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

Abstract K-bentonite (tephra) layers are exposed as thin beds within Late Devonian-Early Carboniferous carbonates of the Yilanli formation at four different locations in northwestern, Turkey. Clays separated from K-bentonites in the Gavurpinari, Yilanli Burnu (Bartin) and Cimsir Cukurlari (Sapca) quarries and the Gudullu-Gokgol 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 Gavurpinari and Yilanli 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, Δ°2θ) and polytype data indicate high-grade diagenesis for illite-bearing site, and low-grade diagenesis for I-Sm-bearing sites. Lattice d060 values (Ǻ) 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 Illites have relatively lower tetrahedral Al substitution and higher octahedral Fe and Mg substitutions compared to those of I-Sm. Illites with phengitic composition occur in the Gavurpinari and Yilanli Burnu sites, whereas muscovitic composition at Sapca Cimsir Cukurlari site. Chondrite-normalized rare earth element patterns for the Gavurpinari and Yilanli Burnu sites exhibit similar trends, with relatively higher values when compared to trends for the Sapca Cimsir Cukurlari sites. Oxygen (δ 18 O) isotope values of illites and I-Sm range from 17.7 to 21.9‰ (V-SMOW), whereas hydrogen (δD) isotope values range widely from − 10.1 to − 69.9‰ (V-SMOW). The depleted values of δ 18 O for the Gavurpinari site imply geologically sudden crystallization under higher temperature conditions. K/Ar ages of different size illite fractions indicate the presence of older detrital illites (2 μm) together with younger diagenetic fractions (0.5 μm) that correspond with an increase of 2 M 1 polytype in the coarser fraction. Illitization ages of K-bentonites in the Bartin area indicate an Early Permian event corresponding to the Variscan orogeny, whereas the illitization of K-bentonites in the Zonguldak area is Early Jurassic in age, related to the Cimmerian deformation in the region.