K-Ar dating and 18O- D characterization of nanometric illite from Ordovician K-bentonites of the Appalachians: Illitization and the Acadian-Alleghenian tectonic activity

Nanometric (<0.02, 0.02-0.05, 0.05-0.1, 0.1-0.2 mm) illite fractions were separated from Kbentonite samples from northwestern Georgia, and studied by X‑ray diffraction, oxygen and hydrogen isotope geochemistry, and K-Ar dated to more tightly constrain the tectono-thermal history of the Appalachian orogeny. Their XRD patterns are very similar for a given sample with respect to the peak shapes and positions. They are ordered illite-smectite mixed layers with only small variations in the relative proportions of illite and smectite interlayers. The illite crystal thickness distributions also are very homogeneous across the various size fractions of the same sample, but crystallite thickness varies from sample to sample. It can be concluded from the a-b2 diagram that illitization occurred in all fractions by simultaneous nucleation and crystal growth, except for one sample. In that sample, a period of growth without nucleation was detected on top of the nucleation and growth episode.