Heavy-Mineral Assemblages of Continental Margins as Indicators of Plate-Tectonic Environments

Abstract

ABSTRACT Heavy-mineral analyses of fifty Quaternary sediments from the North Sea, Red Sea, East China Sea, South China Sea, and Vancouver Island area (western seaboard of Canada) supplemented by over 1000 published analyses of sediments from many other sites in the world define accessory clastic mineral assemblages indicative of the principal plate-tectonic settings (excluding transform plate boundaries) associated with continental margins. Assemblages of all continental margins studied differ significantly from those of the intraoceanic, island-arc, and deep marginal-sea assemblages by possessing relatively high contents of zircon, tourmaline, garnet, epidote, amphibole (as well as other less common minerals), derived chiefly from metamorphic and sialic intrusive rocks. This suite is accompan ed by olivine, iddingsite, and brown (titanium-rich) clinopyroxene in regions containing rifting-type volcaniclastic sediments (i.e., near divergent plate boundaries), and with orthopyroxene, green clinopyroxene, and green-brown hornblende in arc-type volcaniclastic deposits (areas near convergent plate boundaries). On passive continental margins, both volcaniclastic suites are absent or present in negligible amounts.