İnceleme alanı bölgesel tektonik birliklerden Pontidler’in batı bölümünde (Batı Pontidler) içinde yeralır İstanbul’un kuzeyinde boğazın her iki yakasında, doğu-batı uzanımlı olan inceleme alanında stratigrafik olarak tabanda Paleozoyik ve Triyas yaşlı sedimanter kökenli temel kayaları (alt çökel istif) bulunur. İnceleme konusu olan Üst Kretase yaşlı volkanik topluluk temel kayalarıyla tektonik dokanaklı olup, batıda Kısırkaya (Sarıyer)’den doğuda Şile’ye kadar olan alanda ve genellikle Karadeniz’e kıyı alanlarda yüzlekler verir. Temel kayaları ve volkanitler üzerinde diskordan olarak bulunan Tersiyer yaşlı kayalar ise bölgedeki üst çökel istifi oluşturur. Volkanitler İntra Pontid zonu veya muhtemelen İzmir-Ankara zonu boyunca gelişmiş olan okyanusal litosferin Üst Kretase’de kuzeye doğru İstanbul zonu altına dalmasıyla oluşan Pontid magmatik yayı olarak gelişmiştir. Volkanitler alttan üste Bozhane, Sarıyer, Garipçe ve Kısırkaya formasyonlarından oluşur. Volkanitler; bazalt, bazaltik andezit, andezit, dasit ve riyodasit bileşiminde volkanoklastikler, lavlar, epiklastik çökeller ve bu birimleri değişik açılarda kesen aynı bileşimli dayklardan oluşur. Garipçe formasyonu içinde yaygın bulunan blok-kül akıntıları, matriks destekli veya tane destekli, kütle akıntıları şeklinde izlenir. Bu özellikleriyle volkanik ürünlerin tek bir volkanik bacadan değil de çoklu bacalardan ve dayk/dom sistemlerinden türedikleri söylenebilir. Volkanitlerde kuzeye, istifin üstüne doğru gittikçe bazik karakter artar. Volkanitlerde genellikle hyaloplitik, hyaloporfirik ve mikrolitik porfirik dokular izlenir. Volkanitlerin baskın mineralleri plajioklas ve piroksen (ortopiroksen+klinopiroksen)’dir. Bunların yanında bazı kaya örneklerinde, biyotit, amfibol, olivin, birincil kuvars ve opak (manyetit+ilmenit) minerallerine de rastlanır. İkincil mineral olarak kalsit, epidot, klorit, kuvars, kil mineralleri ve hematit gelişmiştir. Anahtar Kelimeler: Volkanoklastik, üst kretase, Garipçe, Şile.The study area is located approximately 10 km to the north of İstanbul. This region forms part of the Istanbul Zone in the western Pondides. The basement rocks in the region are represented by Paleozoic to Early Carboniferous sandstone – shale succession, discordantly overlain by Triassic sedimentary rocks. During the Upper Cretaceous the northern branch of Neotethys was subducting beneath the Istanbul zone and gave rise to magmatic arc volcanism. This volcanism produced different type of volcanic rocks such as basalt, basaltic andesite, andesite, dacite and rhyodacite and less rhyolite. Volcanic seguences exposed especially along the east and west coasts of the Black Sea in the North of Istanbul. The basement rocks are overlain by Upper Cretaceous volcanic rocks unconformably. The uppermost sedimentary facies composed of gravel, sand, silt and clay deposits Neogene in age overlay the basement rocks and Upper Cretaceous volcanic association. Volcanic association in the region have been stratigraphically differentiated four different formations from the bottom to the top. These are: Bozhane, Sarıyer, Garipçe and Kısırkaya formations. Bozhane formation comprises of turbidites siliciclastic sedimentary rocks and lava blocks on the top of it. It is 280 m. in thickness. This formation is seen only the east part of study area. Sarıyer formation comprises felsic and intermediate lava, flow breccias and layered tuffs. It has outcrops both in the west and east part of the study area. It passes Garipçe formation to the top conformably. Garipçe formation consists of mostly volcaniclastics flow units, less lava and dyke systems cutting all these rocks. Garipçe formation is followed by Kısırkaya formation in the uppermost of the volcanites. Lithologically, volcanic rocks are composed of piroklastik breccia, flow breccia, hyloclastite, pyroclastic flow deposits, felsic tuff and volcanic originated sandstone with limestone interbedded in accordance with their facial features and field characteristics. Flow deposits comprise of grain supported mass flows and matrix supported mass flows. These flow units are commonly poorly sorted and sometimes ununiformly bedded. They are deposited as a mixture of blocks and fine grained matrix. Each flow unit represents an individual pyroclastic flow deposit travelled downslope flowing from side of dome or from the side vents and filled mostly valleys. In this study, the stratigraphic facial features have been identified according to the field characteristics of lavas, dykes, volcanoclastics and volcanogenic sedimentary deposits. Lava flows are less than volcaniclastics in the study area. Volcanic sequence represents different thickness in different localities in response to the paleotopography, the type of volcanism, and its effusive characteristics. Lavas have different texture and mineralogical characteristics. The variety of texture is related to the pressure and temperature conditions during the crystallisation of magma and its composition. Volcanics show hyaloplitic, hyaloporphyritic, microlitic porphyritic textures. Most of them are highly hydrothermally altered according to the alteration degree, texture and mineralogical composition and lost their original textures. Most of the mineralogical composition of basalts are characterized by calcic plagioclase feldspar and pyroxene (orthopyroxene + clinopyroxene) + olivine + Fe-oxides. The most common phenocryst assemblage of andesitic and basaltic andesitic lavas are: plagioclase + orthopyroxene + clinopyroxene + Fe-Ti oxides. Some andesites contain amphibole and/or biotite. In the dacites, the predominant phenocryst assemblage is plagioclase + amphibole + biotite + quartz + Fe-Ti oxides. Plagioclase phenocrysts are usually clouded by alteration and show resorbed cores, secondary overgrowths and normal or reverse zoning, indicating disequilibrium processes. Biotite and quartz have undergone magmatic corrosion.in some rock types.In some rock samples biotite is covered by opaque minerals espcially rim of biotite phenocrysts. Most samples have a fine-grained glassy groundmass including small grains of plagioclase and pyroxene microlitesand some magnetit. Epidote, calcite, serisite, chlorite, quartz and hematite some clay minerals are the secondary minerals. Alterationproducts have been developed in response to the infiltrating hydrothermal solutions. Alteration type is mostly related to the textural and mineralogical character of the parent rock. The volcanic rocks served as the parent rock where the residual kaolin and sedimentary clay deposits were formed. Keywords: Volcanoclastics, upper cretaceous, Garipçe-Sarıyer, Şile
