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Petrogenesis of the Moose Lake Area of the Acasta Gneiss Complex, Slave Craton, Northwest Territories, Canada

  • Author / Creator
    Belosevic, Michael B.K.
  • The Acasta Gneiss Complex (AGC) is a late Hadean to early Archean terrane in
    northwestern Canada known to contain the oldest evolved rocks on Earth. However, only a
    small portion of the AGC has been studied in detail. This study aims to explore one of the
    poorly explored portions of the AGC, the Moose Lake area, using a combination of bedrock
    mapping, petrographic analysis, whole-rock geochemistry, U-Pb geochronology, and isotope
    tracer analysis.
    Bedrock mapping indicates that the Moose Lake area can be divided by a gradational
    contact into a higher-strain western domain, and a lower-strain eastern domain. The area has
    undergone multiple phases of deformation in which the regional gneissosity has been folded
    in a later deformational event into map-scale folds with fold axes shallowly plunging to the
    southwest. Like at the original discovery site of the AGC at the Acasta River, the Moose
    Lake area is composed primarily of tonalitic-granodioritic gneisses, containing variable size
    layers and boudins of metagabbro and amphibolite. The bulk of the area is made up of
    tonalitic – granodioritic gneisses that formed during two main pulses of magmatism: one at
    3.72 – 3.68 Ga, and the other at 3.60 – 3.55 Ga. In contrast to previous work, a significant
    portion of the Moose Lake area is composed of a large homogeneous pluton of foliated
    granite which was emplaced at 3374.6 ± 8.5 Ma. The oldest felsic rock at Moose Lake
    (~3700 Ma) has strong heavy rare earth element depletion, which indicates magma formation
    at great depths. In turn, this suggests that the transition from shallow- to deep-level meltgeneration processes, which had previously been proposed to have occurred at ~ 3.6 Ga, may
    have happened ~100 Myr earlier. Unlike all the other 3.70-3.55 Ga granitoid gneisses at
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    Moose Lake, one of the ~3.6 Ga tonalites has geochemical signatures indicative of shallow
    melt generation similar to the oldest lithologies in the AGC.
    Many of the ‘metamorphic’ zircons found in the Moose Lake area record an age of ~
    3.30 Ga, which does not coincide with the age of any known age of plutonic activity in the
    AGC. Thus, 3.30 Ga may represent the time of regional metamorphism or regional
    hydrothermal activity in the AGC which led to subsolidus zircon growth.
    Mafic rocks, which are widespread in the AGC, are generally not well dated because
    of the absence or poor quality of zircon in these rocks. However, two of the metagabbro
    samples of my study contain abundant, good-quality zircon. On the basis of their zoning
    patterns and Th/U ratios, most of these zircons are interpreted to be of igneous origin and
    record a robust age of 3740.7 ± 2.3 Ma. These zircons also record an initial epsilon hafnium
    isotope value (εHfi) of -4.7 ± 0.24, a surprisingly unradiogenic value for a mafic rock.
    Assimilation-fractional crystallization (AFC) modelling suggests that it is difficult for a
    primary mafic magma with an initial chondritic Hf isotope composition to acquire such a
    negative εHf value through assimilation of known felsic- or intermediate-composition rocks
    in the AGC, without also increasing the magma SiO2 content above that observed in the
    metagabbro. Alternative models for generating this Hf isotope composition in the
    metagabbro include partial melting of an enriched mantle that had a long-term (Hadean)
    history of low Lu/Hf or assimilation of enriched, early Hadean-age mafic rocks that had
    evolved to strongly negative εHf by 3.74 Ga

  • Subjects / Keywords
  • Graduation date
    Spring 2021
  • Type of Item
    Thesis
  • Degree
    Master of Science
  • DOI
    https://doi.org/10.7939/r3-mk28-nf22
  • License
    This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.