The Paleoproterozoic Thompson nickel belt, a segment of the northwest margin of the Archean Superior craton in Manitoba, is one of the largest nickel-producing regions in the world and the second largest Ni-Cu-PGE (platinum group elements) mining camp in Canada after Sudbury. The nickel sulfide ores are hosted in or associated with ultramafic sills of komatiitic affinity that were emplaced into sulfur-rich metasedimentary rocks of the Ospwagan Group and subsequently strongly deformed and metamorphosed (amphibolite to granulite facies) during the Trans-Hudson orogeny. A U-Pb zircon age of 1880.2 ± 1.4 Ma for a metaperidotite (olivine orthopyroxenite) from the upper part of a boudinaged ultramafic sill at the Pipe 2 deposit is interpreted as the age of crystallization of the intrusion, the first age from an ultramafic sill containing mined nickel sulfide in the Thompson nickel belt. This age confirms a direct petrogenetic relationship between Ni-Cu-PGE mineralization and widespread mafic-ultramafic magmatic activity (Molson igneous events) at ca. 1.88 Ga along the Superior province craton margin (e.g., Thompson nickel belt, Fox River belt, Cape Smith belt, New Quebec orogen). At the south pit of the Thompson mine, a U-Pb zircon age of 1860.7 ± 0.7 Ma for a composite mafic intrusion (garnet amphibolite to layered metagabbro) identifies a new age of mafic magmatic activity in the Thompson nickel belt that is unrelated to nickel sulfide mineralization. The thermal effects of the Trans-Hudson orogeny are recorded by a range of zircon dates and ca. 1760 Ma metamorphic titanite from the South pit intrusion and are consistent with interpretations that regional metamorphism occurred as a single progressive event. The results of this study open up the possibility of using the high-precision U-Pb geochronology of mineralized ultramafic rocks to precisely constrain the duration of mafic-ultramafic magmatism in the Thompson nickel belt and assess potential age differences between ultramafic sills across and along strike of this world-class nickel metallotect.
The end-Triassic extinction (ETE) event represents one of the 'big five' episodes of mass extinction. The leading hypothesis for the cause of the ETE is the intrusion of voluminous magmas of the Central Atlantic Magmatic Province (CAMP) into carbon-rich sediments of two South American sedimentary basins, around 201.5 Ma. The timing of dikes and sills emplacement, however, must be considered in light of age models from CAMP rocks occurring in North America. In this work, we present new high-precision ages for critical samples in NE Brazil (201.579 ± 0.057 Ma) and Canada (201.464 ± 0.017 Ma), in order to evaluate how the South and North American magmatic events compare at the 100-ka level, and to the ETE timing. We also discuss inter-laboratory reproducibility of high-precision CAMP ages, including the 230Th disequilibrium corrections that are made to zircon U-Pb dates. Our findings in this newly discovered extension of the CAMP large igneous province in NE Brazil support the hypothesis that the CAMP may be responsible for the ETE through the triggering of greenhouse gas release from magma-evaporite interactions (contact metamorphism) in the South American basins.
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