Decompression Reactions and P--T Conditions in High-grade Rocks, Northern Labrador: P--T--t Paths from Individual Samples and Implications for Early Proterozoic Tectonic Evolution

The boundary between the Archaean Nain and Rae Provinces, knownas the Early Proterozoic Torngat orogen, has been examined inthe Saglek Fiord area of northern Labrador. Torngat orogen, up to 40 km wide, is principally composed ofgranulite and amphibolite facies gneisses that are characterizedby transcurrent sinistral shear zone fabrics that were subsequentlypartially reworked in an event involving east-directed thrusting.This paper documents the Early Proterozoic metamorphic historyof some gneisses from the Saglek Fiord area of Torngat orogen.Petrographic and thermobarometric studies show that evidenceof both peak thermal conditions and subsequent decompressionreactions are preserved and that portions of this part of theP–T path (here called the P–T vector to emphasizethe direction of P–T changes), are preserved in individualsamples. Highest P and T ({small tilde} 10 kb and 800?C) arerecorded by cores of coexisting minerals in equigranular aggregates,whereas rims record variable degrees of post-peak re-equilibration.Substantial decompression accompanied by cooling (down to {smalltilde}5 kb and 650?C) is recorded by symplectites developedat garnet rims adjacent to clinopyroxenes. These symplectitesare interpreted to have developed during erosion and upliftimmediately following the peak metamorphic event. A tectonic model incorporating the structural and metamorphicobservations is presented. It is suggested that the Proterozoic-Archaeanboundary in the Saglek area developed through crustal thickeningand north-south oriented transcurrent shearing followed by east-directedthrusting, possibly through a continuous process of obliquecollision.