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