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Geochemical and textural investigations of the Eoarchean Ukaliq supracrustals, Northern Québec (Canada)

Abstract : Structural, geochronological and geochemical studies of pre-3.75 Ga rocks of volcano-sedimentary protoliths in the Inukjuak domain in the Superior Province in Québec have been mostly focused on the Nuvvuagittuq Supracrustal Belt (NSB). The Porpoise Cove outcrops – at the southwestern limit of the NSB – are the de-facto “type locality” for the supracrustals of the Inukjuak Complex. Yet, it remains unclear whether the NSB rocks are geochemically distinct from, or are in fact common to, a host of other supracrustal enclaves locked in the dominantly gneissic Inukjuak domain. Here, we report detailed textural and geochemical studies for a suite of rocks from the Ukaliq Supracrustal Belt (USB), located approximately 3 km northeast of the NSB. We find that the USB and NSB have a similar protracted metamorphic history; both experienced amphibolite grade metamorphism and contain granitoid gneiss sheets, quartz-magnetite rocks (banded iron-formation s.l.) and quartz-biotite schists within amphibolitized rocks of basaltic affinity with local retrogressions. If the various Inukjuak supracrustal belts were once a part of a larger coherent (now dismembered) terrane, they should show similar emplacement ages and source chemistry. New zircon Usingle bondPb geochronology from five gneissic units and two quartz-biotite (metasedimentary) schists reveal the oldest emplacement ages across all units of each individual rock type to be 3.68 ± 0.07 Ga (granitoid gneisses) and 3.65 ± 0.06 Ga (quartz-biotite schists). These new ages are similar to those documented as likely minimum emplacement ages of the NSB determined by Usingle bondPb geochronology. Zircons from the quartz-biotite schist were also analyzed by ion microprobe for their Usingle bondPb geochronology and were found to yield statistically identical, albeit more precise, ages than those obtained by LA-ICP-MS. Possible detrital zircons from the USB quartz-biotite schists were analyzed by ion microprobe for their coupled δ30SiNBS28 and δ18OVSMOW values with respective values between −0.75 and − 0.07‰ (δ30SiNBS28), and + 5.61 and + 6.59‰ (δ18OVSMOW). The δ18OVSMOW values, which are on average above mantle-derived zircon, indicate contribution of altered, non-mantle, derived material into the source of the rocks that weathered to form the quartz-biotite schists. Zircon mineral inclusions (quartz, feldspar, apatite, biotite, muscovite and other unrecognized Fe/Al/Si rich phases), along with the major- and trace element contents for the rocks were analyzed to substantiate this interpretation. Together with δ30SiNBS28, δ18OVSMOW, our results suggest that lithologies like authigenic silica and serpentinized rocks re-melted to form the parent melts that gave rise to zircons found in the USB quartz-biotite schists. Additional Lusingle bondHf studies reported here on the same zircons also show similarities with NSB zircons. The εHf values showed a positive correlation with the measured Usingle bondPb age from −22.7 ± 0.8 to +1.9 ± 1.1. The Lusingle bondHf system also reveals that the USB, extracted at ca. 3.8 Ga, carries isotopic signatures of an older Hadean reservoir, having been formed from an Eoarchean mafic melt that incorporated them. Taken together, this supports a co-genetic origin for the NSB and the USB.
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https://hal.uca.fr/hal-03101296
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Submitted on : Monday, January 11, 2021 - 9:56:48 AM
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Wriju Chowdhury, Dustin Trail, Martin Guitreau, Elizabeth Bell, Jacob Buettner, et al.. Geochemical and textural investigations of the Eoarchean Ukaliq supracrustals, Northern Québec (Canada). Lithos, Elsevier, 2020, 372-373, pp.105673. ⟨10.1016/j.lithos.2020.105673⟩. ⟨hal-03101296⟩

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