SILICATE-OXIDE EQUILIBRIA IN THE WILSON LAKE TERRANE, LABRADOR, AND IMPLICATIONS FOR GRANULITE FACIES METAMORPHISM UNDER HIGH FO2 CONDITIONS

The Wilson Lake terrane of central Labrador is part of the Grenville Structural Province and is comprised of an allochthonous block of high T metamorphic rocks separated from lower-grade parautochthonous rocks by a ductile shear zone. The granulite-grade metamorphism in the allochthon is a result of the Labradorian Orogeny (1700-1635 Ma) with structural and metamorphic reworking associated with the Grenville Orogeny (1100-900 Ma). Tectonic emplacement of the allochthon is generally thought to be a Grenvillian feature. The presence of titanhematite ± magnetite in all major rock units is evidence for relatively high oxygen fugacity (fO2) during metamorphism. Subsequent cooling and exsolution of titanhematite is responsible for the stable magnetic remanence in the area.

The allochthon contains the stable association of Opx + Sil + Qtz, and Spr + Qtz, and corresponds to an aeromagnetic high, caused by a stable remanence carried by grains of titanhematite closely associated with the granulite facies mineral assemblages. Some samples from the allochthon contain a silica-undersaturated assemblage that approaches the [Crd]-absent invariant point in the FMAS system under high fO2 conditions. The P-T-fO2 conditions represented by the [Crd]-absent invariant point are such that XMg(Grt) > XMg(Spl), which requires the stability of Spl + Qtz. The phase relationships and the absence of Grt + Crd assemblages in the allochthon further suggests that the [Crd]-absent invariant point is stable in the field of Spr + Qtz. The partitioning sequence XFe3+(Spr) > XFe3+(Spl) > XFe3+(Opx) > XFe3+(Grt) extends the stabilities of Spr and Spl relative to Opx and Grt, shifting the [Crd]-absent invariant point to lower T. The allochthon shows evidence for extensive partial melting. Textural evidence and phase equilibrium arguments suggest that the titanhematite is a product of dehydration melting of Bt.

Mylonite in the basal shear zone of the allochthon and samples from the leading edge of the allochthon contain lower temperature mineral assemblages (Ky + Crd + Grt ± Sil) than those generally in the allochthon. The rocks associated with the parautochthon also show evidence for lower-grade conditions, including an abundance of hydrous minerals, and are characterized by an aeromagnetic low, consistent with a lesser degree of partial melting.