Mineralogy, geochemistry, and genesis of bentonites in Upper Cretaceous pyroclastics of the Bereketli member of the Reşadiye Formation, Reşadiye (Tokat), Turkey

Abstract Bentonites have been found widespread within the Bereketli member of the Cretaceous Resadiye Formation, central Anatolia. These bentonites were hypothesized to have been formed by the in-situ alteration of parent rhyolitic, dacitic, and trachyandesitic pyroclastics in a shallow marine environment during early diagenesis shortly after deposition. Both Na- and Na-Ca-smectites were observed. The bentonites contained heulandite-clinoptilolite, kaolinite, plagioclase/K-feldspar, mica, quartz, opal-CT, calcite, and in some samples, goethite. The chemical analyses of smectite-rich clay fractions were montmorillonitic in composition. The average structural formula is: (Ca0.13Na0.40K0.05)(Al2.98Fe0.41Mg0.50Ti0.04Mn0.002)(Si7.90Al0.10)O20(OH)4. Smectite was formed from the devitrification of glass shards and dissolution of pyroclastic feldspars, and mica during early diagenesis under alkaline conditions. Certain whole-rock major and trace element signatures supported the formation of smectite from pyroclastic progenitors. The breakdown of pyroclastic hornblende and biotite was inferred from the enrichment in Fe2O3 and MgO in the clay fractions. The devitrification of glass to clay and the dissolution of K-feldspar and plagioclase contributed to the observed negative correlation of SiO2 vs. Al2O3, Fe2O3 + TiO2, and MgO as well as the negative Eu anomaly. These bentonites originated from rhyolitic to trachyandesitic pyroclastic rocks based on the Sc/Th and Co/Th, Th/U, Nd/Sm, Zr/Hf, La/Sc, Nb/Ta, Zr/Hf ratios. The variabilities of the UCC-normalized elemental results were most pronounced for the elements whose solubilities were high in the conversion of tuff to bentonite (alkali and alkaline earth elements). The variabilities seen in the UCC normalized lanthanide elements likely reflected the lanthanide contents of the non-clay minerals in the bentonite and pyroclastic parent materials. These bentonites are presently mined as important raw materials for the drilling and molding industries in Turkey.