Nature of clays in Late Cretaceous dacitic rocks in the eastern Sakarya Zone (NE Turkey): a geochemical and isotopic approach

Authors: FERKAN SİPAHİ, MEHMET ALİ GÜCER, MÜNÜR BURHAN SADIKLAR

Abstract: Late Cretaceous felsic rocks are common in the eastern Pontides (NE Turkey). These rocks developed as an island arc from the Jurassic to the Miocene; they also host numerous volcanogenic massive sulfide deposits. The Late Cretaceous dacites that outcrop around Zigana Mountain (Gümüşhane, NE Turkey) were exposed to intensive hydrothermal alteration. Widespread hydrothermal alteration of Late Cretaceous dacite in Zigana Mountain has led to the formation of well-developed clay minerals. Their main alteration products are sericite, chlorite, and carbonates (ankerite and calcite). The clay minerals identified by X-ray diffraction and differential thermal analysis include illite, chlorite, small amounts of kaolinite, and smectite. The polytype of illite is 2M₁. Illites formed as a result of hydrothermal alteration of feldspars; they are also Fe- and Mg-poor. Chlorite is characterized by decreasing Fe/(Fe+ Mg) ratio with increasing alteration grade and has a trioctahedral structure. Chlorite geothermometer calculations (100-300°C) reflect the activities of hydrothermal solutions. Chlorite forms in more alkaline conditions compared with illite, as its formation is associated with the presence of carbonates. Oxygen and hydrogen stable isotope values, K-Ar dating of illites, and textural and chemical evidence suggest that clays might have formed mainly by alteration of minerals in dacitic rocks with hydrothermal fluids of magmatic origin during the Campanian under acidic?weak alkaline conditions.

Keywords: Clay minerals, Late Cretaceous altered dacites, hydrothermal alteration, K-Ar dating, eastern Pontides (NE Turkey)

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