Central Slave Basement Complex Evolution

✦ The surface rocks in this picture, unfortunately with some lichen cover, are part of the Central Slave Basement Complex (CSBC) and were sampled during fieldwork for the CSBC project. Except where covered by lakes, these 3.2 to 3.7 billion year old rocks can be easily sampled with a rock hammer. (Photo: Steve Shirey)

Central Slave Basement Complex Project

Slave Craton Map

Geologic map of the Slave craton highlighting the major domains that contain basement gneisses. Map modified from St.Onge et al. (1988); Bleeker et al. (1999a,b). Major basement complexes are shown along with crustal province boundaries inferred from Nd and Pb isotope data (Davis and Hegner, 1992; Thorpe et al., 1992). Red stars indicate key locations sampled in this study. (Caption and Figure from Reimink, J. R., Pearson, D. G., Shirey, S. B., Carlson, R. W., Ketchum, J. W. F. (2019) Onset of new, progressive crustal growth in the central Slave craton at 3.5 Ga. Geochemical Perspectives Letters, 10, 7-12. http://doi.org/10.7185/geochemlet.1907.)

Personnel

Jesse Reimink (Project Leader), Carnegie Institution for Science

Richard Carlson, Carnegie Institution for Science

Graham Pearson, University of Alberta

Steven Shirey, Carnegie Institution for Science

Project Description

Only a small fraction of the crust existing on Earth today dates to the first billion years of Earth's existence. What Earth's original crust looked like, and what processes led the planet to form continental crust and underlying mantle that could resist 4 billion years of later tectonic reworking are largely unknown. Answers to these questions can provide key information on the dynamic processes operating in Earth's interior shortly after Earth formation.

This project will collect and analyze gneisses from a much wider spatial and temporal coverage from the central Slave basement in order to define the extent of reworking of Hadean material and its importance in the formation of this fragment of ancient crust.

We will make use of the 146Sm-142Nd and 182Hf-182W radioactive isotope systems, whole rock 147Sm-143Nd, Lu-Hf, and U-Pb isotopic measurements, and U-Pb and Hf isotope data for zircons to investigate the role Hadean (> 4 billion year (Ga) old) crust played in the genesis of the Slave craton of the Northwest Territories, Canada. Data of this nature for rocks from the best-studied locality in the Slave craton, the Acasta Gneiss Complex, show that these circa 3.8-4.02 Ga rocks contain much older components, approaching ages suggesting derivation from what may be the first crust formed on Earth. While the Acasta Gneiss Complex has seen extensive study, it represents but a small fraction of the basement rocks of the Slave craton.

Demonstration of the existence and extent of these ancient components in old crustal rocks across a broad region of the Slave craton will lead to new advances:

illumination of the nature of Earth's earliest crust prior to the time when significant crustal fragments began to be preserved
establishment of the role such ancient crust played in forming the oldest preserved crustal sections
potential detection of whether Earth experienced an enhanced bombardment of extraterrestrial material circa 3.9 Ga
information on the connection between crust formation and the development of a thick section of underlying mantle
the potential for a fundamental change in the mechanism of continent formation occurred circa 3 Ga.

Central Slave Basement Complex Rocks

✦ Pink and grey banded gneisses from Point Lake, NWT, Canada —the oldest rocks studied in the Central Slave Basement Complex. Such obvious intermixing of contrasting rock types is one of the challenges in studying some of the oldest rocks on Earth. Rocks such as these are hallmarks of the oldest continental crust on Earth and are a direct record of what the Earth was like almost 4 billion years ago. (Photo: Jesse Reimink)

Not all the samples taken from the CSBC look as variable in composition as the rocks from Point Lake pictured above. Those from the several hundred million year old younger Big Bear Lake, Brown Lake and Patterson Lake localities are more homogeneous grey gneisses. It will be interesting to see if the petrogenetic processes vary with age and position southward in the Slave craton.

Major and trace element compositions

✦ Ca-Na-K ternary plot of the major element compositions of gneissses from the CSBC and Acasta Gneiss Complex versus various types of tonalite-trondhjemite-granodiorite (TTG) suites.

✦ Strontium-yttrium versus lanthanum-ytterbium ratios versus ratios for gneissses from the CSBC and Acasta Gneiss Complex. See legend in left fig.

✦ Lanthanum-ytterbium ratios versus versus niobium-tantalum ratios for gneissses from the CSBC and Acasta Gneiss Complex. See legend in left fig.

The CSBC and Acasta show a range in composition but in general plot towards the fields of lower pressure tonalite-trondhjemite-granodiorites (TTG's) meaning that they formed at crustal depths of melting somewhat shallower than the garnet stability field. As isotopic data is added to the major and trace elements, constraints on the petrogenetic models will allow us to see whether the continental crust in this region is generated by geodynamic processes that start to look like plate tectonics or not.

Hf isotopic composition of zircon shows new crustal additions at 3.5 Ga

✦Zircon Hf isotope data from Slave basement gneisses. Orange symbols are analyses from the CSBC, blue are new analyses from the AGC, grey circles are a compilation of Acasta Hf isotope data (sources in the text), and small circles are single detrital zircon analyses from sediments of the Slave Craton Cover Group sequence (Pietranik et al., 2008). Note that detrital zircon analyses are separated by sediment location (see Supplementary Information). Grey eld is the evolution of Hadean ma c protocrust, while orange and blue lines show our interpretations (sloped lines for time-integrated isotope evolution, vertical lines for mixing) for systematic petrogenetic differences between the two portions of the Slave basement gneisses. The Archean depleted mantle evolution line shown here is the connector line between a chondritic source at 4.4 Ga and modern MORB εHf values of +17. Calculated with respect to this model evolution for the depleted mantle, the inset shows the maximum crustal residence times (depleted mantle model ages calculated using a source ¹⁷⁶Lu/¹⁷⁷Hf of 0.015, and then subtracting the U-Pb crystallisation age from this model age) for all Slave craton Hf isotope data. The transition from long to short crustal residence times occurs at 3.6 Ga. (Caption and Figure from Reimink, J. R., Pearson, D. G., Shirey, S. B., Carlson, R. W., Ketchum, J. W. F. (2019) Onset of new, progressive crustal growth in the central Slave craton at 3.5 Ga. Geochemical Perspectives Letters, 10, 7-12. http://doi.org/10.7185/geochemlet.1907.)

A clear distinction is seen in the data set (above) between Acasta Gneiss Complex (AGC) rocks (light blue) that were derived from mafic crustal precursors and Central Slave Basement Complex (CSBC) rocks (orange) that were derived from depleted mantle sources. The former were internally recycled and represent reprocessing of older crust whereas the latter represent additions of new crust to the continent that were subtly overprinted by crustal contamination. New additions to the continental crust in the Mesoproterozoic rule out the wholesale recycling of an early-formed continental crust as advocated by some workers.

Reimink, J. R., Pearson, D. G., Shirey, S. B., Carlson, R. W., Ketchum, J. W. F. (2019) Onset of new, progressive crustal growth in the central Slave craton at 3.5 Ga. Geochemical Perspectives Letters, 10, 7-12. http://doi.org/10.7185/geochemlet.1907.

Reimink, J. R., Chacko, T., Carlson, R. W., Shirey, S. B., Liu, J., Stern, R. A., et al. (2018). Petrogenesis and tectonics of the Acasta Gneiss Complex derived from integrated petrology and 142 Nd and 182 W extinct nuclide-geochemistry. Earth and Planetary Science Letters, 494, 12–22. http://doi.org/10.1016/j.epsl.2018.04.047