Akademik Veri Yönetim Sistemi
JOURNAL OF AFRICAN EARTH SCIENCES, cilt.241, 2026 (SCI-Expanded, Scopus)
K-bentonites occur within the Jurassic volcano-sedimentary succession of the S & cedil;enko & uml;y Formation in the G & uuml;m & uuml;s & cedil; hane area (NE T & uuml;rkiye). Clay mineral assemblages are dominated by mixed-layer illite-smectite (I-Sm), accompanied by subordinate chlorite-smectite (Ch-S) and minor illite, possible kaolinite, and imogolite, whereas plagioclase, quartz, calcite, and pyrite constitute the main non-clay phases. The predominance of R1 I-Sm, with lesser R0 and R3 ordering, together with high but incomplete illitization (70-90%) and the presence of 1Md-1M illite polytypes, indicates moderate to advanced burial diagenesis. Geochemical discrimination reveals that the precursor ash was derived from intermediate to felsic-arc-related volcanism. Alteration indices further suggest moderate to advanced chemical modification of the ash layers, reflecting leaching of mobile elements and progressive K fixation during diagenesis. K-Ar ages indicate that the principal phase of burial-related illitization was probably reached to the Early Cretaceous, broadly coeval with the deposition and burial of the carbonate succession of the Berdiga Formation, although alteration likely began earlier after ash deposition. These altered ash layers are therefore most plausibly related to Jurassic intra-arc rift volcanism in the Eastern Pontides magmatic arc, followed first by seawater/pore-water alteration and then by burial-controlled mineral transformation. Stable isotope data from I-Sm show a narrow range of delta 18O values (14.8 to 15.6%o) but a wider spread of delta D values (-70 to-99%o), consistent with authigenic clay formation under relatively low water/rock ratios. When considered together with mineralogical data, these isotope compositions suggest clay formation at around 75-100 degrees C. The coexistence of I-Sm and Ch-S in the same samples is best explained by local chemical heterogeneity, particularly variations in Fe-Mg availability, superimposed on a common burial-related thermal regime.