Doğu pontid kuşağında Topçam, Çambaşı (Ordu-Kd Türkiye) yöresi geç mesozoyik yaşlı granitlerin petrojenezi: Yitim ortamında gelişen a tipi granit oluşumu

Abstract

Doğu Pontidler, özellikle Geç Mesozoyik ve Erken Senozoyik boyunca çok yoğun bir magmatik aktivite ile karakteristiktirler. Bu plütonik kütlelerin çoğu I-tipi granitoyid karakteri sunmaktadırlar. Bu çalışma ile, bölgede, I-tipi granitoyidler ile varlığı belirlenen A-tipi karakterli Çambaşı ve Topçam plütonlarının kaynak karakteristikleri ve jeodinamik evrimleri ortaya konulmaya çalışılmıştır. Geç Kretase (Kampaniyen) yaşlı plütonlar, granit, siyenit, kuvars monzonit ve monzonit bileşimlidirler. Yaygın olmamakla birlikte kayak kapanımları içerirler. Genellikle yüksek Ga/Al oranları ile düşük Mg# (<42) değerleri ile karakteristiktirler. Plütonlar, baskın olarak metalumin karakterli olmakla birlikte, (A/CNK=0.88 to 1.00) peralumin karaktere doğru değişim sunmaktadırlar. Şeşonitik ve ultrapotassik seri özelliği sunarlar. Kayaçlar, çoklukla örümcek diyagramlarında, HNTE?lerce zenginleşme sunarken önemi derecede negative Eu (Eu/Eu*=0.33 to 0.92), Ba, Nb, Sr ve Ti anomalileri göstermektedirler. Bu değerler, alt kabuk ve litosferik manto kaynağı ergiyikleri ile uyumludur. Tüm bulgular ile, plütonların oluşum ve evrimleri için, astenosferin yükselimi ve kısmi ergimeden once zenginleşmiş kıta altı litosferinin ergimesiyle birlikte bazik magma oluşumu modelini önerebiliriz. Oluşan bazik magma, alt kabuğun altına sokularak, kabuğun bu kesiminin ergimesine neden olur. Kıta altı derinliklerinde, oluşan her iki ergiyik bir miktar etkileşerek hibrit bir ergiyik meydana getirmişlerdir. Oluşan hibrit ergiyik fraksiyonel kristallenme ve bir miktar kirlenme ile evrimleşip, kabuğun sığ derinliklerine yükselerek A-tipi karakterli Çambaşı ve Topçam plütonlarını oluştururlar. Bu çalışma ile elde edilen tüm bulgular, söz konusu plütonların, Geç Kretase boyunca var olan yay gerisi ekstansiyonuna bağlı olarak kabuğa sokulum yapabildiklerine işaret etmektedir. Doğu Pontidlerde bu zaman dilimindeki yay gerisi ekstansiyon, Pontidlerin daha kuzeyinde bir yay gerisi havza olarak Karadenizin açılmasına imkan sağlamıştır.Anahtar Kelimeler: Hibrit A-Tipi Granitoyid, Alt Kabuki Ergimesi, Yitim, Yay-Gerisi Ekstansiyon, Doğu Pontidler, KD Türkiye

Anatolia is considered a geologically complex domain in the Alpine-Himalayan fold-thrust belt. It is a continental block migrating westward in response to the continuing north-south convergence between Eurasia and Africa-Arabia during the late Cenozoic. This convergence is accommodated by two major strike-slip faults: the North Anatolian Fault (NAF) and the East Anatolian Fault (EAF). Turkey is formed by four major tectonic blocks that are separated by three main high pressure (HP) belts . The basement rocks beneath the Eastern Pontides of Sakarya zone are early Carboniferous metamorphic terranes and late Carboniferous granitoid and late Carboniferous to early Permian shallow marine to terrigeneous sedimentary rocks. The basement rocks beneath the Eastern Pontides of Sakarya zone are early Carboniferous metamorphic terranes and late Carboniferous granitoid and late Carboniferous to early Permian shallow marine to terrigeneous sedimentary rocks. Late Triassic lamprophyre dykes and stocks are rarely observed in the Eastern Pontides. The basement rocks are overlain by early to middle Jurassic clastic and carbonate sedimentary rocks intercalated with basalts and pyroclastic rocks. This time interval is attributed to the subduction-related rifting in the region and mantle-derived basic volcanic rocks. Late Jurassic granitoids and their volcanic equivalents emplaced into the volcano-sedimentary rocks of Şenköy Formation during middle-late Jurassic. These granitoids are regarded as the products of an arc-continent collision event, in response to closure of Paleotethys during middle Jurassic and the accretion of the Pontides to Laurasia in the north. In the late Cretaceous, the region was a south facing magmatic arc due to northward subduction of Neotethys along the southern border of Sakarya Zone. The magmatic arc is characterized by a more than 2 km-thick volcano-sedimentary sequence with local granitoid intrusions in the northern part of the Eastern Pontides. The southern part represents a fore-arc phase where flyschoid sedimentary rocks with limestone olistoliths were accumulated. The early Paleocene plagioleucitites was referred to last products of the northward subduction. Paleocene time in the region is attributed to continent-continent collision between the Pontides and the Tauride-Anatolide block due to the complete closure of Neotethys. Propose a Paleocene to early Eocene (ca. 55 Ma) collision, causing crustal thickening. The Eastern Pontides have a quiescent period during the Paleoce. Early Cenozoic adakitic rocks, pointing syn- to post-collision phase have been determined in the region. Middle Cenozoic times are recorded by volcanic and granitoid rocks.These granitoid bodies formed in a post-collisional setting. Post Eocene terrigeneous units are observed in the area. Neogene alkaline volcanics are referred to post-collision extensional tectonic setting. The Eastern Pontides is characterized by the many of intrusive bodies formed throughout the late Mesozoic-early Cenozoic. Most of these show I-type granitoid character. A number of petrogenetic models have been proposed for their generation: 1) generation from basaltic magma through fractionation, 2) partial fusion of refractory lower crustal granulites, 3) partial fusion of intracrustal tonalitic I-type granitic rocks at high temperature and 4) partial fusion of a non-melt depleted, anhydrous, alkali-feldspar rich granulitic rocks. Recently, mixing source model has gained considerable favor. Late Cretaceous and middle Eocene granitoids around the Eastern Pontide region are mostly I-type, accompanied by A-type plutons.Granitoids are regarded as one of the key elements to understand petrogenetic evolution and geodynamic processes and they are divided into S-, I-, M-, and A-types based on their chemical and mineralogical features. It was originally considered A-type granitoids formed in anorogenic setting , recent studies have clearly pointed out that A-type granitoids could occur in various geodynamic setting ranging from within-plate setting to plate boundaries ). A-type granitoids have a unique compositional features such as higher K2O+Na2O than typical I-type rocks in the granitoid family and they have been extensively worked due to their unusual composition and tectonic settings Although they offer useful information on their tectonic environments, petrogenetic processes responsible for their genesis is still debated. Fundamentally, it is not known what type of tectonic process controls alkalinity in felsic systems and enrichment in the protolith prior to partial fusion. It appears that the generation of A-type granitoids may involve multiple processes, tectonic settings and mantle/crust proportions. The plutons in the area of the Eastern Pontides has an A-type composition and was emplaced in a magmatic arc. Therefore, study of the plutons can provide important clues for understanding the petrogenetic processes caused by regional extension and mantle upwelling. Also, it provides an early geological record of back-arc extensional events in the Eastern Pontides. we compare the plutons with similar outcrops across the world and present new SHRIMP zircon U-Pb ages, whole-rock geochemical results and Sr and Nd isotopic data for these rocks. (Zircon grains were extracted by standard magnetic and heavy liquid techniques. U-Pb dating was performed at the Beijing SHRIMP Laboratory, Electron microprobe analyses of mineral phases were conducted on polished thin sections of representative samples using a JEOL 8600 electron microprobe at the University of Georgia, USA. Major, trace and rare earth element contents were determined at the commercial ACME Laboratories Ltd in Vancouver, Canada. Sr and Nd isotopic analyses were carried out at the Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing).Finally, we propose a tectonomagmatic model for the generation of the A-type granitoids in the Eastern Pontides, NE Turkey. The Çambaşı and Toçam plutons are located in the southern part of the eastern Pontides and surrounded by a wide contact aureole of Cenomanian-Turonian andesitic rocks of the Çağlayan Formation and of early-middle Jurassic volcano-sedimentary rocks of the Şenköy Formation. The pluton forms outcrops with a length of less than 17 km and a maximum width of 8 km. It is a part of the composite Kaçkar Batholith, which yielded various radiometric ages to constrain the emplacement of granitoid bodies along an E-W trend in the Eastern Pontides. The plutons are made of a variety of rock types, including granite, syenite, quartzmonzonite and monzonite. All of the rock units share several common petrographic and textural features and are therefore described together. All of the contact relations between rock types are transitional. This study explains source characteristics and geodynamic setting of Çambaşı and Topçam A-type plutons showing spatial relationships with I-type granitoid from the region. Late Cretaceous plutons are composed of granite, syenite and quartz monzonite and monzonite with rarely mafc microgranular enclaves. The rocks have elevated Ga/Al ratios and low Mg# (<42). The plutons are dominantly metaluminous (A/CNK=0.88 to 1.00) to peraluminous. They belong to shoshonitic and ultra-potassic series. In spidergrams the rocks are enriched in LREE and exhibit significant negative Eu (Eu/Eu*=0.33 to 0.92), Ba, Nb, Sr and Ti anomalies, suggesting to lower crustal parental magma with a contribution of lithospheric mantle. We hypothesize that upwelling of asthenosphere triggers melting of chemically enriched mantle and basic magma formed. Melting of lower crust was provided by the underplated magma. Mixing between lower crust- and mantle-derived melts 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 at depths of lower crust. Then the hybrid melt ascended to shallower crustal level and underwent a limited fractionation process to generate a variety of rock types. Our data suggest that the A-type pluton likely formed at an extensional environment of active continental margin throughout the late Cretaceous. Such an extensional event led to opening of Black Sea as a back-arc basin further north of the Eastern Pontides. We performed U-Pb age, mineral chemical analyses, major-trace elements, and Sr-Nd isotopic determinations in order to assess the origin of A-type pluton from the Eastern Pontides and the tectonic setting in which it formed.Based on the all data existed, we suggest that upwelling of asthenosphere triggers melting of chemically enriched mantle and basic magma formed. Melting of lower crust was provided by the underplated magma. Mixing between lower crust- and mantle-derived melts 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 at depths of lower crust. Then the hybrid melt ascended to shallower crustal level and underwent a limited fractionation process to generate a variety of rock types. Our data suggest that the A-type Çambaşı ve Topçam pluton likely formed at an extensional environment of active continental margin throughout the late Cretaceous. Such an extensional event led to opening of Black Sea as a back-arc basin further north of the Eastern Pontides.Keywords: Hybrid A-Type Granitoid, Lower Crustal Melting, Subduction, Back-Arc Extension, Eastern Pontides, NE Turkey