Latest Archean continental growth in a post-collisional setting: Evidence from 2.5 Ga high-K calc-alkaline granitoids in the Rae craton, Northwest Territories, Canada

Abstract At least 30% of preserved continental crust was generated in the late Archean (3.0–2.5 Ga). Constraints on how late Archean crust formed are therefore important to understanding continental growth. We present whole-rock elemental and Nd isotope data and zircon U-Pb-Hf isotope data for late Archean plutonic (mostly granitoid) rocks in an understudied portion of the southwestern Rae craton. Three plutonic suites with crystallization ages of ca. 2.71–2.69 Ga, 2.58–2.57 Ga, and 2.52–2.49 Ga are documented. The 2.5 Ga suite is characterized in detail because the tectonic setting of 2.5 Ga magmatism in the Rae craton is debated and magmatic rocks of this age are globally understudied. Data from the older suites provide constraints on the isotopic and chemical composition of the local basement. The 2.5 Ga rocks range from mafic to felsic (47–74 wt% SiO2), are high-K calc-alkaline and light rare earth element enriched, exhibit negative Nb anomalies, and yield restricted ranges of initial εNd (−0.5 to +0.6) and weighted mean initial εHf (−1.1 to +1.5) values. Mafic 2.5 Ga magmas derived from a subduction-enriched mantle source that had roughly chondritic isotopic compositions at the time of magma generation. The unradiogenic composition of the mantle source can be explained by: (1) metasomatism of depleted mantle by low Sm/Nd and Lu/Hf fluids or melts well before 2.5 Ga, such the source evolved from a suprachondritic to chondritic isotopic composition by 2.5 Ga, or (2) metasomatism of depleted mantle by melts of unradiogenic (εNd2.5 Ga and εHf2.5 Ga ≤ 0) subducted sediments shortly before or during the 2.5 Ga magmatism. More evolved 2.5 Ga magmas were produced through some combination of fractional crystallization of mantle-derived magmas, assimilation of preexisting crust, and partial melting of juvenile 2.5 Ga crust. Rare ca. 2.57–2.56 Ga zircon xenocrysts in the 2.5 Ga suite are evidence of minor crustal contamination. Neodymium isotope modeling permits, but does not require, assimilation of the local 2.7–2.6 Ga crust. However, the data do require that all components of the suite are, at least in part, juvenile crustal additions. The 2.5 Ga rocks are geochemically similar to post-collisional I-type plutonic rocks from the Phanerozoic Caledonian and Variscan orogens, and their emplacement in a post-collisional setting is consistent with existing constraints on the tectonic evolution of the Rae craton. This study exemplifies how the timing of some significant crustal growth events may not be accurately recorded in depleted mantle model ages, and how integrating isotopic and geochemical data with local geological context helps to identify such events. The results also suggest, along with other studies, that substantial late Archean continental growth may have occurred in post-collisional settings.