Clay mineralogy, chemistry, and diagenesis of Late Devonian K-bentonite occurrences in northwestern Turkey

Authors: ASUMAN GÜNAL TÜRKMENOĞLU, ÖMER BOZKAYA, MEHMET CEMAL GÖNCÜOĞLU, ÖZGE ÜNLÜCE, İSMAİL ÖMER YILMAZ, CENGİZ OKUYUCU

Abstract: Thin beds of tephra (K-bentonites) formed by the diagenesis of volcanic ash are exposed within the limestone-dolomitic limestone successions of the Yılanlı formation at Zonguldak and Bartın in northwestern Turkey. They were deposited on the Middle Devonian-Lower Carboniferous shallow carbonate platform of the Zonguldak terrane. In this study, K-bentonite samples collected from Gavurpınarı and Yılanlı Burnu limestone quarries are investigated in order to reveal their mineralogical and geochemical characteristics and diagenetic evolution. Illite is the major clay mineral in the studied K-bentonites. Additionally, kaolinite and mixed-layer illite-smectite are identified in some samples. The nonclay minerals calcite, dolomite, quartz, gypsum, feldspar, pyrite, and zircon are also found. Crystal-chemical characteristics (Kübler index, d₀₆₀ values, and polytypes of illites) from two different sampling locations do not show significant variations. Kübler index values for the Yılanlı Burnu and Gavurpınarı sampling locations, 0.47-0.93 (average: 0.71 $\Delta$°2$\theta$) and 0.69-0.77 (average: 0.72 $\Delta$°2$\theta$), respectively, indicate that illites were affected by high-grade diagenetic conditions. The swelling (or smectite) component (~5%), crystallite size (N = 10-20 nm), and polytype (2M₁ > 1M_d) data of illites support the same conditions. Illite d₀₆₀ values of 1.491-1.503 Å correspond to a range of octahedral Mg+Fe values of 0.27-0.51 atoms per formula, indicating a composition between end-member muscovite and phengite unit. Trace and rare earth element-based chemical classification of the K-bentonite samples revealed that composition of original volcanic ash is basaltic. Illitization took place by fixation of K from volcanic minerals and ash, and diffusion of elements (Mg+Fe) into and out of the beds during diagenesis. Mineralogical-chemical data point out that these K-bentonites evolved in high-grade diagenetic conditions (approximately 100-150 °C) from the products of volcanic eruptions of disputed sources and distances during the Late Devonian time.

Keywords: Devonian, tephra, K-bentonite, illite, diagenesis, NW Turkey

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