The application of reflectance determinations on coalified and graphitized plant fragments to metamorphic studies

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Abstract
In many sedimentary rocks, small quantities of plant fragments (phytoclasts) are present. During prograde metamorphism, they undergo coalification and graphitiza‐tion concomitant with the recrystallization of the inorganic detritus. The degree of coalification, or rank, can be quantified by measuring the reflectance of the phytoclasts. Measurement of this optical parameter has been applied mainly to rank‐variation analyses in sedimentary basins and, to a lesser degree, to oil and gas exploration, stratigraphical and structural problems, and to contact and burial metamorphic rocks as an indicator of rank. In this paper, the potential of reflectance studies of phytoclasts in metamorphic rocks, is further developed. It is shown that phytoclast reflectivity is a more sensitive indicator of thermal perturbations than are silicate minerals, and that it is a useful indicator of grade in areas where index minerals are not developed. Furthermore, examples are given showing that isorank lines delineated from reflectance data reveal, on a regional scale, a metamorphic imprint similar to that of the silicate assemblages, and that metamorphic variables other than pressure and temperature may be involved in metamorphic reactions.