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

Abstract

Newly discovered K-bentonite beds, interstratified with limestones/dolomitic limestones of the Upper Devonian–Lower Carboniferous Yılanlı Formation, are exposed in the northwestern Black Sea region of Turkey, around Zonguldak and Bartın. K-bentonite samples collected from four different locations: the Gavurpınarı and Yılanlı Burnu quarries from the Bartın area, the Çimşir Çukurları quarry from the Şapça area, and the Güdüllü and Gökgöl 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 Yılanlı 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 Bartın (Gavurpınarı and Yılanlı Burnu) and Güdüllü K-bentonites were deposited at around the boundary between the Frasnian and Famennian, whereas those in the Şapça and Gökgöl 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 Bartın area display a high degree of illitization and consist mainly of illite indicating high-grade diagenesis, whereas illite-smectite-rich samples from the Şapca and Gökgöl tunnel locations reflect relatively lower diagenetic conditions. According to their geochemical compositions, two groups of K-bentonites were distinguished, one with alkali basalt (Bartın area and Güdüllü locations) and one with trachyte affinities (Gökgöl tunnel and Şapça 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.

Weak and intermittent anoxia during the mid-Tournaisian (Mississippian) anoxic event in the Montagne Noire, France

“…The potential source of the ash layers and cryptotephra could be within-plate explosive rift-related volcanism, such as ash layers in the Tournaisian found at the Zonguldak terrane in Turkey (cf. Göncüoglu et al 2016). A very large anomalous Hg spike has recently been found in the Carnic Alps with maximum values reaching 3650 ppb, generally >1000 ppb (Rakociński et al 2022).…”

“…the carbonatite Kola-Dnieper Igneous Province (Motuza et al 2015), subduction-related arc volcanism (e.g. Acquafredda et al 1991; Lardeaux et al 2014), extensional and rift-related volcanism (Alexandre et al 2004; Bryan et al 2004; Göncüoglu et al (2016) and basaltic intraplate volcanism (e.g. Nesbor, 2004).…”

Occurrence of anatase in reworking altered ash beds (K-bentonites and tonsteins) and discrimination of source magmas: a case study of terrestrial Permian–Triassic boundary successions in China

“…In closed conditions such as in marine phreatic environments, alteration of volcanic materials does not lead to fractionation of the immobile elements, and consequently their ratios remain unchanged (Bertine, 1974). For this reason, ratios of immobile elements can be used for testing correlations and interpreting source magmas and tectonic environments (Pearce & Peate, 1995; Göncüoğlu et al , 2016). The Nb/Y vs Zr/TiO 2 diagram of Winchester and Floyd (1977) is widely used to interpret source magmas of volcanic ash beds (Huff & Türkmenoğlu, 1981; Batchelor & Clarkson, 1993; Kiipli et al , 2017).…”

“…Alteration of volcanic ash involves the devitrification of ash to clay minerals and leaching of alkalis and silica but uptake of Mg, Fe and Ca during the weathering process (Christidis, 1998; Arslan et al , 2010), and the clay species yielded by the ash beds are largely facies-dependent (Hong et al , 2019). Al 2 O 3 and TiO 2 are highly immobile chemical components during chemical weathering, and their concentrations will increase with greater degree of weathering due to the net loss of mobile components (Göncüoğlu et al , 2016). Ash beds of the paludal Tucheng section consist mainly of kaolinite and K-S minerals, while those of the shallow-water Yanlou section and the lacustrine Chahe section consist mainly of mixed-layer I-S minerals (Fig.…”

“…Alteration of volcanic ash may result in notable changes in the abundances of mobile elements such as K, Na, Ca, Mg and Si (De La Fuente et al , 2000), whereas Al, Ti, high-field-strength elements such as Zr, Nb, Hf and Ta and rare earth elements ( REEs ) are nearly immobile and remain largely unaffected during diagenesis. For this reason, the concentrations and ratios of these elements can be used to draw inferences regarding the source-magma chemistry and tectonic setting of the altered ash beds (Zielinski, 1985; Göncüoğlu et al , 2016). However, the distributions of these so-called immobile elements in ash beds can be affected by depositional and diagenetic processes ( e.g.…”