Determination of Tectonic Setting of Sandstone-Mudstone Suites Using SiO2 Content and K2O/Na2O Ratio

Several previous studies have shown that sandstones from different tectonic settings possess characteristic chemistry, particularly content and ratio. Systematic variation of these variables with change in grain size in New Zealand greywackes shows that such discrimination can be extended to the finer-grained members (siltstones, mudstones) of sedimentary sequences. Compilation of published data from ancient sedimentary suites defines passive margin (PM), active continental margin (ACM), and oceanic island arc (ARC) tectonic setting fields on a simple bivariate plot. These fields are confirmed by published analyses of modern sands and muds of known tectonic setting, which also reveal distinctive sand-mud trends in sediments from the forearc environment of island arcs, where muds show greater ratios and than the sands. Results gained from New Zealand terranes are in good agreement with published paleoenvironmental interpretations. Quartzose Greenland terrane sediments are of PM type, and quartz-intermediate samples from the Torlesse, Haast Schist, and Miocene terranes are classified as ACM. Quartz-poor volcanogenic sandstones from the Caples, Maitai, and Murihiku terranes fall in the ARC field, and argillites plot at larger and near or in the ACM field, supporting proposed arc settings. The New Zealand data illustrate well the effects of grain size, petrologic evolution, and maturation on the chemistry of sediments, and overall results demonstrate the value of sampling the entire range of grain sizes when chemical discrimination is to be attempted. The chemical approach is a useful complement to petrographic analysis, and the two methods combined are a powerful tool for examination of provenance and determination of tectonic setting.