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Rb-Sr Isotopic and elemental studies of the origin of glauconite in the Permian northern Karoo Coal Fields, South Africa: Evidence for a thermal Mid-Jurassic influence

Department of Geology, Rand Afrikaans University, P.O. Box 524, Auckland Park 2006, South Africa, e-mail: jmb{at}na.rau.ac.za; bc{at}na.rau.ac.za

I. McLachlan

P.O. Box 307, Parow 7499, South Africa, e-mail: mclachlai{at}petroleumagencysa.com

Rb-Sr isotopic analyses of glauconite from Permian sedimentary rocks associated with coal seams in the northern Karoo basin yield ages ranging from ~181 to ~178 Ma. These ages, being significantly younger than times of deposition of the host rocks, are interpreted to reflect the time of recrystallization and glauconite maturation during the Middle Jurassic. These ages are very similar to the time of widespread volcanism and intrusion of dolerite dykes and sills into Karoo sedimentary strata at ~180 Ma. Some link, whether just heating or hydrothermal activity leading to maturation of glauconite is implied. Rb-Sr ages between ~400 and ~750 Ma for detrital biotite and muscovite from these same sedimentary rocks limit the maximum temperature of the thermal activity to <~250°C, and vitrinite reflectance and the presence of oil in the sediments indicate a maximum temperature of <120°C, well below the ~230°C block temperature of glauconite in the Rb-Sr system. Therefore the second possibility seems more probable.