The application of detrital zircon grains from Pleistocene eskers for reconnaissance bedrock mapping in the Acasta Gneiss Complex

• Author / Creator

  Grayson, Bilak, S.

• The Acasta Gneiss Complex (AGC) is a ~2400 km2 Hadean-Mesoarchean terrane that contains the oldest known zircon-bearing rocks on Earth. Despite its importance for early Earth geology, only a small fraction (~50 km2) has been mapped in detail. To determine the lateral extent of ancient rocks and assess relative proportions of diverse-age rock units in the vast, little-studied parts of the AGC, I used a novel approach based on detrital zircon grains recovered from a series of eskers that transects the complex. The zircon grains were derived from the subglacial erosion of AGC and surrounding bedrock during the late Pleistocene and are therefore a proxy for the ages of exposed bedrock in the study area. U-Pb dates on ~3600 detrital zircons from three grain-size fractions (50 – 1mm, 12 – 2mm, and ≤ 250µm) from four separate esker transects yielded age distributions that coincide with ages of regionally mapped AGC bedrock, and the adjacent Wopmay Orogen and Slave craton. Based on detrital zircon age distributions and new reconnaissance-scale mapping, I infer that 3.37 Ga granitoids are a volumetrically significant component of the unmapped AGC. Esker zircons >3.7 Ga are present in several esker samples but at low abundance, which suggests that Eoarchean and Hadean rocks are a volumetrically subordinate component of the exposed AGC. However, the data also show that unmapped rocks at least as old as 3.95 Ga are present close to the inferred eastern limit of the AGC. I also outline several practical considerations for future detrital zircon esker-based bedrock exploration projects. Coarser grain size fractions reflect more proximal bed rock units, and thus are more useful for approximating rock unit boundaries, while a sample spacing of 5 to 10 km provides sufficient spatial resolution to observe gradual changes in detrital zircon age spectra across the esker transect.

• Subjects / Keywords

  • Zircon
  • Geochronology
  • Acasta Gneiss Complex

• Graduation date

  Fall 2019

• Type of Item

  Thesis

• Degree

  Master of Science

• DOI

  https://doi.org/10.7939/r3-zzse-zc64

• License

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