Composition of modern deep-sea sands from arc-related basins

Summary Petrographic and chemical examination of modern deep-sea sands shows a clear distinction between active and passive plate-tectonic settings. Among active settings, sands from forearc basins of island arcs and basins from strike-slip continental margins can be distinguished, but those from other arc-related settings overlap considerably in composition. In particular, sands from continental margin subduction zones and those from the backarc of island arcs appear to be indistinguishable. Chemically, arc-related sands are very close in composition to ancient greywacke sandstones. For instance, sodium almost always exceeds potassium. Thus it is not necessary for sodium to be added to sands diagenetically to make greywackes, as has been suggested.