ASSESSING DETRITAL GARNET CRYSTAL CHEMISTRY AS A PROVENANCE TOOL – AN EXAMPLE FROM THE FRENCH BROAD RIVER

Determining the crystal chemistry of detrital garnet is a widely utilized quantitative tool for determining the provenance of sediments. The composition of garnet generally falls within one of the two isomorphous series: Ugrandite (Ca3(Al,Cr,Ti,Fe)2(SiO4)3 or pyralspite (Mg,Fe,Mn)3Al2(SiO4)3. Garnet most commonly occurs in metapelites but may also be found in granites, pegmatites and ultramafic peridotites. The elemental composition of garnet is a function of both protolith bulk composition as well as P-T history. Consequently, the major / trace element composition and chemical zonation of garnet each provide potentially valuable information for characterizing/ reconstructing the sediment donor lithologies. To assess the utility of detrital garnet crystal chemistry as a provenance indictor, we compared quantitative electron microprobe analyses and x-ray element maps for detrital garnet from modern river alluvium to garnet separated from known potential source rocks. Analyzing detrital garnet from modern river alluvium provides a robust assessment of the utility of garnet crystal chemistry as a provenance proxy because the petrology and geochemistry of the source rocks are well known. Modern sediments were collected from the French Broad River in North Carolina and Tennessee. In this portion of the southern Appalachian Mountains, the French Broad drains the following potential source rocks: variably metamorphosed Grenville ortho- and paragneisses, Taconic-metamorphosed pelites and mafic volcanic rocks of the Ashe metamorphic suite, and Paleozoic plutons, all potentially deformed and/or metamorphosed during the Allegheny Orogeny. Preliminary analyses of garnet compositions from the Ashe metamorphic rocks show direct correlation to detrital garnet isolated from river alluvium known to be derived from the Ashe suite. Typical garnet compositions from the Ashe rocks are: gneisses Alm+Sp(75-77)Gross (5-7)Pyr(15-18), schists Alm+Sp(80-85)Gross(7-10)Pyr(7-12); detrital garnet composition fall within these ranges. Additional garnet analyses from alluvium-source rock pairs will be discussed. These data from modern environments have important implications for sandstone provenance investigations where the primary goal is to identify the source region of an ancient sediment.