RADIOGENIC ISOTOPE SYSTEMATICS OF THE TANCO AND SILVERLEAF GRANITIC PEGMATITES, WINNIPEG RIVER PEGMATITE DISTRICT, MANITOBA

Abstract

Isotopic ages for minerals of the Archean lithium–cesium–tantalum (LCT)-family Tanco and Silverleaf granitic pegmatites in southeastern Manitoba range from about 2660 Ma to 2600 Ma. Etched tantalite yields U–Pb ages between ca. 2640 and 2620 Ma, and the six oldest ages form an array indicating 2641 ± 3 Ma, which is interpreted as a major period of crystallization. Fragments of zircon megacrysts give U–Pb ages ranging from ~2660 to 2606 Ma, which may reflect lead loss after crystallization associated with subsolidus fluids from the pegmatite. A Pb–Pb age on megacrystic apatite and associated feldspar gives 2657 ± 18 Ma. Rb–Sr ages on lepidolite from Silverleaf (2620 ± 13 Ma) and Tanco (2631 ± 12 Ma) overlap and agree with the Rb–Sr age of muscovite from Tanco (2620 ± 20 Ma), using a 87 Rb decay constant of 1.395 × 10 −11 yr −1 . Pb–Pb data from K-feldspar and albite show highly evolved unsupported Pb compositions that scatter along or between a single-stage isochron of about 2640 Ma and a mixing line between the composition of common Pb at about 2640 Ma on the Stacey and Kramers growth curve and the composition of radiogenic Pb generated at around 2640 Ma. This indicates that feldspars exchanged Pb with radioactive minerals like tantalite both early, during the period of pegmatite emplacement and cooling, and in geologically recent times. Early exchange is consistent with the observation of progressive structural modification of feldspars, whereas disturbed Rb–Sr systematics indicate late exchange of these elements between feldspar and its environment. The peraluminous composition of late granites and pegmatites in this suite and high initial 87 Sr/ 86 Sr ratio (> 0.8) of the Tanco pegmatite suggest an origin from melting of metasedimentary rocks beneath the Bird River greenstone belt. These rocks are most likely associated with the English River Subprovince exposed to the northeast.