Geochemistry and assemblage accretion of metavolcanic rocks in the Beardmore–Geraldton greenstone belt, Superior Province

Abstract

The Beardmore–Geraldton greenstone belt lies between the Wabigoon volcanic arc (Onaman–Tashota terrane) and the Quetico metasedimentary subprovince and thus has an important bearing on the accretionary model that has been proposed for the amalgamation of these terranes. This paper presents geochemical evidence for the petrogenetic affinities of the volcanic units of the western half of the Beardmore–Geraldton greenstone belt. These data suggest that the metavolcanic rocks of the greenstone belt form a series of distinct packages. Trace element data are used to demonstrate the similarities and differences of each unit of lavas and to characterize their source region and likely tectono-magmatic setting. The data indicate that three separate fragments of volcanic crust representing oceanic crust, arc crust, and back-arc crust formed in a small arc system and were juxtaposed prior to collision with the Wabigoon arc. These fragments of crust were then accreted to the Wabigoon arc where sedimentation was followed by thrusting and folding resulting in shortening of the belt. Delamination of the volcanic units is thought to have been responsible for the preservation of just the pillow lava sequence of oceanic crust. Lower crustal or crust–mantle delamination of the Wawa arc and underplating of the Quetico are thought to have been responsible for the late and long-lived, high-grade metamorphic event in the Quetico and such "flake tectonics" are thought to have been an important process in the interaction between the Wabigoon, Quetico, and Wawa subprovinces.