Mineralogy and geochemistry of alkali feldspars from the Tanco pegmatite, southeastern Manitoba

Abstract

The alkali feldspar paragenetic sequence in the Tanco pegmatite at Bernic Lake, Manitoba, spans the entire crystallization and cooling history of the pegmatite, and includes six different K(Rb)-feldspar and six different albite varieties. Total feldspar modal abundances approach 50% for the entire pegmatite volume. Blocky K-feldspar, aplitic albite and cleavelandite together account for more than 95% of total feldspar volumes. The other, late, low-temperature feldspars include (Rb,K) or (K) adularia and albite in veins and cavities, mostly contained in inner zones associated with pollucite and petalite. Feldspars collected from each of the nine major zones of the pegmatite have been assessed in terms of composition and structural state, along one east-west transect and two north-south transects. The large and economically significant Tanco pegmatite represents an extreme in terms of petrologic processes involving fractional and disequilibrium crystallization from granitic melts enriched in volatiles and fluxing components (H2O, B, F, P), rare-alkalis (Li, Rb, Cs) a d HFSE (Ta, Nb, Zr, Hf). (Abstract shortened by UMI.)