Borosilicate‐ and phengite‐bearing veins from the Grenville Province of Labrador: evidence for rapid uplift

Abstract

Quartzo-feldspathic veins emplaced within a migmatite terrane near Wilson Lake in the Grenville Province of central Labrador record a metamorphic event not evident in the host rocks. The discordant veins are undeformed and have undisturbed primary igneous/hydrothermal textures. Most of the veins contain euhedral kyanite, as well as aggregates of kyanite, K-feldspar, phlogopite and minor dumortierite which are likely pseudomorphs after primary phengite. The reconstructed phengite compositions range from 3.1 to 3.2 Si per 11 oxygen formula unit. The pseudomorph assemblage is interpreted as the product of phengite + quartz melting under H 2 O-undersaturated conditions, 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, dumortierite, prismatine and grandidierite, but no kyanite. The borosilicate assemblages constrain the P-T conditions of vein crystallization to ‡10 kbar and c. 750-850°C. Vein emplacement is constrained to T £ 875°C at the same pressures, which is well within the kyanite zone. Because the host rocks and veins must have experienced the same P-T history following vein emplacement, the presence of unreacted sillimanite in the host migmatites implies insufficient time for host rock equilibration. Slow reaction rates because of anhydrous conditions are not a likely explanation given the abundance of biotite and hornblende in the host rocks. The ductility implied by the breakdown of a hydrous phase (phengite) and the production of an H 2 Oundersaturated melt in the veins contrasts with the apparently brittle behaviour of the host rocks. The absence of deformation since the time of vein emplacement, even at temperatures above 750°C, suggests that the deep crust in this part of Labrador had a very short residence time under conditions of the kyanite zone. Rapid decompression from those conditions is consistent with quartz + phengite melting and accounts for the relatively brittle behaviour of the terrane as it was uplifted.

Melting of eclogite facies sedimentary rocks in the Belomorian Eclogite Province, Russia

“…Magnesiodumortierite is noted in other metamorphic complexes, for example, in the UHP rocks from the Dora Maira Massif in the Western Alps (Schertl et al ., ) and in the cordierite–orthoamphibole‐bearing rocks from the high‐grade Bamble Sector in South Norway (Visser & Senior, ). Blue dumortierite occurs in polymineralic pseudomorphs resembling those described here in borosilicate‐ and phengite‐bearing veins from a migmatite terrane near Lake Wilson in the Grenville Province of central Labrador (Korhonen & Stout, ).…”

Stability of sapphirine + quartz in the oxidized rocks of the Wilson Lake terrane, Labrador: calculated equilibria in NCKFMASHTO

“…, 1986; Herd et al. , 1987; Currie & Gittens, 1988; Korhonen & Stout, 2005). The changes in M H2 O on Fig.…”

“…Rivers et al. , 1989; Gower & Krogh, 2002), although a significant Labradorian component has been identified in other areas, including the Wilson Lake terrane (Korhonen & Stout, 2005). These areas that have largely escaped penetrative Grenvillian‐aged metamorphism characterize the Orogenic Lid of Rivers (2008).…”

“…Coronas of secondary orthopyroxene, cordierite and sillimanite commonly separate sapphirine and quartz, suggestive of near‐isobaric cooling from peak conditions (Currie & Gittens, 1988; Korhonen & Stout, 2004b). The cooling path extended into the kyanite stability field, and accounts for kyanite–garnet–cordierite assemblages in the leading edge of the massif and abundant kyanite in the basal shear zone and in thin borosilicate‐bearing veins (Korhonen & Stout, 2004b, 2005). These borosilicate‐ and kyanite‐bearing veins also provide evidence for rapid decompression from lower crustal conditions (750–850 °C, ≥10 kbar; Korhonen & Stout, 2005).…”

“…The cooling path extended into the kyanite stability field, and accounts for kyanite–garnet–cordierite assemblages in the leading edge of the massif and abundant kyanite in the basal shear zone and in thin borosilicate‐bearing veins (Korhonen & Stout, 2004b, 2005). These borosilicate‐ and kyanite‐bearing veins also provide evidence for rapid decompression from lower crustal conditions (750–850 °C, ≥10 kbar; Korhonen & Stout, 2005). U–Pb ages from zircon and monazite in the metapelitic assemblages in the massif (Currie & Loveridge, 1985; Corrigan et al.…”