Petrogenesis of the Acasta Gneiss Complex, Northwest Territories, Canada

• Author / Creator

  Reimink, Jesse R

• This contribution presents the results of a new mapping and geochemical investigation of well-preserved rock units in the Acasta Gneiss Complex (AGC), Canada, and provides additional support for a broadly mafic Hadean Earth, in which the first evolved crust formation took place in a geodynamic environment unlike modern subduction zones. Rocks in the AGC, with crystallization ages spanning a billion years, document a transition in crust-forming processes starting in a shallow petrogenetic setting with the involvement of plagioclase that eventually evolves to a deep-seated regime dominated by partial melting in the presence of residual garnet. As part of this mapping campaign, we identified and mapped a well-preserved rock unit with an igneous crystallization age of 4.02 Ga, a unit informally termed the Idiwhaa tonalitic gneiss (Idiwhaa meaning “ancient” in the local aboriginal language). The unique geochemical signatures of the Idiwhaa unit are distinct from typical Archean (4.0–2.5 billion year old) evolved crust. Specifically, the strong iron-enrichments and flat rare-earth-element patterns contained in this unit imply a distinct environment of formation; these signatures suggest a petrogenesis involving high degrees of crystal fractionation from a relatively water-poor basaltic magma at shallow levels. Additionally, well-preserved zircons in this unit allow a further investigation of the geochemical evolution as zircon can retain geochemical information through high-temperature metamorphism. In particular, changes in oxygen isotope ratios between two phases of zircon growth, both with ~4.02 Ga crystallization ages, indicate assimilation by the Idiwhaa magma of pre-existing rocks that had been altered by high temperature surface waters. Combined with the whole-rock signatures discussed above, these data imply petrologic processes strikingly similar to those operating on modern Iceland, a setting that has long been proposed as a suitable analog for crust formation on the early Earth. Because of the presence of very well preserved zircons in the Idiwhaa unit, we are also able to evaluate the composition and age of pre-existing crust into which the Idiwhaa magma intruded, as well as the differentiation state of the mantle from which the magma was derived. To this end, Hf isotope data were obtained from two igneous phases of 4.02 Ga zircon material, as well as the highest precision dating yet undertaken on AGC rocks. These isotopic data, combined with the whole-rock data described above, suggest that there was little or no interaction with typical Archean continental crust during the formation of the Idiwhaa unit. This result, along with evolved Hf isotope signatures contained in zircon with mantle-like oxygen isotope ratios, suggests interaction with very ancient mafic crust or an early-enriched mafic reservoir. The combination of early enriched reservoirs and no evidence for interaction with evolved, continental crust suggests that the Hadean Earth may have been dominated by mafic rocks. Whole-rock major- and trace-element data combined with zircon uranium-lead and oxygen isotope data obtained from younger rocks from the AGC, including rocks formed at ~3.95, 3.75, 3.5–3.6 and 3.4–2.9 Ga, document a transition in crust-forming and likely tectonic processes over that time interval. Specifically, the earliest components of the AGC (~4.02 Ga) formed in an oceanic plateau-like environment and produced primarily mafic- to intermediate-composition rocks with flat REE patterns. Episodic melting of this thickened and partially hydrated pile at ~3.95 and 3.75 Ga at moderate depths produced tonalitic magmas with somewhat more fractionated REE patterns. The volumetrically dominant 3.5–3.6 Ga rocks of the AGC have steep REE patterns similar to those found in Archean granitoids worldwide. These patterns require deep-seated melting in the presence of abundant residual garnet in the source rock and possibly occurred in a subduction-like setting. Thus, the AGC as a whole records a gradual transition from an oceanic plateau setting to a subduction-like setting over an ~400 million year time interval. Similar models have been proposed for other ancient gneiss terranes, but this process occurred a few hundred million years earlier in the AGC.

• Subjects / Keywords

  • Archean tectonics
  • Acasta Gneiss Complex
  • Zircon
  • Hadean Earth
  • Iceland

• Graduation date

  Spring 2016

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R3FB4WV19

• 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.