Paleoproterozoic metamorphic and deformation history of the Thompson Nickel Belt, Superior Boundary Zone, Canada, from in situ U-Pb analysis of monazite

The Thompson Nickel Belt is a 35 km × 200 km northeast-trending segment of the northwest margin of the Archean Superior craton in Manitoba, bounded to the west by the Paleoproterozoic Reindeer Zone. The belt was metamorphosed and deformed during the Trans-Hudson orogeny, resulting in an elongate, nested metamorphic zonal pattern, parallel to the overall trend of the belt. The metamorphic pattern consists of an inner middle amphibolite-facies zone, flanked on both sides by upper amphibolite-facies zones, flanked in turn by granulite-facies zones. U-Pb data were collected by laser ablation-multiple collector-inductively coupled plasma-mass spectrometry from metamorphic monazite in polished thin sections from each of the metamorphic zones. In the middle amphibolite-facies and upper amphibolite-facies zones, monazite in metapelite is interpreted to have developed during the dominant, foliation-forming deformation event and yields ages of ca. 1776 and 1779 Ma, respectively. Metapelite from the eastern granulite-facies zone contains ca. 1834 Ma monazite grains intergrown with sillimanite that is pseudomorphous after andalusite and grew during the dominant foliation-forming event. Metawacke from the eastern granulite-facies zone contains ca. 1826 Ma and 1792 Ma monazite grains that are interpreted to be contemporaneous with the main foliation-forming event and were unstable in the presence of melt, and a third population of ca. 1748 Ma monazite grains. In the western granulite-facies zone, metapelite contains ca. 1803 Ma monazite grains intergrown with sillimanite that grew during the dominant foliation-forming event, and meta-iron formation contains ca. 1752 Ma monazite grains characterized by oscillatory zoning and associated with melt pseudomorphs. A tectonic model is presented in which the initiation of collision between the Reindeer Zone and the Superior craton begins around ca. 1850-1840 Ma. With continued convergence, rocks now present in the western and eastern granulite-facies zones were progressively buried and heated to middle amphibolite-facies conditions by ca. 1803 and 1792 Ma, respectively. As the collision progressed, rocks now present in the upper amphibolite-facies and middle amphibolite-facies zones were buried and subjected to lower amphibolite-facies metamorphic conditions by ca. 1779 and 1776 Ma, respectively. Progressive regional metamorphism may have been punctuated by local thermal events related to magmatism, such as at ca. 1830 Ma in the eastern granulite-facies zone. Post-peak metamorphic cooling and differential uplift of crustal blocks related to transpressional deformation is recorded by ca. 1752 and 1748 Ma monazite grains from the western and eastern granulite-facies zones, respectively.