METAMORPHISM IN THE PALEOPROTEROZOIC TORNGAT OROGEN, LABRADOR - PETROLOGY AND P-T-T PATHS OF AMPHIBOLITE-FACIES AND GRANULITE-FACIES ROCKS ACROSS THE KOMAKTORVIK SHEAR ZONE

The Paleoproterozoic Torngat Orogen in northern Labrador developed as a result of collision between the Archean Nain and Southeast Rae crato ns. Early phases involved contractional deformation and crustal thicke ning, whereas later phases were dominated by transcurrent and subseque nt near-vertical displacements. The axial region of the orogen is occu pied by two major structures, the Abloviak (ASZ) and Komaktorvik (KSZ) shear zones. The gneisses of the Nain Province east of the KSZ are ch aracterized by Archean granulite-facies assemblages; these become prog ressively overprinted westward by static amphibolite- to granulite-fac ies assemblages of Paleoproterozoic age toward the KSZ. P-T determinat ions on the Avayalik dykes and Archean mafic gneisses define a P-T arr ay from ca. 11 kbar -750 degrees C to ca. 6 kbar -600 degrees C. The K SZ is underlain by variably reworked Archean gneisses, mafic and pelit ic supracrustal rocks, the Avayalik dykes, anorthosite, and members of the Paleoproterozoic diorite - tonalite - granodiorite (DTG) suite. T he peak grade of metamorphism is upper amphibolite to granulite facies ; however, P-T determinations from various microstructural settings re cord a substantial part of the post-peak history. In a range of rock t ypes, an array is defined from ca. 11.7 kbar -720-800 degrees C to ca. 6.5 kbar -540 degrees C. P-T data from west of the KSZ define an arra y from ca. 10 kbar -786 degrees C to ca. 4 kbar -500 degrees C. In any of the geographic zones above, P-T arrays from each rock type are par allel to the composite array defined by data from all zones. Furthermo re, vectors connecting P-T determinations from cores to rims of coexis ting minerals are parallel to the overall trend. The data strongly sug gest that the Avayalik dykes are most resistant to resetting during co oling, and that mafic orthogneisses, mafic supracrustal rocks and peli tic supracrustal rocks are progressively less robust. Available U-Pb g eochronological data suggest that the KSZ and its bordering segments f ollowed different P-T-t paths. From west to east, there is an overall younging in ages of metamorphic zircon, and almost 80 million years of tectonic and metamorphic activity along the KSZ is recorded by ages d etermined on titanite. The geographic variation in metamorphic ages is interpreted to result from migration of zones where active deformatio n (and availability of fluid) allowed continued equilibration and grow th of metamorphic mineral assemblages.