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

Abstract

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 iii 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