Assessing detrital garnet chemical composition as a quantitative provenance tool: A multivariate statistical approach

Chemical analysis of detrital garnet is a widely utilized quantitative tool for determining the provenance of sediments. The goal of this study is to investigate the ability of the compositions of detrital garnet to provide a direct linkage to the specific source rocks from which the garnet was derived. To achieve this goal, we analyzed detrital garnet isolated from alluvial samples collected from the French Broad River and several of its tributaries, a watershed that drains portions of the multiply deformed southern Appalachian Orogen, USA. To test whether potential linkages between detrital and potential source-rock garnet could be achieved, garnet crystals were chemically characterized from a wide variety of lithologies that crop out in the French Broad watershed, and thus are the most likely local sources of detrital garnet. Mahalanobis distances measured on canonical discriminant functions successfully differentiated garnet compositions among the 18 analyzed source rocks. These metrics were then applied to ∼ 2,300 detrital-garnet compositions to link each grain to its potential source rock. Approximately 94% of the detrital garnet crystals were linked to their potential source regions. Thus only 6% of the detrital crystals were treated as "unclassified." We interpret the "unclassified" high Ca garnets to have been derived from minor submap- scale lenses and boudins of calc-silicate or skarns from exposed Ocoee or Ashe metaclastic units within the French Broad River watershed. The "unclassified" high-Mn detrital garnet grains are interpreted to have been derived from the various felsic granitiods and pegmatites that intrude the region of study.