Existence of a marginal basin within the Circum-Superior Belt: geochemical evidence from the Churchill-Superior boundary in Manitoba, Canada

Abstract

Mafic volcanic rocks incorporated within the Churchill-Superior Boundary Zone at Ospwagan Lake, Moak Lake, Assean Lake and the Fox River Belt constitute part of the Circum-Superior Belt. Major and trace element data have been obtained from mafic and ultramafic volcanic rocks from four localities. The data provide a means of comparing and contrasting the geochemistry of the rocks from the portion of the Circum-Superior Belt surrounding the Archean Superior Province in Manitoba with other units from elsewhere in the belt, and with rocks from the adjacent Proterozoic Churchill Province. Major element geochemical data are consistent with the rocks having originally formed in a oceanic environment. Trace element and rare earth element data suggest that the environment was a marginal basin. Al 2O 3/TiO 2 ratios indicate that the magmas were derived from a mantle source by small degrees of partial melting. High field strength element patterns show that the mantle source region giving rise to the rocks was relatively uniform in composition. Trace element, mid-ocean ridge basalt (MORB) and chondrite-normalized patterns indicate similarities in alkali element enrichment for the rocks at Asean and Ospwagan Lakes. Virtually all the rocks have Ti, Zr, Nb, Y and P levels < MORB, which is consistent with the mantle source region having undergone a small degree of partial melting. Zr/P ratios for the rocks from the Fox River Belt are consistently lower than those from the other localities, and the Assean Lake rocks show slight enrichment in light rare earth elements, which may suggest local compositional variability in the mantle source regions supplying the magmas to the Fox River Belt and the Ospwagan, Assean and Moak Lake areas.