Lu–Hf garnet dating and the timing of collisions: Palaeoproterozoic accretionary tectonics revealed in the Southeastern Churchill Province, Trans‐Hudson Orogen, Canada

Abstract

Dating the onset of continental collision is fundamental in defining orogenic cycles and their effects on regional tectonics and geodynamic processes through time. Part of the Palaeoproterozoic Trans‐Hudson Orogen, the Southeastern Churchill Province (SECP) is interpreted to result from the amalgamation of Archean to Palaeoproterozoic crustal blocks (amalgamated as the central Core Zone) that diachronically collided with the margins of the North Atlantic and Superior cratons, resulting in two bounding transpressive orogens: the Torngat and New Quebec Orogens. The SECP exposes mainly gneissic middle to lower orogenic crust in which deformation and amphibolite to granulite facies metamorphism and anatexis overprinted the early geological features classically used to constrain the timing of collisional events. To enable improved tectonic models for the development of the SECP, and the Trans‐Hudson as a whole, we investigated granulite facies supracrustal sequences from the Tasiuyak Complex (TC) accretionary prism and the western margin of the North Atlantic Craton—that is, Saglek Block (upper plate)—using a multi‐chronometer approach coupled with trace element geochemistry. In particular, the use of garnet Lu–Hf geochronology provides an important minimal time constraint for crustal thickening and collision. Garnet growth in the TC is constrained at 1885 ± 12 Ma (Lu–Hf), indistinguishable from U–Pb age of prograde monazite at 1873 ± 5 Ma. Zircon growth during melt crystallization occurred at 1848 ± 12 Ma. Garnet from the overriding Saglek Block is dated at 2567 ± 4.4 Ma (Lu–Hf) and indicates that gneissic rocks from the upper plate did not record the metamorphic imprint of the Torngat Orogeny. The diachronicity of the integrated metamorphic record across the strike of the SECP is explained by the location of terrane boundaries, consistent with the westward growth of the Churchill plate margin through sequential amalgamation of narrow crustal blocks during accretionary tectonics from c. 1.9 to 1.8 Ga.

Retentiveness of rare earth elements in garnet with implications for garnet Lu‐Hf chronology

“…With no clear exception, and regardless of rock composition, mineral assemblage or metamorphic grade, Lu‐Hf garnet ages are similar to, or older than, the accessory mineral U–Pb ages that are interpreted to date peak metamorphism (Figure 1b). The data thus do not reflect down‐skewing of Lu‐Hf ages, as predicted for garnet‐clinopyroxene assemblages (Bloch et al, 2015), and in fact can be readily interpreted to reflect the nature of garnet as a prograde mineral that typically forms earlier in the petrogenetic history of metamorphic rocks than zircon or monazite (e.g., Anczkiewicz et al, 2007; Godet et al, 2021; Guilmette et al, 2018; Guilmette et al, 2023; Smit, Hacker, & Lee, 2014) and may even retain age information on metamorphic events that accessory minerals appear to have missed (e.g., Lihter et al, 2022; Thiessen et al, 2019). Although these data generally indicate the robustness of Lu‐Hf ages, they do not exclude that Lu diffusion affects such ages in individual cases.…”

Lithospheric unzipping explaining hot orogenesis during continental subduction

“…Arc magmatism swept across the core zone from the upper plate towards the lower plate, always predated by Barrovian metamorphism (Godet et al, 2021). Maximum pressures recorded were on the order of 11 kbar for granulite facies rocks (Charette et al, 2021;Godet et al, 2021), with a preserved Barrovian sequence on the western edge of the orogen (Godet et al, 2020).…”

“…These units do not reveal evidence of early orogenic HP-UHP metamorphism but display predominantly Barrovian metamorphism that commonly reaches granulites facies temperatures but rarely reaches pressures above 8-10 kbar. These conditions overlap with or predating arc magmatism, and even though the continental fragments that constitute the orogen have markedly different geological histories and are interpreted to represent individual microcontinent, there is a near total absence of ophiolites or oceanic material between these accreted terranes (Corrigan et al, 2009;Godet et al, 2021;St-Onge et al, 2006;Weller et al, 2013, and references therein). More specifically, the South-East Churchill Province branch (Corrigan et al, 2018;Godet et al, 2021;Wardle et al, 2002) of the Trans-Hudson Orogen (Figure 8) is comprised of upper amphibolite to granulite facies crystalline basement units of 50-100 km wide, the Core Zone, separating the lower plate Superior craton and its rifted margin volcanic-sedimentary sequences (the Labrador Trough) from the upper plate North Atlantic Craton (Figure 8).…”

“…More specifically, the South-East Churchill Province branch (Corrigan et al, 2018;Godet et al, 2021;Wardle et al, 2002) of the Trans-Hudson Orogen (Figure 8) is comprised of upper amphibolite to granulite facies crystalline basement units of 50-100 km wide, the Core Zone, separating the lower plate Superior craton and its rifted margin volcanic-sedimentary sequences (the Labrador Trough) from the upper plate North Atlantic Craton (Figure 8). Arc magmatism swept across the core zone from the upper plate towards the lower plate, always predated by Barrovian metamorphism (Godet et al, 2021). Maximum pressures recorded were on the order of 11 kbar for granulite facies rocks (Charette et al, 2021;Godet et al, 2021), with a preserved Barrovian sequence on the western edge of the orogen (Godet et al, 2020).…”

Tectono‐Metamorphic Evolution of the Northern Dom Feliciano Belt Foreland, Santa Catarina, Brazil: Implications for Models of Subduction‐Driven Orogenesis

“…To investigate the above‐described controversy, we have undertaken a multidisciplinary study focusing on the pressure–temperature–time–deformation (P–T–t–D) paths of supracrustal rocks of the northern Dom Feliciano Belt foreland. By dating the growth of P–T‐sensitive metamorphic minerals, in particular garnet, it is possible to constrain the timing and conditions of specific tectonic processes (Anczkiewicz et al., 2014; Baxter & Scherer, 2013), enabling reliable estimates of the timing and duration of crustal thickening during orogenesis (Godet et al., 2021; Leech et al., 2005; Smit et al., 2014). Thus, detailed structural and petrographic observations, thermodynamic modeling, Lu–Hf and Sm–Nd isotopic dating of garnet, U–Pb dating of monazite, and Ar–Ar dating of micas were utilized to constrain the conditions and timing of major tectonic events.…”