Geological, mineralogical and geochemical characteristics of Mississippian K-bentonites from southern Turkey: A correlation with coeval tephras from Gondwana-derived terranes

Abstract

Five thin (5–15 cm) and yellowish green tephra (K-bentonite) layers are exposed along two sections in the south of Akşehir (Konya, southern Turkey) within the Early Carboniferous (Mississippian) red dolomite-dolomitic limestone succession of the Sultandağ Unit in the central Tauride Belt. K-bentonites exhibit typical porphyritic texture with euhedral plagioclase phenocrysts replaced by fine-grained white K-micas and sparry dolomite crystals. The intensively sericitized volcanic matrix has vitrophyric texture and contains hydroxidized euhedral pyrite crystals. Illite is the only clay mineral in all K-bentonite samples. Non-clay minerals are dolomite, calcite, quartz, pyrite, rutile and zircon. Illites have sheet-like flakes (1–4 μm sizes) showing well-developed orientations and partly slaty cleavage. Illite Kübler index values (KI, Δ°2θ) of K-bentonites reflect high-grade diagenetic to anchizonal conditions indicating temperatures that reached up to 200 °C. Illite d060 values (1.4990–1.5007 Å) corresponding to dioctahedral composition display differences in coexisting slates and shales of the same age. Illite polytypes are characterized by the association of 2M1, 1M and 1Md. The presence of 1M illites is indicative of a volcanic (tephra) origin. Immobile element compositions suggest that tephras have been derived from trachyandesitic volcanism. K-bentonites exhibit some differences from the coeval pelitic occurrences in the Hadim-Bozkır areas, S Turkey, and the Late Devonian-Early Carboniferous K-bentonites in Bartın and Zonguldak areas of the Istanbul-Zonguldak Terrane, NW Turkey. A geochemical evolution of the studied K-bentonites suggests that they were possibly originated from the Early Carboniferous (Mississippian) intra-continental back-arc magmatism at the northern margin of Gondwana-derived Tauride-Anatolide microplate.