Archean to Early Paleoproterozoic Tectono-Magmatic Evolution of the Western Rae Craton
Creators: Neil, Benjamin
Publisher: name: University of Alberta Library
Year: 2024

<p>The Rae craton is one of the largest Archean–Paleoproterozoic age terranes on Earth and preserves &gt;1.6 billion years of the planet’s history. However, significant uncertainty remains regarding how and when the crust of this craton was created and reworked. This thesis investigates the nature and evolution of continental crust in the western Rae craton, with a focus on the Nonacho Lake area (“Nonacho area”) of the Northwest Territories. The techniques used in this study include bedrock mapping, whole-rock geochemical and isotopic analyses, and accessory mineral trace-element and isotopic analyses. <br>
Granitoid and orthogneiss rocks (“granitoids”) with zircon U-Pb dates &gt;3.0 Ga and whole-rock Sm-Nd depleted mantle model ages &gt;3.2 Ga are shown to be widely distributed throughout the western Rae craton. They define a large (~1000 x 100 km) basement block referred to herein as the Perry River terrane (PRT). The PRT primarily comprises 3.3–3.1 Ga sodic tonalites and granodiorites that have chondritic to suprachondritic zircon initial εHf (εHfi) values and mantle-like zircon oxygen isotope compositions. These granitoids were derived from partial melting of mantle-derived basaltic rocks that had resided in the crust for a relatively short period of time, potentially at the base of a magmatic arc. The 3.3–3.1 Ga PRT granitoids appear to reflect a global period of enhanced continental growth. The juvenile nature of most 3.3–3.1 Ga granitoids worldwide argues that only small amounts of older continental crust were present, and thus available for magmatic reworking, on Earth at that time. This observation is at odds with models that suggest large volumes of continental crust were established on the early (&gt;3.5 Ga) Earth. <br>
The Nonacho area basement comprises variably deformed and heterogeneous granitoids, and rare metasedimentary rocks. Zircon U-Pb geochronology documents at least seven episodes of granitoid magmatism in the Nonacho area, at: ca. 3.25, 2.78, 2.71–2.68, 2.58–2.57, 2.52–2.48, 2.43–2.38 and 2.32–2.31 Ga. Zircon and monazite U-Pb age and trace-element data show that the protoliths to the metasedimentary rocks were deposited after ~2.52 Ga and underwent high-grade metamorphism/migmatization during the Arrowsmith orogeny at ~2.38 Ga. Geochemical and isotopic data indicate that the granitoids emplaced prior to ~2.45 Ga primarily represent juvenile crustal additions, whereas those emplaced after ~2.45 Ga reflect reworking of older (Archean) continental crust. <br>
The geochemical and petrogenetic characteristics of the ca. 2.52–2.48 (2.5), 2.43–2.38 (2.4) and 2.32–2.31 (2.3) Ga granitoids from the Nonacho area were studied in detail. The 2.5 Ga granitoids range from mafic to felsic, comprise a high-K calc-alkaline series, are light rare earth element (LREE)-enriched and have “subduction-like” trace-element patterns. They also yield consistently chondritic whole-rock initial εNd (εNdi) and near-chondritic zircon εHfi values. The homogeneous chondritic isotopic compositions suggest that all components of the suite (mafic to felsic) represent, at least in part, juvenile crustal additions. The mafic 2.5 Ga magmas are interpreted to have derived from a subduction-enriched lithospheric mantle source. Geochemical comparisons with possible Phanerozoic-age analogues suggest that the 2.5 Ga granitoids were emplaced in a post-collisional setting, potentially associated with the accretion of the PRT to the Rae craton. <br>
Both the ca. 2.43–2.38 and 2.32–2.31 Ga granitoid suites are characterized by &gt;69 wt.% SiO2 and negative εNdi values (-1 to -11). The former are strongly heavy rare earth element (HREE)-depleted, characterized by steeply-slopping REE patterns and in some cases contain abundant inherited zircon. The latter have comparatively shallow-slopping REE patterns, are strongly LREE-, Th- and Zr-enriched, contain little-to-no inherited zircon, and yield relatively high zircon saturation and Ti-in-zircon temperatures. The 2.4 Ga granitoids are interpreted to reflect crustal thickening and partial melting of garnet-bearing rocks in the mid- to lower-crust during the collisional phase of the 2.4–2.3 Ga Arrowsmith orogeny. The 2.3 Ga granites are interpreted to reflect high-temperature partial melting of tonalitic/granodioritic rocks at relatively shallow levels, associated with post-orogenic extension and the intrusion of hot mantle-derived magmas. Geochemically analogous Phanerozoic or Paleoproterozoic granitoids from orogenic and post-orogenic settings are identified for all three of the 2.5–2.3 Ga granitoid suites, which is consistent with the operation of “modern-style” plate tectonics by at least the end of the Archean.  </p>