Contrasting geochemistry of two pegmatite fields in Manitoba: Products of juvenile Aphebian crust and polycyclic Archean evolution

Abstract

The geochemistry of the Proterozoic Wekusko Lake pegmatite field, in the Flin Flon-Snow Lake greenstone belt of the Churchill Province, is remarkably primitive, compared with the Kenoran Cat Lake-Winnipeg River pegmatite field in the Superior Province and its analogs elsewhere in Archean cratons. Comparable pegmatite types at Wekusko Lake are slightly depleted in Li, considerably impoverished in Rb, Cs, Be, P and F, and poor in Sn, Nb and Ta relative to those in the Cat Lake-Winnipeg River terrane. Eliminating differences in plutonic evolution, level of erosion and, to a degree, in the effectiveness of volatile components, the principal reason for the difference in the geochemical signature of the pegmatite fields is the nature of the crust and its evolution in the two regions. The Proterozoic greenstone belt resulted from a single-cycle metamorphism and melting of juvenile volcanic-sedimentary sequences derived from depleted mantle ( ϵ Nd = +5) with minimal, if any, contribution from pre-existing crust. Consequently, the source rocks of the granite-pegmatite evolution were highly primitive. In contrast, the host terrane of the Kenoran pegmatite field contains pre-2800 Ma crustal components of several generations, and a 2780 Ma tonalitic basement. The greenstone belt lithologies and associated batholithic granitoids are derived in part from a heterogeneous, near-chondritic to REE- and LILE-enriched mantle ( ϵ Nd = +2), in part from the pre-existing crust. This polycyclic crustal evolution generated highly enriched protoliths from which the fertile granites and their pegmatite aureoles evolved. The low level of geochemical evolution in the Wekusko Lake field is not necessarily characteristic of other Proterozoic rare-element pegmatites. In the Svecofennian terrane of Sweden and Finland, developed in part over Archean basement and with considerable Archean components in its lithologies, the fertile granites and their pegmatite aureoles reached the same fractionation levels as in the Kenoran provinces.