Abstract

A compilation of all types of magmatic deposits in Precambrian terranes of northeast North America revealed a continental-scale metallic zonation. Nickel is homogeneously distributed. Palladium and chromite are centered on the Archean craton, as predicted by the Groves hypothesis, whereas low-Ti Cu-U–rare earth element iron oxide deposits occur just south and east of the Archean and Paleoproterozoic margin, within the Grenville province. Ti is concentrated along this margin in association with anorthosite. Nickel is still abundant there, but palladium and chromite are remarkably absent. This Grenvillian Ni-Pd-Cr paradox suggests that the fractionation process invoked in the classical plume model is inadequate to explain the geochemical characteristics of the anorthosite parental magmas. The crustal-tongue melting model seems to better explain the metallic zonation observed at the margin of the Archean and Proterozoic terranes.

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