Geological features and geochemical characteristics of Late Devonian-Early Carboniferous K-bentonites from northwestern Turkey

摘要

Newly discovered K-bentonite beds, interstratified with limestones/dolomitic limestones of the Upper Devonian-Lower Carboniferous Yilanli Formation, are exposed in the northwestern Black Sea region of Turkey, around Zonguldak and Bartin. K-bentonite samples collected from four different locations: the Gavurpinari and Yilanli Burnu quarries from the Bartin area, the Cimsir Cukurlari quarry from the Sapca area, and the Gudullu and Gokgol highway tunnel section near Zonguldak city were investigated using optical microscopy, X-ray diffraction and inductively coupled plasma mass spectrometry in order to reveal their mineralogical and geochemical characteristics and understand their origin and evolution. The K-bentonites occur at different levels in the Yilanli Formation as 2-40 cm-thick, greenish to yellowish beds cropping out several hundred metres along strike. Preliminary biostratigraphic data suggest that the protoliths of the Bartin (Gavurpinari and Yilanli Burnu) and Gudullu K-bentonites were deposited at around the boundary between the Frasnian and Famennian, whereas those in the Sapca and Gokgol sections are slightly younger (Devonian-Carboniferous boundary interval). The lithofacies types of the host carbonate rocks suggest an 'epeiric' shallow carbonate platform environment. Illite and mixed-layer illite-smectite were the major clay minerals in the K-bentonites. The K-bentonites from the Bartin area display a high degree of illitization and consist mainly of illite indicating high-grade diagenesis, whereas illite-smectite-rich samples from the Sapca and Gokgol tunnel locations reflect relatively lower diagenetic conditions. According to their geochemical compositions, two groups of K-bentonites were distinguished, one with alkali basalt (Bartin area and Gudullu locations) and one with trachyte affinities (Gokgol tunnel and Sapca locations). Geochemical fingerprinting of K-bentonites by trace and rare earth element (REE) data suggest that tephras with alkali basalt composition have been derived by a source formed in a 'continental back-arc' setting, whereas the source of K-bentonites with trachytic precursors is related to 'continental within-plate rifting'. An evaluation of the global Late Devonian and Devonian-Carboniferous volcanism suggests that the bentonite precursors may be related to late-Variscan magmatism in Laurussia.