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The Athabasca Granulite Terrane and Evidence for Dynamic Behavior of Lower Continental Crust

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The Athabasca Granulite Terrane and Evidence for Dynamic Behavior of Lower Continental Crust EA46CH14_Dumond ARI 25 April 2018 12:58 Annual Review of Earth and Planetary Sciences The Athabasca Granulite Terrane and Evidence for Dynamic Behavior of Lower Continental Crust Gregory Dumond,1 Michael L. Williams,2 and Sean P. Regan3 1Department of Geosciences, University of Arkansas, Fayetteville, Arkansas 72701, USA; email: [email protected] 2Department of Geosciences, University of Massachusetts, Amherst, Massachusetts 01003, USA; email: [email protected] 3Center for Earth and Environmental Science, State University of New York, Plattsburgh, New York 12901, USA; email: [email protected] Annu. Rev. Earth Planet. Sci. 2018. 46:353–86 Keywords The Annual Review of Earth and Planetary Sciences is continental crust, garnet, granulite, lithosphere, melt, shear zone online at earth.annualreviews.org https://doi.org/10.1146/annurev-earth-063016- Abstract 020625 Deeply exhumed granulite terranes have long been considered nonrepre- Copyright c 2018 by Annual Reviews. sentative of lower continental crust largely because their bulk compositions All rights reserved Annu. Rev. Earth Planet. Sci. 2018.46:353-386. Downloaded from www.annualreviews.org do not match the lower crustal xenolith record. A paradigm shift in our un- Access provided by University of Arkansas - Fayetteville on 06/29/21. For personal use only. derstanding of deep crust has since occurred with new evidence for a more felsic and compositionally heterogeneous lower crust than previously rec- ANNUAL > 2 REVIEWS Further ognized. The 20,000-km Athabasca granulite terrane locally provides a Click here to view this article's >700-Myr-old window into this type of lower crust, prior to being exhumed online features: • Download figures as PPT slides and uplifted to the surface between 1.9 and 1.7 Ga. We review over 20 years • Navigate linked references • Download citations of research on this terrane with an emphasis on what these findings may • Explore related articles tell us about the origin and behavior of lower continental crust, in general, • Search keywords in addition to placing constraints on the tectonic evolution of the western Canadian Shield between 2.6 and 1.7 Ga. The results reveal a dynamic lower continental crust that evolved compositionally and rheologically with time. 353 EA46CH14_Dumond ARI 25 April 2018 12:58 INTRODUCTION The origin, character, and evolution of lower continental crust remain intensely debated (Burgmann¨ & Dresen 2008, Rudnick & Gao 2014, Hacker et al. 2015). The debate persists because disparate data sets remain difficult to reconcile, e.g., geophysical observables (Ross et al. 2004, Bai et al. 2010, Copley et al. 2011, Schulte-Pelkum et al. 2017), xenolith data (Rudnick & Taylor 1987, Villaseca et al. 1999, Hacker et al. 2000, Mahan et al. 2012), and the record gleaned from deeply exhumed granulite terranes (Fountain & Salisbury 1981, Percival et al. 1992, Mahan et al. 2011). Conventional views of the lower continental crust have held that it is predominantly mafic (Rudnick 1992; Rudnick & Fountain 1995; Rudnick & Gao 2003, 2014). Recent work, however, suggests that heat flow and seismic velocity constraints can be satisfied by a range of end-member bulk compositions and that most lower continental crust need not be mafic (Figures 1 and 2) (Hacker et al. 2015). Lower continental crust in a variety of tectonic settings displays a range of seismic wavespeeds with P-wave velocities (VP) = 6.0–7.7 km/s (Hacker et al. 2015). Hacker et al. (2015) derived average VP for four end-member lower crustal compositions that ranged from 6.8 to 7.2 km/s with densities of 2.9 to 3.2 g/cm3. These results are consistent with a compositionally heterogeneous lower continental crust and imply that some exhumed granulite terranes may be more representative of lower continental crust than previously thought. We review more than 20 years of research on one of Earth’s largest granulite terranes, the >20,000-km2 Athabasca granulite terrane in the western Canadian Shield (Figures 3 and 4), and summarize evidence bearing on several important processes, including (a) compositional evolution of lower continental crust; (b) lower crustal densification, with implications for seismic velocity, 0.9 a b 0.8 0.7 0.6 X X L L B B LL 0.5 L L E L U U L E 0.4 0.3 Molar Mg number 0.2 0.1 Negative Positive Eu anomaly Eu anomaly 0 40 45 50 55 60 65 70 75 021 34 Annu. Rev. Earth Planet. Sci. 2018.46:353-386. Downloaded from www.annualreviews.org Access provided by University of Arkansas - Fayetteville on 06/29/21. For personal use only. wt% SiO2 Eu/Eu* B Average bulk continental crust (Rudnick & Gao 2003) Athabasca granulite terrane U Average upper continental crust (Rudnick & Gao 2003) Arc-related granitoids (Regan et al. 2017b; this study); n = 11 L End-member average lower continental crust compositions (Hacker et al. 2015) Chipman mafc dikes (Flowers et al. 2006a); n = 16 X Median deep crustal xenolith (Hacker et al. 2015) Mafc granulites (Flowers et al. 2008, Dumond et al. 2017); n = 16 Median post-Archean granulite terrane (Hacker et al. 2015) Eclogites (Baldwin et al. 2004, Dumond et al. 2017); n = 7 Median Archean granulite terrane (Hacker et al. 2015) Felsic granulites (Baldwin et al. 2006, Dumond et al. 2015; this study); n = 32 Granulite database of Huang et al. (2013); n = 2,481 E Average composition of the East Athabasca mylonite triangle Figure 1 ∗ Bulk compositions from the Athabasca granulite terrane: (a)SiO2 versus molar Mg number and (b) Eu/Eu (Eu anomaly) versus molar Mg number. The field of gray crosses corresponds to the granulite database of Rudnick & Presper (1990) that was updated by Huang et al. (2013). 354 Dumond · Williams · Regan EA46CH14_Dumond ARI 25 April 2018 12:58 Surface heat fow ab50 mW/m2 51 mW/m2 67 wt% SiO 2 67 wt% SiO 1.6 mW/m3 2 1.6 mW/m3 2.8 wt% K O 2 2.8 wt% K O 10.5 ppm Th 2 10.5 ppm Th UPPER CRUST 2.7 ppm U 2.7 ppm U 12 km UPPER CRUST 14 km 64 wt% SiO2 1.0 mW/m3 2.8 wt% K2O 6.5 ppm Th 1.3 ppm U MIDDLE CRUST 23 km 64 wt% SiO2 0.7 mW/m3 1.5 wt% K O 53 wt% SiO 2 2 5.6 ppm Th 3 0.2 mW/m 0.7 ppm U LOWER CRUST 0.6 wt% K2O 1.2 ppm Th 0.2 ppm U LOWER CRUST 40 km 40 km 17 mW/m2 11 mW/m2 Mantle heat fow Figure 2 Continental crustal columns illustrating two end-members for the lower continental crust. (a) Three-layer model with mafic lower crust derived by Rudnick & Gao (2003, 2014). (b) Two-layer model with felsic lower crust derived by Hacker et al. (2011). Figure modified from Hacker et al. (2015) with permission. crustal root growth, and foundering; (c) dynamic lower crustal rheology, with periods of weak flow and periods of high strength; and (d ) mechanisms for stabilization and destabilization of continental lithosphere. These processes affected the terrane at a range of timescales, with some portions of the terrane hypothesized to have experienced long-term (>650 Myr) lower crustal residence (Williams & Hanmer 2006, Flowers et al. 2008). This contrasts with other domains in the terrane that have been inferred to record short-term (10s to >100 Myr) residence in the deep crust (Martel et al. 2008, Bethune et al. 2013). This review begins with a summary of the geological record of the Annu. Rev. Earth Planet. Sci. 2018.46:353-386. Downloaded from www.annualreviews.org Athabasca granulite terrane, including hypotheses for the tectonic evolution of several domains Access provided by University of Arkansas - Fayetteville on 06/29/21. For personal use only. within the terrane and their implications for the tectonic history of the western Canadian Shield (Figures 3 and 4). Observations from the Athabasca granulite terrane are then used to place constraints on the characteristics and evolution of deep continental crust in general. GEOLOGICAL BACKGROUND The western Canadian Shield of North America consists of a number of Archean cratonic provinces, including the Churchill Province (Figure 3) (Hoffman 1988). The western extent of the Churchill Province is delimited by the Great Slave Lake shear zone and the 2.0–1.9 Ga Taltson-Thelon orogen (Figure 3b) (Hoffman 1987, Ross 2002). The eastern boundary is marked by the 1.9–1.8 Ga Trans-Hudson orogen (Figure 3b) (Ansdell 2005, St-Onge et al. www.annualreviews.org • The Athabasca Granulite Terrane 355 EA46CH14_Dumond ARI 25 April 2018 12:58 b Wopmay a orogen n Cfz TWBd e g o r Slave o e Province n n o o el z h b ? ? T Cd Tfz North - n ic America so Churchill n lt o CHd Hudson a t T Province–Rae c te Tsz ? ? ? Bay N 60°N 500 km CLsz See Churchill GSLsz Figure 4 Province–Hearne BBsz GRsz Intracratonic basins SHd LLsz Paleoproterozoic orogens Aeromagnetic highs in AB covered basement CBsz Trans-Hudson Western orogen Aeromagnetic lows in Bufalo limit of extent o covered basement Head f exposed Sa sk Undiferentiated high-pressure, terrane VRsz Canadian d c e r a high-temperature granulite gneiss NFsz r r t Shield e o f n n Archean greenstone belts I TNB Central Hearne domain (? = extent uncertain) Hearne Archean cratonic provinces Snowbird domain km Lithoprobe deep seismic refection line 0 200 Jones et al. (2002) teleseismic/ Superior magnetotelluric line Sask Province craton Eastern limit of Cordilleran deformation 110°W Figure 3 (a) Location of the western Canadian Shield in North America. (b) Simplified geologic map of the western Canadian Shield with data compiled from Hoffman (1988), Tella et al.
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