Effects of Kaolinitization on the K-Ar Sytematics in Weathered Muscovite From the Cretaceous and Tertiary Kaolin Deposits of Georgia

Detrital muscovite in the Cretaceous and Tertiary kaolin deposits of central Georgia has been studied to determine K-Ar and ⁴⁰Ar/³⁹Ar apparent age values and examine the effect of weathering and kaolinitization on the K-Ar radiometric clock. K-Ar dates of muscovite separates range from 166 +/- 4 Ma to 374 +/- 8 Ma. Both the muscovite dates and K₂O contents increase with increasing stratigraphic age. ⁴⁰Ar/³⁹Ar spot analysis of a single large grain from the base of the Tertiary deposits reveals an apparent-age profile through the grain from the margin to the center. The oldest date, from near the center of the grain is 292 +/- 13 Ma; the youngest, from nearest the edge of the grain is 223 +/- 10 Ma. K-Ar analyses also reveal that all of the muscovite is significantly depleted in K, which has been removed during intense weathering in the sedimentary weathering environment. Hand sample and petrographic analyses show unremarkable detrital muscovite grains exhibiting no obvious signs of kaolinitization. All the grains are delaminated to very thin sheets and the edges of all grains are rounded. All samples analyzed appear to be muscovite. Electron microprobe analysis reveals that the Tertiary muscovites tend to have lower K₂O contents at their weathered margins than elsewhere. The altered zones are approximately 10-15 μ m wide in grains with diameters ranging from 150 to 300 μ m. Some larger grains (300-600 μ m diameter) show random patterns of K-depletion, possibly related to the presence of fractures or other imperfections. These K-depleted zones imply the presence of a non-muscovite weathering product mineral. X-ray diffraction analysis of random powder mounts confirms electron-microprobe implications of the presence of a secondary weathering product. All samples analyzed are shown to contain both muscovite and kaolinite. The peak intensities of kaolinite relative to muscovite in the samples increase with decreasing grain size. Furthermore, the deepest and most weathered samples analyzed show the greatest relative kaolinite peak intensities; whereas, the most shallow, least weathered samples have the lowest. X-ray diffraction analysis of a single large grain also shows the presence of kaolinite, though with a much lower relative intensity. Powdered samples of 150-300 μ m sized separates from the same sample have significantly greater kaolinite peak intensities. The general agreement of the measured K-Ar dates from these separates (282 +/- 6 Ma) and the ⁴⁰Ar/³⁹Ar spot dates (223 +/- 10 to 292 +/- 13 Ma) from the same large grain and the variability of ⁴⁰Ar/³⁹Ar spot dates across the grain indicate that Ar is lost preferentially over K, and not simply due to edge diffusion as would be expected.