The source of quartz in sedimentary rocks has been a topic of intense interest to sedimentary petrologists for more than 100 years, with the major effort devoted to sand-size grains. However, at least half of all detrital quartz is of silt size and the origin of these grains has proved to be a more difficult problem. PI will examine grain size, petrographic and oxygen isotopic changes in coexisting quartz and illite during the transition from shale to slate to phyllite in pelites of the Stanley Formation (Mississippian) in the Ouachita Mountains of Arkansas. In the core area of the Ouachitas these pelites have developed slaty cleavage and in the central part of the core they have developed the sheen associated with growth of phyllosilicate minerals that characterizes phyllites. The results of this study, when correlated with published vitrinite reflectance data, will establish (1) the temperature at which detrital quartz in shales reequilibrates with the sediment pile during very low- grade metamorphism; (2) the relationship between oxygen isotopic values in quartz and metamorphic grade in the slate-phyllite series; (3) the conditions during very low-grade metamorphism in which grain size and proportion of polycrystalline quartz increase; (4) the relationship between field appearance (shale, slate, phyllite) of pelites and the various petrographic and isotopic variations.
