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Illitization of Late Devonian-Early Carboniferous K-bentonites from Western Pontides, NW Turkey: Implications for their origin and age

K-bentonite (tephra) layers are exposed as thin beds within Late Devonian-Early Carboniferous carbonates of the Yilanh formation at four different locations in northwestern, Turkey. Clays separated from K-bentonites in the Gavurpman, Yilanh Burnu (Barth) and Cimsir Cukurlari (Sapca) quarries and the highway tunnel section near Zonguldak were analyzed by X-ray diffraction, optical, scanning and high-resolution transmission electron microscopy and inductively coupled plasma-mass spectrometry. The clay mineralogy is dominated by illite and mixed-layer illite-smectite (I-Sm) along with subordinate amounts of kaolinite, dolomite, calcite, quartz, feldspar, and gypsum. Morphologically, platy shaped illite is the major clay mineral in the Gavurpman and Yilanh Burnu sites, while sponge-like to platy shaped mixed-layer illite-smectites occur in the Sapca Cimsir Cukurlari and Gokgol sites. Illite Kubler index (KI, Delta degrees 20) and polytype data indicate high-grade diagenesis for illite-bearing site, and low-grade diagenesis for I-Sm-bearing sites. Lattice d060 values (A) of illite and I-Sm reflect a dioctahedral composition, with relatively larger d060 values in the Yilanli Burnu site, which is related to Mg incorporation into the octahedral layer from dolomitic limestone host-rocks