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

• Published in: Journal of Petrology
• Main Authors: MENGEL, FLEMMING, RIVERS, TOBY
• Format: Text
• Language: English
• Published: Oxford University Press 1991
• Subjects: Articles; Nain; Saglek Fiord
• Online Access: http://petrology.oxfordjournals.org/cgi/content/short/32/1/139; https://doi.org/10.1093/petrology/32.1.139

The boundary between the Archaean Nain and Rae Provinces, known as the Early Proterozoic Torngat orogen, has been examined in the Saglek Fiord area of northern Labrador. Torngat orogen, up to 40 km wide, is principally composed of granulite and amphibolite facies gneisses that are characterized by transcurrent sinistral shear zone fabrics that were subsequently partially reworked in an event involving east-directed thrusting. This paper documents the Early Proterozoic metamorphic history of some gneisses from the Saglek Fiord area of Torngat orogen. Petrographic and thermobarometric studies show that evidence of both peak thermal conditions and subsequent decompression reactions are preserved and that portions of this part of the P–T path (here called the P–T vector to emphasize the direction of P–T changes), are preserved in individual samples. Highest P and T ({small tilde} 10 kb and 800�C) are recorded 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 {small tilde}5 kb and 650�C) is recorded by symplectites developed at garnet rims adjacent to clinopyroxenes. These symplectites are interpreted to have developed during erosion and uplift immediately following the peak metamorphic event. A tectonic model incorporating the structural and metamorphic observations is presented. It is suggested that the Proterozoic-Archaean boundary in the Saglek area developed through crustal thickening and north-south oriented transcurrent shearing followed by east-directed thrusting, possibly through a continuous process of oblique collision.