A comparison of chrome-spinels in ophiolites and mantle diapirs of Newfoundland

Abstract

Ultramafic bodies of Newfoundland occur as two recognizable types, obducted ophiolites in the west and intrusive "mantle diapirs" in the east. The Bay of Islands bodies in the west form a classic ophiolite assemblage comprising, generally from base to top, Iherzolite (primitive mantle), harzburgite (residuum of partial melting), cumulus dunite, gabbro, trondhjemite, sheeted diabase dikes, pillow lava (all representing the magma produced by partial melting), and deep-water sediments. Chromite occurs mainly in cumulus layers and lenses in the cumulus dunite horizon, and as disseminated grains in the harzburgite. The Gander River bodies in the east occur as a discontinuous belt in sedimentary rocks as young as Silurian and show a range of contact relations from faulted to cold intrusive and hot intrusive. They consist predominantly of clinopyroxenite, with variable proportions of dunite, gabbro, diabase and diorite. These features, in conjunction with independent tectonic models, lead to an interpretation of these bodies as mantle diapirs arising from an eastward-dipping subduction zone beneath a lower Paleozoic marginal ocean basin, although further data may show that some of them represent marginal basin crust, in which case they could display similarities to the Bay of Islands ophiolites. The chrome-spinels in these bodies always occur in a dunite fraction and range from disseminated to podiform to banded. The major chemical difference between the spinels in the two types of occurrence is that those of the diapirs have substantially higher Cr:Al ratios than the cumulus spinels of the Bay of Islands. Magnesium is slightly lower in the diapir spinels, but Fe and Ti show no significant differences. The harzburgite (residual mantle) spinels of the Bay of Islands are chemically similar to those of the diapirs, enhancing the interpretation of the latter as residual mantle material. © 1975.