Geology | 2019
High-grade metamorphism flying under the radar of accessory minerals
Abstract
Uranium-lead (U-Pb) accessory mineral petrochronology has been increasingly used to constrain the timing of tectonometamorphic events. However, because mafic rocks commonly lack minerals with a high U/Pb ratio, they may be underrepresented in the chronologic record. This study on polymetamorphic mafic granulites from the Archean Rae craton (northern Canada) provides a striking example of a metamorphic cycle that has been entirely overlooked. We utilized Lu-Hf garnet geochronology and equilibrium phase diagram modeling to characterize two high-pressure granulite-facies mineral assemblages that affected the 2.6 Ga protolith. Zircon and garnet recrystallization occurred at 1.87 Ga within a gneissic foliation, while a coarse-grained garnet precursor nucleated 230 m.y. earlier during a stage of high heat flow within thickened lower crust, the latter of which is nearly absent in the zircon and monazite age record except for rare igneous occurrences. Combined garnet geochronology and petrological modeling reinforce a ca. 1.9 Ga age for high-grade overprinting in the southern Rae craton and clearly show within the same sample that U-Pb accessory minerals did not grow during a newly identified 2.11 Ga granulite-facies event. INTRODUCTION Uranium-bearing accessory minerals are common in crustal rocks and widely used to constrain the timing and duration of igneous, metamorphic, and deformation events. Petrochronology using minerals with a high U/Pb ratio (high-U/Pb) enables important linkages between age records and petrological processes (e.g., Kohn et al., 2017); nevertheless, these records may be incomplete, especially in mafic rocks where high-U/Pb minerals may be lacking. Zircon and monazite, for instance, may not be stable together, such that single stages are recorded by one or the other depending on rock composition (Kooijman et al., 2017) and some stages may not be recorded at all (Kelsey et al., 2008; Yakymchuk and Brown 2014; Kohn et al., 2015). Additionally, in rocks with a polymetamorphic history, linkages between accessory mineral petrochronology and a sample’s petrological record may be ambiguous (e.g., Dragovic et al., 2016). Herein, we demonstrate that direct dating of the petrogenetic indicator mineral garnet can provide crucial time constraints for poorly recorded metamorphic processes in high-pressure (HP) polymetamorphic mafic rocks. The southeastern Rae craton (northern Canada; Fig. 1A) exemplifies a polymetamorphic amphiboliteto granulite-facies region (Berman et al., 2007) where U-bearing accessory minerals have been used to constrain the timing of numerous tectonometamorphic events that occurred between initial crust formation at 2.7– 2.6 Ga to stabilization after 1.82 Ga (Fig. 1B; e.g., Berman et al., 2013; Bethune et al., 2013; Dumond et al., 2015). HP (>13 kbar) relict assemblages in mafic rocks that remained intact after 1.9 Ga pervasive deformation and recrystallization are argued to have crystallized during collisional orogenesis at 2.55 and/or 1.9 Ga (e.g., Flowers et al., 2008; Martel et al., 2008). However, these assemblages have only been dated indirectly by U-Pb zircon (e.g., Baldwin et al., 2004; Flowers et al., 2008) or monazite in metasedimentary rocks adjacent to the undated HP mafic units (Dumond et al., 2017). Importantly, the highest metamorphic pressures are commonly preserved by mineral assemblages within mafic units (e.g., O’Brien et al., 2001; Carswell et al., 2003; Baldwin et al., 2004), so direct dating of their crystallization ages is necessary for ascertaining accurate reconstructions of their tectonic histories. To better evaluate the timing and nature of HP metamorphism during the complex history of the Rae craton, we have dated two texturally distinct garnet generations in a HP mafic granulite. Using Lu-Hf garnet geochronology with complementary petrological modeling, garnet dates presented herein correspond to a HP stage that has remained largely undetected in the U-Pb mineral record of the southeastern Rae craton. REGIONAL GEOLOGY AND SAMPLE CONTEXT Southern Rae craton basement comprises Neoarchean (ca. 2.7–2.5 Ga) intermediate gneiss and intrusions with minor mafic gneiss metamorphosed between 2.60 and 2.50 Ga (e.g., Davis et al., 2015; Dumond et al., 2015; Regis et al., 2017). The Arrowsmith orogeny (Fig. 1B) mainly affected the northwestern margin of the southern Rae craton from 2.54 to 2.28 Ga (e.g., Berman et al., 2013). From 2.28 to 2.00 Ga, regional extension is manifested by sedimentation, anorthosite magmatism, and formation of mafic dikes (Rainbird et al., 2010; Card et al., 2014; Regan et al., 2017). These processes did not favor zircon growth (Fig. 1B), and, thus, their records are dwarfed by those of other events. Widespread 1.94–1.89 Ga regional metamorphism during the Taltson and Snowbird orogenies (Fig. 1B; Berman et al., 2007; Bethune et al., 2013) preceded melt generation and exhumation facilitated by crustal-scale shear zones that occurred between 1.90 and 1.76 Ga ( Flowers et al., 2006; Mahan et al., 2006; Regis et al., 2017; Thiessen et al., 2018). To better understand the aforementioned HP relicts, we analyzed a mafic granulite (sample CITATION: Thiessen, E.J., et al., 2019, High-grade metamorphism flying under the radar of accessory minerals: Geology, v. 47, p. 568–572, https:// doi .org /10 .1130 /G45979.1 Manuscript received 4 January 2019 Revised manuscript received 5 March 2019 Manuscript accepted 2 April 2019 https://doi.org/10.1130/G45979.1 © 2019 The Authors. Gold Open Access: This paper is published under the terms of the CC-BY license. Published online 16 April 2019 Downloaded from https://pubs.geoscienceworld.org/gsa/geology/article-pdf/47/6/568/4707817/568.pdf by guest on 12 August 2019 Geological Society of America | GEOLOGY | Volume 47 | Number 6 | www.gsapubs.org 569 15ET249) that has comparable metamorphic assemblages to other HP (>13 kbar) rocks preserved in the southern Rae craton (e.g., Baldwin et al., 2004; Mahan et al., 2008). Our sample was collected from a belt of highly-tectonized granulite units within the Wholdaia Lake shear zone (Thiessen et al., 2018), a >300-km-long crustalscale shear zone that exhumed midto lowercrustal rocks adjacent to the Snowbird tectonic zone (STZ) between 1.90 and 1.86 Ga. Sample 15ET249 from this study has 2.6 and 1.9 Ga U-Pb zircon age components and contains a rare relict metamorphic domain (M1) that was not recrystallized at 1.9 Ga during pervasive ductile shearing along the Wholdaia Lake shear zone. PETROGRAPHY AND MINERAL COMPOSITIONS The M1 domain contains coarse-grained, granoblastic garnet + clinopyroxene + quartz + ilmenite ± plagioclase with >5-mm-diameter garnet porphyroblasts (Fig. 2A). These minerals typically display polygonal boundaries and do not preserve a solid-state tectonic fabric. Garnet (Grt1) occurs as a roughly equant porphyroblast with an irregular outer margin surrounded by 1-mm-diameter Grt1 fragments suggesting a larger initial crystal size by at least 10 vol% (Fig. 2A). Major element composition profiles in Grt1 are generally flat; however, Grt1 margins and two internal fractures have relatively low Mn (Fig. 2B), suggesting a locally focused secondary equilibration event, which is the opposite of what is expected for resorbed garnet margins (e.g., Kohn and Spear, 2000). Lutetium concentrations vary between 0.5 and 2.5 ppm across Grt1; the higher concentrations correlate with zones of low Mn (Fig. 2B). Plagioclase is closely associated with Grt1 grain bound aries and was likely produced during decompression and Grt1 consumption. Amphibole occurs along fractures and is considered a late, retrograde phase. The M2 domain envelops the M1 domain and consists of a fine-grained <1 mm granoblasticpolygonal assemblage of garnet + plagioclase + clinopyroxene + quartz + ilmenite + titanite (Fig. 2A), which defines a prominent gneissic foliation. Quartz occurs as lobate inclusions in garnet (Grt2) or within interstitial spaces. Major element compositions are generally homogeneous within all major phases and are comparable to M1 phase compositions (Table DR1 in the GSA Data Repository1). PHASE EQUILIBRIUM MODELING AND GARNET CHRONOLOGY Pressure-temperature (P-T ) isochemical phase diagrams were constructed (Fig. 3; Figs. DR2–DR3 in the Data Repository) for sample 15ET249 to constrain equilibrium conditions recorded by M1 and M2. We segmented the thin section into the two texturally distinct domains and combined modal phase proportions with their representative compositions to calculate independent effective bulk compositions for each domain (Table DR1). Based on the calculated stable equilibrium assemblage field of the observed major silicates Grt1 and clinopyroxene (>1 vol%) and complemented by isomodes for garnet (originally ~35 vol% or higher assuming resorption) and plagioclase (initially plagioclase free), minimum P-T conditions for the M1 assemblage are 12 ± 1 kbar and >700 °C (Fig. 3). Accessory rutile was not preserved in the M1 assemblage and may have been lost during pervasive overprinting in the ilmenite stability field. However, it should be noted that the stability of minor phases is not accurately predicted with this technique (e.g., Forshaw et al., 2019). Three 1–3 mm fragments of the Grt1 por phyroblast shown in Figure 2, together with a M1 matrix clinopyroxene, were subjected to Lu-Hf analysis, yielding a statistically valid isochron regression (Fig. 3; Table DR2) with an apparent age of 2111 ± 3 Ma (mean square weighted deviation [MSWD] = 0.41). The M2 assemblage equilibrated at P-T conditions of 8–11 kbar and >750 °C (Fig. DR3). Lu-Hf analyses of two separates of clean, single 1 mm Grt2 crystals, with no anchoring mineral or whole-rock point, lay along a 1.87 Ga reference isochron and yield a two-point date of 1870 ± 40 Ma (Fig. 3), which is within error of metamorphic zircon age components in the same sample (1890 ± 33 Ma; Thiessen et al., 2018).