CRUSTAL THINNING AND AMCG-TYPE MAGMATISM: A CASE STUDY FROM THE MAURICIE REGION, QUÉBEC, AND IMPLICATIONS FOR THE EVOLUTION OF THE MÉKINAC-MORIN GRANULITE DOMAIN

The Adirondack Highlands and their potential northern extension in Canada (Morin and Mékinak domains) are characterized by high-T and medium- to high-P granulites, as well as voluminous anorthosite massifs and associated anhydrous melts that were emplaced during the interval 1.16-1.13 Ga. In the Mauricie region (northeastern flank of the Morin-Mékinac domain), structural analyses, as well as U-Pb and 40Ar/39Ar hornblende ages on accessory minerals, suggest that the granulites were exhumed during the interval 1.09-1.06 Ga by a combination of erosion and oblique extension along a zone of crustal transtension (Tawachiche shear zone). The Tawachiche shear zone and it’s hanging wall were also the focus of AMCG-type magmatism dominated by ca. 1.08-1.06 Ga gabbro-norite, mangerite and charnockite. During this crustal extension event, ‘cold’ upper-crustal level rocks that preserve a pre-Grenvillian (ca. 1.39 Ga) high-T, medium- to low-P metamorphic imprint were down-dropped and juxtaposed against the ‘hot’ footwall rocks (Central Granulite Terrane (CGT)), producing relatively rapid cooling in the footwall and moderate re-heating in the hanging-wall. On the northwestern side of the CGT, relatively cold crust was similarly preserved in the Frontenac Terrane, which was also the locus of crustal- and mantle-derived melts of ca. 1.09-1.07 Ga age. The above structural, magmatic and tectonothermal constraints suggest that: 1) the Carthage-Colton, Labelle and Tawachiche shear zones may have formed detachments via which the granulite core was exhumed, and 2) the CGT may have formed a core complex-like structure during the Ottawan Orogeny, and was likely underlain or flanked by relatively thin lithospheric mantle.