A MULTIVARIATE STATISTICAL ANALYSIS OF DETRITAL GARNET MAJOR ELEMENT CHEMISTRY AS A TOOL FOR QUANTITATIVE SEDIMENT PROVENANCE

Determining the chemical composition of detrital garnet is a widely utilized quantitative tool for determining the provenance of sediments. The elemental composition of garnet, a function of protolith bulk composition and P-T history, generally falls within two isomorphous series: ugrandite (Ca₃(Al,Cr,Ti,Fe)₂(SiO₄)₃ or pyralspite (Mg,Fe,Mn)₃Al₂(SiO₄)₃. Given the potential for substitution of several different cation species in the dodecahedral and octahedral sites in the garnet structure, detrital garnet mineral chemistry has the potential to provide valuable information for characterizing/reconstructing possible sediment sources. In our current study we investigate the ability of detrital garnet compositions to constrain potential source rock lithologies, and thus ultimately provide a linkage to specific source rocks.

To assess the utility of detrital garnet mineral chemistry as a provenance indicator, we measured the elemental composition of garnet crystals from modern river alluvium from the French Broad River in North Carolina and Tennessee. This river was chosen as it drains multiple domains of the southern Appalachians: Grenville basement, which underwent high grade Grenvillian metamorphism and variable Paleozoic metamorphism; greenschist to granulite facies metapelites of the Ashe Metamorphic Suite metamorphosed during Taconian orogenesis, and Paleozoic plutons. All of these deformed and/or metamorphosed regions are potential garnet-donor sources. To test possible links between detrital garnet and potential sources, garnet was chemically characterized from a wide variety of lithologies that crop out within the French Broad watershed. Mahalanobis distances measured on principal components (PCA) and on canonical discriminant functions successfully differentiated garnet compositions among the analyzed source rocks. These metrics will be applied to detrital garnet compositions to link each grain to its potential source rock(s).

The results from this study of garnet from modern alluvium will have important implications for provenance investigations of ancient clastic sedimentary rocks where the primary goal is to identify the source regions of the ancient sedimentary units. It could also shed light on the characteristics of original sources that are no longer exposed.