Onset of new, progressive crustal growth in the central Slave craton at 3.55 Ga

doi: 10.7185/geochemlet.1907 Ancient rock samples are limited, hindering the investigation of the processes operative on the Earth early in its history. Here we present a detailed study of well-exposed crustal remnants in the central Slave craton that formed over a 1 billion year magmatic history. The tonalitic-granodioritic gneisses analysed here are broadly comparable to common suites of rocks found in Archean cratons globally. Zircon Hf isotope data allow us to identify a major change positive εHf starting at ~3.55 Ga. The crust production processes and spatial distribution of isotopic compositions imply variable interaction with older crust, similar to the relationships seen in modern tectonic settings; specifically, long-lived plate margins. A majority of the Slave craton might have been formed by a similar mechanism. Received 1 November 2018 | Accepted 20 February 2019 | Published 28 March 2019 1. Department of Terrestrial Magnetism, Carnegie Institution for Science, Washington DC, 20015, USA 2. Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, AB, Canada T6G2E3 3. Northwest Territories Geological Survey, Yellowknife, NT, Canada X1A1K3 * Corresponding author (email: jreimink@carnegiescience.edu) Introduction The growth and preservation of Earth’s continental crust is an important planetary differentiation process. The Eoto Mesoarchean is an important era when crustal preservation was just beginning to become successful. Crust from this period is the only vestige that remains to evaluate early geodynamics, the evolution of the atmosphere and ocean chemistry, and ultimately, the origin of life. Though many recent models suggest that a large fraction of the present crust formed in the Archean (e.g., Dhuime et al., 2015), details of the timing of its extraction from the mantle and the mechanism of continental growth remain unclear, in part because of an incomplete rock record in many areas. Here we study an ancient crustal terrane that preserves an exceptionally long record of crustal production, allowing us to track the geodynamic processes of crust formation for over 1.5 Gyr. Archean cratons are blocks of stable continental crust >2.5 billion years old, forming the cores of Earth’s most ancient continents. The Slave craton of the Northwest Territories, Canada is an archetypal example, with extensive exposures of Mesoto Palaeoarchean rocks, making it ideally suited for tracing crust formation and reworking in the Archean. The core of the Slave craton—the Central Slave Basement Complex (CSBC; Bleeker et al., 1999a,b)—consists of extensive exposures of basement gneiss thought to extend even farther beneath younger granitoids (Davis and Hegner, 1992; Thorpe et al., 1992; Fig. 1). Although other cratons are also endowed with basement gneiss complexes (e.g., Moyen and Martin, 2012), the Slave craton is exceptional for the age range of exposed crust within a coherent geographic area. The detailed petrologic and tectonic framework for the formation of the Acasta Gneiss Complex (AGC) from 4.02 to 3.4 Ga (e.g., Reimink et al., 2018 and references therein) can be used to explore how the younger, more geographically expansive, CSBC is related to the AGC in the subsequent billion years of crustal evolution. The new data presented here from both younger AGC samples (3.4−2.94 Ga) as well as a suite of 3.4–2.9 Ga CSBC samples, allow direct examination of the evolution of the broader Slave craton. Geology and Geochemistry of the Slave Basement Gneisses The definition of the CSBC (Bleeker et al., 1999a) encompasses the zone of exposed basement gneisses in the central to south-central portion of the craton, including the Acasta Gneiss Complex. However, we refer to the CSBC and AGC independently to compare their formational mechanisms. The Acasta Gneiss Complex, on the westernmost margin of the Slave craton (Fig. 1), has received by far the most scientific interest due to the presence of the oldest known zircon-bearing