Kornerupine-quartz symplectites in paragneiss, Lac Ste-Marie, Quebec, Canada: pseudomorphic replacement of a hexagonal phase, probably osumilite

Kornerupine has been reported from several localities in the Grenville Prov ince. It is most abundant and accessible within quartz - sillimanite - K-fe ldspar - tourmaline Fe/Ti-oxides - garnet paragneiss, forming a 300 m by 15 00 m lens in diopside- and chondrodite-bearing marble at Hinck's bridge alo ng the Gatineau river near Lac Ste-Marie. Biotite-rich zones within the len s contain six-sided prisms, up to 7 cm in diameter and 30 cm long. These pr isms consist of symmetrically zoned (coarse rim and finer grained centre) s ymplectitic intergrowths of komerupine-quartz overgrowing the biotite folia tion, with minor sillimanite (as fibrolitic inclusions in kornerupine), K-f eldspar, and scattered trails of ilmenite. The kornerupine-quartz symplectite in the six-sided prism appears to be a r eplacement of an early single hexagonal phase. Habit and bulk composition o f the prism point to osumilite as the precursor. Boron-bearing fluids durin g amphibolite- to granulite-grade metamorphism stabilised tourmaline-quartz intergrowths which crystallized by pseudomorphic replacement of osumilite. Numerous dark green tourmalines are found as relict inclusions in quartz ( rare in kornerupine) in core and margin of a prism. In a later stage, korne rupine-quartz symplectites were formed by replacement of the tourmaline wit h loss of alkalis. On the margin of the prisms kornerupine is in turn parti ally in contact with (replaced by) fine-grained, light green, euhedral tour maline. The inferred mineral assemblages may reveal an early portion of the P-T-t path for the region. Kornerupine - quartz assemblages are not often described, whereas granite pegmatites commonly contain tourmaline - quartz symplectitic intergrowths.