HEAVY-MINERAL ASSEMBLAGES OF CONTINENTAL MARGINS AS INDICATORS OF PLATE-TECTONIC ENVIRONMENTS

Heavy-mineral analyses of fifty Quaternary sediments from the North Se a, Red Sea, East China Sea, South China Sea, and Vancouver Island area (western seaboard of Canada) supplemented by over 1000 published anal yses of sediments from many other sites in the world define accessory elastic mineral assemblages indicative of the principal plate-tectonic settings (excluding transform plate boundaries) associated with conti nental margins. Assemblages of all continental margins studied differ significantly from those of the intraoceanic, island-are, and deep mar ginal-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 rock s. This suite is accompanied by olivine, iddingsite, and brown (titani um-rich) clinopyrqxene in regions containing rifting-type volcaniclast ic sediments (i.e., near divergent plate boundaries), and with orthopy roxene, green clinopyroxene, and green-brown hornblende in are-type vo lcaniclastic deposits (areas near convergent plate boundaries). On pas sive continental margins, both volcaniclastic suites are absent or pre sent in negligible amounts.