TOTTEN, MATTHEW W., Department of Geology and Geophysics, University of New Orleans, New Orleans, LA; and HARVEY BLATT, School of Geology and Geophysics, University of Oklahoma, Norman, OK

ABSTRACT: Growth of Silt-Size Quartz during Diagenesis/Low-Grade Metamorphism of Pelitic Rocks

The smectite-illite transformation has been cited by many authors as a source of silica during diagenesis of mudrocks. However, illites themselves undergo chemical changes as they recrystallize into micas during high-grade diagenesis/low-grade metamorphism. Average compositions of phyllosilicates from the literature suggest that an equivalent amount of silica is available from transformation of illite to muscovite 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 of pelites from the Stanley Shale (Mississippian) in the Ouachita Mountains of Oklahoma and Arkansas was analyzed using standard petrographic techniques. The data obtained were related to illite crystallinity and vitrinite relectance as reported by Guthrie et al (1986) and Houseknecht and Matthews (1985).

Both the percentage of quartz and the mean grain size of the quartz and feldspar fraction increase with increasing illite crystallinity. The growth in quartz is especially apparent in the finest size fractions. A corresponding decrease in the silica content of the clay-mineral fraction is

also observed. Development of quartz polycrystallinity occurs across the same interval.

The results of this study are consistent with reported differences between quartz in schists and their shale precursors, and suggest that release of silica during diagenesis of phyllosilicates continues after the smectite-illite transformation. 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.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.