Backarc origin for Neoarchean ultrahigh-temperature metamorphism, eclogitization, and orogenic root growth Gregory Dumond1*, Michael L. Williams2, Julia A. Baldwin3, and Michael J. Jercinovic2 1Department of Geosciences, University of Arkansas, Fayetteville, Arkansas 72701, USA 2Department of Geosciences, University of Massachusetts, Amherst, Massachusetts 01003-9297, USA 3Department of Geosciences, University of Montana, Missoula, Montana 59812-1296, USA ABSTRACT modeling of Thompson et al. (2001) and provide evidence for crustal Contraction of continental crust during orogeny results in elevated root growth associated with high-P metamorphism of mid-oceanic ridge topography at the surface and a root at depth. Thermomechanical basalt (MORB)–like mafc intrusions. Intraplating of these intrusions models suggest that root growth is enhanced by thickening of ther- facilitated partial melting and UHT metamorphism of peraluminous supra- mally softened thin lithosphere. A >400 km2 region of Archean gneiss crustal host rocks (Dumond et al., 2015). We use Th–U–total Pb monazite in the Athabasca granulite terrane in the Canadian shield contains petrochronology to constrain the timing of high-P melting preserved in abundant mafc sills with mid-oceanic ridge basalt–like chemistry. felsic granulite paragneiss in contact with a previously dated eclogite Heat from the sills facilitated melting of supracrustal host rocks along sill that yielded U-Pb zircon dates of 2.54 Ga and 1.90 Ga (Baldwin et a prograde pressure-temperature (P-T) path culminating at P > 1.4 al., 2004). These results are combined with bulk-rock geochemistry for GPa and T > 950 °C in the Neoarchean. A basalt sill, converted to mafc granulites and eclogite near the base of the Upper Deck to infer a eclogite near the base of the domain, exhibits positive Eu anomalies backarc origin for a Neoarchean orogenic root. consistent with plagioclase accumulation at a shallow crustal level prior to burial. Eclogite facies metamorphism previously dated as BACKGROUND 1.90 Ga is here revised to 2.54 Ga based on existing zircon dates from The >400 km2 and >50-km-wide Upper Deck domain is part of the the sill and new monazite dates from the paragneiss that hosts the >20,000 km2 Athabasca granulite terrane in the western Canadian shield sill. The results suggest that upper crustal materials were thermally (Figs. 1A and 1B). The domain consists of (1) garnet-rich kyanite (Ky) softened in a backarc setting prior to burial to lower crustal levels + potassium feldspar (Kfs) ± orthopyroxene (Opx) ± sillimanite (Sil) during orogenic root growth. felsic granulite gneisses (mineral abbreviations after Bucher and Frey, 2002), (2) centimeter- to kilometer-scale sills and dikes of garnet (Grt) + INTRODUCTION clinopyroxene (Cpx) + plagioclase (Pl) + quartz (Qtz) ± hornblende (Hbl) Contraction of continental crust during mountain building leads to mafc granulite gneisses, and (3) a sill of eclogite as much as 15 m thick surface uplift and growth of deep crustal roots. Root survival depends near the base of the domain (Snoeyenbos et al., 1995; Fig. 1B). Field on the amount of isostatic rebound that occurs when contraction ceases, relationships indicate that the felsic granulite gneisses were produced by and is a function of erosional unloading at the surface (Fischer, 2002), partial melting of a biotite (Bt) bearing metasedimentary protolith dur- metamorphic reactions that densify the lower crust and inhibit rebound ing intrusion of a mafc magma intraplate, now represented by the mafc (Williams et al., 2014), and the magnitude of foundering of lower crust granulites and eclogite (Dumond et al., 2015). into the mantle (Ducea, 2011). Roots are well documented seismically Peak conditions for the Upper Deck are constrained by phase equilibria in modern and ancient orogens, e.g., the Southern Alps in New Zealand modeling of felsic granulite bulk compositions indicating temperature, T (Scherwath et al., 2003) and the Trans-Hudson orogen in the Canadian > 950 °C during a period of prograde crustal thickening from P < 0.8 GPa Shield (Lucas et al., 1993). Deeply exhumed feld examples of crustal to P > 1.4 GPa (Dumond et al., 2015). Existing P-T data for the mafc roots, however, are rare and include parts of the Western Gneiss region granulites and eclogite are consistent with these pressures (Baldwin et (Dewey et al., 1993) and the Bohemian Massif (Schulmann et al., 2005). al., 2003, 2007). Constraints on the timing of metamorphism and defor- Thermomechanical models for root growth emphasize the importance mation in the Upper Deck include zircon and monazite dates from felsic of thermal softening prior to thickening, a situation that is most likely in granulites that record partial melting, garnet growth, and lower crustal fow continental rift zones or backarc settings (Thompson et al., 2001). Tec- at 2.61–2.52 Ga (Baldwin et al., 2006; Dumond et al., 2015). Monazite tonic burial of a hot backarc provides a mechanism for ultrahigh-temper- inclusions in high-Ca garnet constrain the timing of UHT–high-P meta- ature (UHT) metamorphism due to an elevated geothermal gradient and morphism at 2.58–2.52 Ga (Dumond et al., 2015). The felsic granulites >800 °C Moho temperature that result from high mantle heat fux across contain polyphase inclusions of Bt + Pl + Qtz + rutile (Rt) included in thin lithosphere (Currie and Hyndman, 2006). Models of root growth high-Ca garnet, indicating that partial melting via the reaction Bt + Pl + achieve UHT conditions at high pressure (P) when thickening occurs Qtz → Grt + Kfs + melt occurred at high P (>1.4 GPa; Dumond et al., across a narrow 100-km-wide zone with a hot 35 °C/km initial geotherm 2015). The ca. 1.9 Ga monazite dates in the felsic granulites are inferred to that is enhanced by additional heat supplied from below (model HH100 indicate crustal reactivation during a second lower P granulite facies event in Thompson et al., 2001). (Dumond et al., 2015). Zircon dates from the mafc granulites and eclogite We present evidence for Neoarchean eclogitization near the base of span 2.54–1.89 Ga (Baldwin et al., 2003, 2004). A zircon U-Pb isotope a buried backarc basin, now preserved as the Upper Deck domain in dilution–thermal ionization spectrometry (ID-TIMS) date of 1.904 Ga was the western Canadian shield (Fig. 1). Our results are consistent with the obtained (Baldwin et al., 2004) for the eclogite that was reinforced by ca. 1.9 Ga in situ sensitive high-resolution ion microprobe (SHRIMP) dates *E-mail: [email protected] on zircon included in clinopyroxene in a single thin section from a second GEOLOGY, October 2017; v. 45; no. 10; p. 943–946 | Data Repository item 2017313 | doi:10.1130/G39254.1 | Published online 4 August 2017 ©GEOLOGY 2017 Geological | Volume Society 45 | ofNumber America. 10 For | www.gsapubs.org permission to copy, contact [email protected]. 943 Figure 1. A: Location of 60°N Upper Deck domain, Athabasca granulite terrane the Athabasca granulite Lake eclogite dextral tranpressive shear zone terrane (star) in northern Athabasca mafic granulite low-angle detachment (block on downthrown side) 07G-030-H2A felsic granulite Saskatchewan, Canada. Honsvall Lake North felsic granulite Currie Lake America protolith 1.6 GPa 2 S1 Eclogite B: Geologic map of the 1.6 GPa 102°W 500 km 1.607G-030-H2 GPa A Upper Deck domain; A Eclogite 3 1.6 GPa pressure estimates are >1.4 GPa Felsic Granulite modified from Dumond et >1.4 GPa >1.2 GPa al. (2015). Circled locations N Eclogite were sampled for mafic Sucker 1 Lake Athabasca granulite and eclogite. C: Bay r zone 106°W Cora Lake Folded contact between shea B 0 10 km Stony Rapids 59°N C felsic granulite and eclog- ite with location of sample 07G-030-H2A.v sample. Single zircon grains from the same sample, however, produced a A B Ca Kα Th Mα high-precision ID-TIMS U-Pb discordia with intercepts at 2.54 Ga and 3 mm HIGH 1.91 Ga (Baldwin et al., 2004), consistent with a polymetamorphic origin 2523 for zircon in the eclogite and similar to zircon U-Pb discordia documented +/- 42 Ma 3-m7 in Upper Deck mafc granulites (Baldwin et al., 2003). residual Grt-rich felsic granulite NEW FIELD OBSERVATIONS FROM THE DATED ECLOGITE Th Mα LOCALITY LOW 2572 A wildfre in A.D. 2006 burned several hundred square kilometers of +/- 10 Ma 3-m8 3-m11 the Upper Deck and revealed new exposures of the eclogite sill. Field 3-m13 observations in 2007 showed that the eclogite and the host felsic granu- 3-m15 3-m12 3-m16 lite gneiss share a penetrative gently southwest-dipping foliation. The Qtz 3-m17 Grt 3-m18 foliation and the contact between the two rocks is folded (Fig. 1C), with 3-m22 retrograde sapphirine occurring axial planar to the fold in both rock types. 3-m10 3-m21 These concordant observations at the original sample locality negate the CDlow-Th inclusions 1900 discordant relationships inferred by Baldwin et al. (2004), and require in high-Ca Grt 1.8 2529 +/- 34 Ma that the felsic granulite and eclogite had a shared history prior to folding 2450 (2σ; MSWD = 0.11) and sapphirine growth. 1.6 2500 3-m1 2550 1.4 Date (Ma) 2600 MONAZITE PETROCHRONOLOGY OF THE ECLOGITE 2650 HOST ROCK 1.2 3-m10 Domain 4 Grt In Monazite petrochronology is a rapidly advancing feld in continental (wt%) 1.0 1.9 Ga Mnz rims at 3 Neoarchean cracks in Grt tectonics with the goal of linking monazite dates to specifc metamorphic O melt loss Neoarchean 2 0.8 Domain 3 processes and deformation events (e.g., Mottram et al., 2014). Here we Y Y-depleted, low-Th Grt In present results from 07G-030-H2A (Figs.