt of northern Turkey. These lithologies are inferred to have formed part of the northern margin of Gondwana during the Palaeozoic. Immobile element geochemistry suggests that the country rock metasediments were derived from upper continental crust and arc-type magmatic rocks. Major and trace element analyses of the metagranite indicate affinities with upper continental crust or a continental margin magmatic arc. Tectonic discrimination of the metagranite is consistent with a rift or post-collisional setting. Niobium depletion relative to Ce on primitive mantle-normalized spidergrams suggests a subduction influence, possibly inherited from Late Precambrian (Cadomian) arc magmatism. Ion microprobe U–Pb dating of zircons from the metagranite yielded a crystallization age of 446 ± 8 Ma (Late Ordovician). An inherited core gave a concordant age of <em>c</em>. 578 Ma, consistent with Cadomian or Pan-African basement. Regional comparisons indicate that the Palaeozoic lithostratigraphy is similar to that of the Taurides (Gondwana) rather than the Pontides (Laurasia). We infer Mid- to Late Ordovician crustal extension along the north Gondwana margin, followed by spreading of Palaeotethys. The entire stratigraphy experienced high-pressure/low-temperature metamorphism during Late Cretaceous closure of the İzmir–Ankara–Erzincan ocean. </p

Constraints on Variscan and Cimmerian magmatism and metamorphism in the Pontides (Yusufeli–Artvin area), NE Turkey from U–Pb dating and granite geochemistry

<p>Metamorphic and igneous rocks exposed in NW-vergent thrust sheets and their autocthonous basement in the NE Pontides were dated by the U–Pb method using zircons, supported by geochemical data for granitic rocks. Two meta-sedimentary units (Narlık schist and Karadağ paragneiss) yielded detrital zircon populations of 0.50–0.65 and 0.9–1.1 Ga, suggesting an affinity with NE Africa (part of Gondwana). The youngest concordant zircon age is Ediacaran for the schist but Devonian for the paragneiss, bracketing the paragneiss depositional age as Mid-Devonian to Early Carboniferous. Metamorphic rims of zircon cores in the paragneiss gave Carboniferous ages (345–310 Ma). The zircon rim data indicate two Variscan metamorphic events (334 and 314 Ma) separated by a hiatus (320–325 Ma). Granite emplacement took place during early Carboniferous, Early Jurassic and Late Jurassic phases. The crystallization age of the early Carboniferous granites (<em>c.</em> 325 Ma) corresponds to a hiatus in the zircon age data that could reflect subduction slab break-off. The Variscan granitic rocks intruded a Gondwana-derived continental terrane that was loosely accreted to Eurasia during early–late Carboniferous time but remained isolated from Eurasian-derived terrigenous sediment. In contrast, the Jurassic granitic magmatism relates to later back-arc extension along the southern margin of Eurasia. </p

Tectonic significance of Late Ordovician granitic magmatism and clastic sedimentation on the northern margin of Gondwana (Tavşanlı Zone, NW Turkey)

Metasedimentary schists and a small crosscutting metagranite underlie a Mesozoic metamorphosed carbonate platform within the regional Anatolide tectonic belt of northern Turkey. These lithologies are inferred to have formed part of the northern margin of Gondwana during the Palaeozoic. Immobile element geochemistry suggests that the country rock metasediments were derived from upper continental crust and arc-type magmatic rocks. Major and trace element analyses of the metagranite indicate affinities with upper continental crust or a continental margin magmatic arc. Tectonic discrimination of the metagranite is consistent with a rift or post-collisional setting. Niobium depletion relative to Ce on primitive mantle-normalized spidergrams suggests a subduction influence, possibly inherited from Late Precambrian (Cadomian) arc magmatism. Ion microprobe U-Pb dating of zircons from the metagranite yielded a crystallization age of 446 +/- 8 Ma (Late Ordovician). An inherited core gave a concordant age of c. 578 Ma, consistent with Cadomian or Pan-African basement. Regional comparisons indicate that the Palaeozoic lithostratigraphy is similar to that of the Taurides (Gondwana) rather than the Pontides (Laurasia). We infer Mid-to Late Ordovician crustal extension along the north Gondwana margin, followed by spreading of Palaeotethys. The entire stratigraphy experienced high-pressure/low-temperature metamorphism during Late Cretaceous closure of the Izmir-Ankara-Erzincan ocean

Lutetian arc-type magmatism along the southern Eurasian margin: New U-Pb LA-ICPMS and whole-rock geochemical data from Marmara Island, NW Turkey

The rocks of Turkey, Greece and Syria preserve evidence for the destruction of Tethys, the construction of much of the continental crust of the region and the formation of the Tauride orogenic belt. These events occurred between the Late Cretaceous and Miocene, but the detailed evolution of the southern Eurasian margin during this period of progressive continental accretion is largely unknown. Marmara Island is a basement high lying at a key location in the Cenozoic Turkish tectonic collage, with a Palaeogene suture zone to the south and a deep Eocene sedimentary basin to the north. North-dipping metamorphic thrust sheets make up the island and are interlayered with a major metagranitoid intrusion. We have dated the intrusion by Laser Ablation ICP-MS analysis of U and Pb isotopes on zircon separates to 47.6 ± 2 Ma. We also performed major- and trace-elemental geochemical analysis of 16 samples of the intrusion that revealed that the intrusion is a calc-alkaline, metaluminous granitoid, marked by Nb depletion relative to LREE and LIL-element enrichment when compared to ocean ridge granite (ORG). We interpret the metagranitoid sill as a member of a mid-Eocene magmatic arc, forming a 30 km wide and more than 200 km long arcuate belt in NW Turkey that post-dates suturing along the İzmir-Ankara-Erzincan Suture zone. The arc magmatism was emplaced at the early stages of mountain building, related to collision of Eurasia with the Menderes-Taurus Platform in early Eocene times. Orogenesis and magmatism loaded the crust to the north creating coeval upward-deepening marine basins partially filled by volcanoclastic sediments.P. Ayda Ustaömer, Timur Ustaömer, Alan S. Collins and Jörg Reischpeitsc

Implications of U–Pb and Lu–Hf isotopic analysis of detrital zircons for the depositional age, provenance and tectonic setting of the Permian–Triassic Palaeotethyan Karakaya Complex, NW Turkey

New zircon U-Pb age data, combined with Lu-Hf isotopic data, are presented here for sandstones of mainly arkosic composition from the Permian-Triassic Karakaya Complex. Predominantly, Carboniferous, Triassic and Devonian zircon age groups are recognised, most of which have a Late Triassic (Carnian-Norian) maximum depositional age. Carboniferous- and Devonian-aged zircon populations exhibit intermediate epsilon (Hf(t)) values (-11 to +2), consistent with formation in a continental margin arc setting where juvenile mantle-derived magma mixed with (recycled) old crust of Palaeoproterozoic Hf model age. In contrast, the Triassic-aged zircon population exhibits higher epsilon (Hf(t)) values (-5 to +4), consistent with mixing of juvenile mantle-derived melts with (recycled) old crust of Neoproterozoic Hf model age. Potential igneous source rocks for the sandstones of the Karakaya Complex exist in the Devonian and Carboniferous granitic rocks of the Sakarya continental basement to the north. Their epsilon (Hf(t)) and corresponding model ages are nearly identical to the age-equivalent zircon populations within the Karakaya Complex sandstones. However, the Triassic granitic rocks of the Sakarya continental crust differ significantly in epsilon (Hf(t)) and corresponding model age from the sandstones of the Karakaya Complex. Late Triassic sandstones from the Tauride continental unit to the south lack the dominant Late Palaeozoic and Triassic zircon populations of the Karakaya Complex sandstones. Triassic granitic bodies and intermediate-composition extrusive rocks in the Tauride continental unit also differ in epsilon (Hf(t)) and corresponding Hf model ages from the Karakaya Complex sandstones. In addition, Late Triassic sandstones of the Kocaeli Triassic unit (A degrees stanbul Terrane) in the north differ strongly from the Karakaya Complex sandstones in zircon population ages and epsilon (Hf(t)). In the regional context, the new zircon age and lutetium-hafnium isotopic data are consistent with derivation of the Late Triassic Karakaya Complex sandstones from a Late Palaeozoic-Triassic continental margin arc located somewhere along the southern margin of Eurasia, although its exact position cannot be pinpointed at present owing to lack of suitable outcrop and comparable isotopic data