Beyond whole-rock geochemistry of shales: The importance of assessing mineralogic controls for revealing tectonic discriminants of multiple sediment sources for the Ouachita Mountain flysch deposits

Abstract

The origin of the Ouachita Mountains has been the focus of significant debate for decades. Considerable confusion also exists concerning the provenance of the Carboniferous flysch of the Ouachitas. Trace-element geochemistry of shales from the Stanley Group delineates the provenance of the sediments and provides clues to the plate tectonic evolution of the southern continental margin during Mississippian time. Th/Sc and Cr/Th ratios indicate a cratonic source for the majority of the Stanley Group sedimentary rocks. However, in several samples, low Th/Sc ratios and high Cr/Th ratios suggest a contribution from a mafic source. Using element ratio diagrams, all of the samples plot along a curve consistent with a two-component mixing model, consisting of a dominant felsic and a subordinate mafic source. The heavy-mineral fraction of these shales sequester many of the trace elements used in whole-rock studies. Monazite is ubiquitous in trace amounts and is the probable site for much of the rare earth elements in the whole rock. The occurrence of monazite almost exclusively in sialic igneous rocks implies that Sm/Nd isotopic signatures are not sensitive to sediment input from more mafic sources. In some Stanley shale samples, chromite and Mn oxides were identified and positively identify an oceanic crustal component as a source of Stanley Group sediment. The results of this study emphasize the importance of determining the mineralogic sites of trace elements, and realization of specific mineralogic contributions from mafic or sialic tectonic provenances.