UNDEFORMED KYANITE- AND BOROSILICATE-BEARING VEINS FROM THE WILSON LAKE TERRANE, LABRADOR: EVIDENCE FOR RAPID EXHUMATION

We describe here some remarkable quartzo-feldspathic veins that have been emplaced within high temperature granulites near Wilson Lake in the Grenville Province of central Labrador. The host granulites are highly deformed and characterized by orthopyroxene + sillimanite + quartz and sapphirine + quartz assemblages. The discordant veins are undeformed and have undisturbed primary igneous/hydrothermal textures. Most of the veins contain euhedral kyanite, as well as mica-shaped aggregates of kyanite, K-feldspar, phlogopite and minor dumortierite. These aggregates are likely pseudomorphs after primary phengite with reconstructed compositions ranging from 3.1 to 3.3 Si/11 oxygens. The pseudomorph assemblage is interpreted to be the product of phengite + quartz melting, which brackets P-T conditions of formation to about 9-16 kbar and 775-875°C.

A parallel vein that is likely of the same generation contains the borosilicate phases prismatine and grandidierite, but no kyanite. Both vein types have undeformed needles of dumortierite up to 3 mm in length. Prismatine, the boron-rich endmember of the kornerupine group, is commonly associated with kyanite in high pressure rocks elsewhere, but grandidierite in other localities appears to be restricted to conditions of the sillimanite zone. Together, the borosilicate assemblages constrain the P-T conditions of vein crystallization to ≥10 kbar and ~ 750-850°C.

Because the host rocks and the veins must have experienced the same P-T history, we conclude that there was insufficient time for the host rocks to equilibrate under the later high pressure conditions recorded by the veins. Slow reaction rates due to anhydrous conditions are not a likely explanation given the abundance of biotite and hornblende in the host rocks. The absence of ductile deformation since the time of vein emplacement, even at temperatures above 750°C, suggests instead that the deep crust in this part of Labrador had a very short residence time under the conditions of the kyanite zone. Rapid decompression from those conditions would account for the relatively brittle behavior of the terrane as it was exhumed.