Emily Palmer

Petrogenesis of the Archean Prestige leucogranite and spatially associated LCT pegmatites, NWT: insights from whole-rock and muscovite trace element geochemistry and apatite U-Pb geochronology

• Date: January 16, 2018
• Time: 2:30 p.m.
• Location: Forestry & Geology, Room 104

Abstract

The Yellowknife pegmatite field is host to LCT-family rareelement pegmatites that are associated with Late Archean granitic magmatism. The Prosperous suite, a large plutonic suite composed of 14 two-mica S-type leucogranites, lies in the southwest quadrant of the pegmatite field. The plutons are spatially associated with the rare-element pegmatites. Although the geology in this domain is the most thoroughly documented in the Slave Province, the ages of the major plutonic suites are poorly constrained. The Sparrow Lake pluton of the Prosperous suite is the only pluton to have previously been reliably dated at 2596 ± 2 Ma. The Prestige pluton, the focus of this thesis, has been classified as a member of the Prosperous suite and has previously been studied because of its high Li contents. Apatite U-Pb geochronology of the Prestige pluton yielded a concordant age of 2608 ± 4 Ma, which is interpreted to represent the crystallization age of the pluton. Based on geochemical and geochronological similarities, the Prestige pluton is interpreted to be a part of the Prosperous suite. The difference in age between the Sparrow Lake and Prestige plutons is attributed to different crustal levels of emplacement and, therefore, different cooling histories. Upper intercept and concordia ages for apatite from the intra- and inter-pluton pegmatites associated with the Prestige pluton yielded overlapping ages of 2588 ± 6 Ma and 2593 ± 6 Ma, respectively. The whole-rock geochemistry of the intra- and inter-pluton pegmatites exhibit similar trace element compositions and enrichment of incompatible elements, with averages of 22 ppm Sn, 9.5 ppm Ta, 19.6 ppm Nb, 21.0 ppm Cs, and 453 ppm Rb. Trace element analyses of muscovite via LA-ICP-MS reveal elevated concentrations of Rb, Cs, and Sn within the granite and pegmatites. In general, the rims of muscovite grains are enriched in Li, Cs, Sn, Nb, and Ta, which is attributed to normal magmatic fractionation processes. The whole-rock and muscovite geochemistry reflect increasing fractionation trends from the intra-pluton to inter-pluton pegmatites, as indicated by decreases in K/Rb, K/Cs, and Sr/Rb ratios. The similarity in age, geochemistry, and geothermometry of the intra- and inter-pluton pegmatites suggests they are comagmatic. Distinct differences in fractionation trends, crystallization ages, and muscovite geochemistry, in addition to a lack of field evidence of gradation, suggests that the Prestige granite is not parental to the spatially associated pegmatites. This study emphasizes the importance of geochronological and geochemical work to determine the petrogenesis of pegmatites to spatially related plutons. The source of the pegmatites in this study is not known; however, it is suggested to be a deep-seated magmachamber that has yet to be identified.