A MULTIVARIATE METHOD FOR DETERMINING THE PROVENANCE AND PROTOLITH OFMETASEDIMENTARY ROCKS - AN EXAMPLE FROM THE FORK MOUNTAIN FORMATION, SOUTHWESTERN VIRGINIA PIEDMONT, USA

Metasedimentary and metavolcanic rocks within the Smith River allochth on, southwestern Virginia Piedmont, are present in two units, the Bass ett and the Fork. Mountain :Formations. The younger Fork Mountain Form ation contains some metabasalt layers but primarily consists of a biot ite gneiss and an overlying high-alumina mica schist; the biotite gnei ss is wedge shaped and thins to the northwest. Fourteen major-oxide an alyses were performed on samples of metasedimentary rocks from the For k Mountain Formation. Compositions of both gneisses and schists fall o n 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 correlatio n between SiO2 and K2O is significant because in igneous rocks these t wo oxides are normally positively correlated. Thus relatively coarse, quartz-rich sediments apparently became the psammitic rocks that forme d the paragneisses, and the finer, clay-rich sediments became che shal es that are protoliths to the schists. A simple explanation of these r elationships is a relatively fine-grained, deep-water sedimentary faci es 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-compo nent mixing is an over-simplification. Some subtle differences in chem istry between the gneisses and schists exist that cannot be explained by linear mixing alone and may imply more than one source region for t he sediments. In addition, PCA and simultaneous solution of mass balan ce equations estimate the following major rock-forming mineralogy for the pelitic sediments, exclusive of quartz: illite -48%, montmorilloni te -20%? chlorite -9%, K-feldspar - 20%. This mineral composition is c onsistent with known patterns of mineral alteration during burial diag enesis. Discriminant function analysis (DFA) was also performed; it su ggests that an apparently large compositional gap is present between t he gneisses and schists. Conventional bivariate scattergrams and PCA, however, indicate that a compositional continuum exists for most oxide s.