ALTERATIONS IN THE NON-CLAY-MINERAL FRACTION OF PELITIC ROCKS ACROSS THE DIAGENETIC TO LOW-GRADE METAMORPHIC TRANSITION, OUACHITA MOUNTAINS, OKLAHOMA AND ARKANSAS

The transformation of smectite to illite has been cited by many author s as a source of silica during diagenesis of mudrocks. Illites themsel ves, however, undergo chemical changes as they recrystallize into mica s during high-grade diagenesis/low-grade metamorphism. Average composi tions of phyllosilicates from the literature suggest that an equivalen t amount of silica is available from transformation of illite to musco vite as from illitization of smectites. The fate of silica released by this process has not been reported, but could be a major contributor to the silt-size quartz population. The quartz and feldspar fraction o f pelites from the Stanley Shale (Mississippian) in the Ouachita Mount ains of Oklahoma and Arkansas was separated using sodium bisulfate fus ions. The mineralogy and the grain-size distribution of this fraction were determined using standard petrographic and X-ray diffraction (XRD ) techniques. Bulk rock samples were analyzed using X-ray fluorescence (XRF) and instrumental neutron activation analysis (INAA) methods. Th e data obtained were related to illite crystallinity and vitrinite ref lectance as reported by Guthrie et al. (1986) and Houseknecht and Matt hews (1985). Both the percentage of quartz and the mean grain size of the quartz and feldspar fraction increase with greater illite crystall inity values. The growth in quartz is especially apparent in the fines t size fractions. A corresponding decrease in the silica content of th e clay-mineral fraction is also observed. Development of quartz polycr ystallinity occurs across the same interval. Whole-rock chemical abund ances show no statistical correlation with thermal maturity. Relative to titanium, both the major-element and the trace-element concentratio ns show little variation. Rare-earth element ratios do not correlate w ith thermal maturity and remain essentially constant. Our results are consistent with reported differences between quartz in schists and the ir shale precursors, and suggest that release of silica during diagene sis of phyllosilicates continues after the smectite-illite transformat ion. This silica precipitates as quartz within the pelite, consistent with the suggestion by Blatt (1987) that metapelites are the source of abundant silt-size quartz. The lack of whole-rock chemical variation with thermal maturity implies closed-system behavior across much of th e pelite-to-metapelite transition.