Petrogenesis of volcanic rocks, and clay mineralogy and genesis of underclays, Şile Region, Istanbul, Turkey

In the Şile Region, the basement rocks are represented by Lower-Middle Devonian limestones, gradually grading upward into shale; Mesozoic rocks composed of Lower Triassic basal conglomerates and sandstones, and Upper Cretaceous volcanic rocks (andesite, basalt, basaltic andesite and pyroclastics) with biomicritic limestones. Cenozoic rocks include Eocene marls, sandstones, shales with limestone blocks. Weathered volcanic rocks are the major parent rocks of the Neogene sequence composed of clays, sands, gravels and of Quaternary alluvial deposits. Three coal seams occur within the Upper Miocene succession with gray-beige to greenish underclay, they occurred cyclically in swamp environment due to fluctuation of lake water level that provided the necessary humic and fulvic acids for the weathering of highly altered calc-alkaline (andesite) volcanic rocks into clay minerals. Geochemical discrimination diagrams revealed that calc-alkaline differentiation series extend from dacite to rhyolite. Primary weathering of andesitic rocks took place in the inorganic system and secondary alteration occurred after transportation and deposition in the organic system in swamp environment, therefore, all major and trace, and most of REE's were mobile. The underclay layers consist of disordered kaolinite, illite, mixed-layer illite-smectite, smectite, gibbsite, quartz and organic matter. They are intensively used as raw material in floor tile manufacturing by ceramic industries.