ABSTRACT

Metasedimentary and metavolcanic rocks within the Smith River allochthon, southwestern Virginia Piedmont, are present in two units, the Bassett and the Fork Mountain Formations. The younger Fork Mountain Formation contains some metabasalt layers but primarily consists of a biotite gneiss and an overlying high-alumina mica schist; the biotite gneiss is wedge shaped and thins to the northwest. Fourteen major-oxide analyses were performed on samples of metasedimentary rocks from the Fork Mountain Formation. Compositions of both gneisses and schists fall on single linear trends on conventional Harker-type scattergrams, which can be explained by sedimentary mixing lines between pure quartz and a pelitic sedimentary assemblage. In addition, the negative correlation between SiO₂ and K₂O is significant because in igneous rocks these two oxides are normally positively correlated. Thus relatively coarse, quartz-rich sediments apparently became the psammitic rocks that formed the paragneisses, and the finer, clay-rich sediments became the shales that are protoliths to the schists. A simple explanation of these relationships is a relatively fine-grained, deep-water sedimentary facies present in the northwest that transgressed to the southeast through time; the source for these sediments would have been to the southeast. Principal components analysis (PCA), ratio-ratio scattergrams, and t-tests for differences of means, however, indicate that this two-component. mixing is an over-simplification. Some subtle differences in chemistry between the gneisses and schists exist that cannot be explained by linear mixing alone and may imply more than one source region for the sediments. In addition, PCA and simultaneous solution of mass balance equations estimate the following major rock-forming mineralogy for the pelitic sediments, exclusive of quartz: illite - 48%, montmorillonite - 20%, chlorite - 9%, K-feldspar - 20%. This mineral composition is consistent with known patterns of mineral alteration during burial diagenesis. Discriminant function analysis (DFA) was also performed; it suggests that an apparently large compositional gap is present between the gneisses and schists. Conventional bivariate scattergrams and PCA, however, indicate that a compositional continuum exists for most oxides.