Geochronology and Petrogenesis of Hadean to Paleoarchean Mafic and Felsic Crust from the Northeastern Superior Province, Canada
The first billion years of our planet’s history is almost devoid of geological records and this scarcity of Eoarchean/Hadean rocks and minerals greatly limits our understanding of how and when the first crust formed on Earth. The Nuvvuagittuq Greenstone Belt (NGB), located in the Hudson Bay terrane...
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| Format: | Thesis |
| Language: | English |
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Université d'Ottawa / University of Ottawa
2021
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| Online Access: | http://hdl.handle.net/10393/41641 https://doi.org/10.20381/ruor-25863 |
| Summary: | The first billion years of our planet’s history is almost devoid of geological records and this scarcity of Eoarchean/Hadean rocks and minerals greatly limits our understanding of how and when the first crust formed on Earth. The Nuvvuagittuq Greenstone Belt (NGB), located in the Hudson Bay terrane of the Northeastern Superior Province, may host the oldest preserved rocks on Earth. It is locally intruded in its southwestern corner by rare 3.76 Ga trondhjemite bands which impose a minimum age for the NGB, but its dominant lithology, a mafic cummingtonite-amphibolite called the Ujaraaluk unit, displays isotopic evidence suggesting it may represent a rare remnant of Hadean mafic crust as old as 4.3 Ga. However, this proposed Hadean age for the NGB has been heavily debated for more than a decade. As potentially the only remnant of crust formed within the first 500 million years of Earth’s history, the NGB could have important implications on our knowledge of the first terrestrial crust. In order to impose tighter geochronological constraints on the NGB, here we present U-Pb zircon and ¹⁴⁷Sm-¹⁴³Nd whole rock data for gneissic gabbro sills that intrude the Ujaraaluk unit as well as U-Pb data for zircons from intruding and surrounding granitoids. A new strategy of sampling for the gneissic gabbros targeted the most evolved plagioclase-rich zones and amphibole-rich cumulative rocks to better constrain their age of magmatic differentiation. The most evolved parts of the sills were also sampled because they are the most likely to contain igneous zircon or baddeleyite that could constrain their crystallization age. Zircons from two compositionally evolved gneissic gabbros yielded U-Pb ages between 2.7 and 2.6 Ga consistent with the timing of Neoarchean metamorphism in the region. A plagioclase-rich layer found within the gabbro sills yielded zircons defining a U-Pb Concordia upper intercept age of 2789 Ma, but their texture and Th/U ratios are more consistent with recrystallization of zircon subsequent to the breakdown of ... |
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