The Calderian orogeny in Wopmay orogen (1.9 Ga), northwestern Canadian Shield Robert S. Hildebrand1,†, Paul F. Hoffman2,§, and Samuel A. Bowring3 1Department of Geosciences, University of Arizona, 1040 E. 4th Street, Tucson, Arizona 85721, USA 2Department of Earth and Planetary Sciences, Harvard University, 20 Oxford Street, Cambridge, Massachusetts 02138, USA 3Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Building 54-1126, Cambridge, Massachusetts 02139, USA ABSTRACT continent collisions, perhaps because the evolutionary model that incorporates progres- attenuated arc crust, coupled with the ex- sive rollback of the lower plate and extension of The Wopmay orogen is a Paleoproterozoic tended crust of the lower plate, never gen- the arc terrane on the upper plate, emplacement orogenic belt formed in part by the accre- erates severely overthickened crust. Rise of of the leading edge of the extended continental tion of Hottah terrane, an east-facing conti- asthenospheric mantle due to slab breakoff arc upon the western margin of Slave craton, nental magmatic arc, to the western margin generates rapid uplift, erosion, and collapse failure of the subducting plate during the colli- of the Archean Slave craton at ca. 1.88 Ga. of the mountains built during collision. sion, and the subsequent development of a new The arc-continent collision was responsible subduction zone, or possible propagation of an for the Calderian orogeny. Just prior to the INTRODUCTION older one, of opposite polarity outboard of the collision, arc volcanism of the Hottah terrane amalgamated collision zone—all within about had migrated trenchward and changed in Located in the northwest part of the Canadian 10 m.y. The overall evolution and short-lived composition from an aphyric calc-alkaline Shield (Fig. 1), the Wopmay orogen provides a nature of the collision are typical of modern arc- to a bimodal tholeiitic suite. The change well-exposed example of a Paleoproterozoic continent collisions (Suppe, 1987; Cloos et al., in magmatism, along with subsidence and collisional belt and postcollisional magmatic 2005), and it appears that arc-continent colli- consequent high-temperature–low-pressure arc. First mapped by Fraser et al. (1960; see also sions are the chief way in which passive margins metamorphism of rocks on the upper plate, Fraser, 1964), it contains one of the fi rst recog- are converted to active margins. is attributed to extension and asthenospheric nized Precambrian continental margins (Hoff- upwelling during rollback of the lower plate. man, 1973) and remains a defi ning example of TECTONIC DIVISIONS OF The upwelling led to regional heating of the Precambrian plate tectonics (Hoffman, 1980; WOPMAY OROGEN crust, melting, and generation of the metalu- Windley, 1995; Condie, 1997; Stanley, 1999). minous to peraluminous Hepburn intrusive Existing models were mostly published Wopmay orogen is divided into fi ve major suite within the arc just prior to accretion of prior to the recognition of a large klippe of zones, from east to west: Coronation margin, Hottah terrane onto the Slave craton. During exotic rocks (Turmoil klippe; Fig. 1) within Turmoil klippe, the Medial zone, Great Bear the collision, a 200-km-long, orogen-parallel the internal zone of the orogen. The volcanic magmatic zone, and Hottah terrane (Fig. 1). swarm of mafi c sills was intruded into the and sedimentary rocks of the klippe were pre- Coronation margin contains basement rocks of lower plate and trench-fill sediments at viously interpreted as initial rift-facies rocks the Archean Slave craton overlain by a tri partite the passive-margin shelf-slope break. The (Easton, 1981a, 1982; Hoffman and Bowring, sedi mentary succession representing three dis- mafi c magmatism may have been induced by 1984; Hoffman et al., 1988; Hoffman, 1989), tinct tectonic regimes: rift, passive margin, and slab breakoff. The Calderian orogeny pro- and the crystalline basement beneath them was foredeep (Hoffman, 1973, 1980, 1984, 1989). vides an excellent example of the magmatic interpreted as their metamorphosed equivalents The supracrustal rocks of the margin were de- history of an arc-continent collision. (St-Onge, 1981, 1984a, 1984b; St-Onge and tached from their basement, folded, and trans- The Calderian orogeny was short-lived. King, 1987a, 1987b; King, 1986). A key fi nd- ported eastward during the Calderian orogeny. Within a few million years, an east-dipping ing was presented by Bowring and Grotzinger The passive margin to foredeep transition, mark- subduction zone formed outboard of the (1992), who reported U-Pb zircon dates for ing the onset of collision, is dated at 1882 ± accreted terrane, creating volcanic rocks ash beds in cratonic cover correlative with the 4 Ma by U-Pb chronology of zircons from a that were erupted atop the eroded Hottah- passive-margin sequence to demonstrate that the volcanic ash bed near the base of the foredeep Slave collision zone. The short duration of Coronation margin was substantially older than sequence (Bowring and Grotzinger, 1992). The the Calderian orogeny seems typical of arc- the supposed initial rift rocks. westernmost zone, Hottah terrane, developed In this contribution, we meld geochronologi- remotely from, but in part contemporaneously cal data with fi eld observations in the internal with, Coronation margin. It consists of calc- zone to reveal that the evolution of the main alkaline vol canic and plutonic rocks erupted on †e-mail: [email protected]. §Current address: School of Earth and Ocean Sci- collisional phase of the orogen, termed the and intruded into early Paleoproterozoic con- ences, University of Victoria, Box 1700, Victoria, Calderian orogeny, represents a typical short- tinental crust (Hildebrand et al., 1983, 1984; British Columbia V8W 2Y2, Canada. lived arc-continent collision. We also present an Hildebrand and Roots, 1985). Turmoil klippe GSA Bulletin; May/June 2010; v. 122; no. 5/6; p. 794–814; doi: 10.1130/B26521.1; 19 fi gures. 794 For permission to copy, contact [email protected] © 2010 Geological Society of America America Bulletin,May/June2010 Geological Societyof 795 111° 68° Paleozoic cover Coronation Gulf younger Proterozoic intrusions (includes Muskox intrusion, Coronation & Gunbarrel sills) younger Proterozoic cover (includes Hornby Bay, Dismal Lakes, Coppermine River, and Rae groups) plutonic rocks of Great Bear magmatic zone volcanic and sedimentary rocks of Great Bear magmatic zone rocks exotic with respect toSlave Coronation margin orogen Wopmay in Calderian orogeny craton and Coronation margin sedimentaryrocks possibly of the forearc region plutonic rocks of the Hepburn intrusive suite Turmoil undivided Akaitcho Group klippe and Bent gneiss Morel sills Turmoil klippe Hottah terrane Calderian foredeep e Medial (Recluse Group) k a Zone L r G r e a t B e a Coronation margin slope Odjick and Rocknest fms Coronation platform Great Bear Odjick and Rocknest fms magmatic zone Coronation rift Supergroup Coronation (Melville Group) A Hottah Coronation basement Lake (Slave craton) 0 50 km 65° 120° Figure 1. Geological sketch map showing the subdivisions of Wopmay orogen discussed in this paper. A—Acasta gneiss in Exmouth anticline. Hildebrand et al. is a large erosional remnant of Hottah terrane surrounded by Paleoproterozoic orogenic belts Overlying the Melville Group, there is a se- that structurally overlies the westernmost parts (Hoffman, 1989; Hoffman and Hall, 1993). quence of shallow-marine sedimentary rocks of Coronation margin (Hildebrand et al., 1990, Regional geological mapping and geochronol- known as the Epworth Group, which is inter- 1991). The klippe contains crystalline basement ogy led to the recognition of a domain of ig- preted to represent a west-facing passive conti- unconformably overlain by metamorphosed neous and metamorphic basement older than nental margin (Hoffman, 1973). The Epworth sedimentary and volcanic rocks, all of which are 3.0 Ga—locally overlain by 2.8 Ga quartz- Group is widely exposed and consists of a basal intruded by the composite Hepburn Batholith ites and iron formation—in the west-central 200–1500 m, westward-thickening succession of (Fig. 1). Great Bear magmatic zone is dominated portion of the craton (Bleeker et al., 1999). storm-dominated siliciclastic rocks known as the by calc-alkaline volcanic and plutonic rocks. It The basement rocks include the well-known Odjick Formation. It is overlain by the Rocknest is a typical magmatic arc built on continental 4.03–3.6 Ga Acasta gneisses (Bowring et al., Formation, a 450–1100 m, cyclic, dolo mitic, crust (Hildebrand et al., 1987b). It is younger 1989; Bowring and Williams, 1999), which are shelf sequence with a rim of supratidal tepee than the Calderian orogeny and forms an overlap exposed in the core of Exmouth anticline within facies fl anked by outer-shoreface grainstones assemblage unconformably overlying the Hottah Wopmay orogen (Fig. 1). Overlying the older and stromatolites, and a sediment-starved sub- terrane, the extreme western edge of Corona- basement, there are 2.73–2.62 Ga supracrustal marine debris apron (Grotzinger, 1986a, 1986b, tion margin, and Turmoil klippe (Hoffman and rocks intruded by diverse suites of granitoids, 1986c). U-Pb zircon geochronology of volcanic McGlynn , 1977; Hildebrand et al., 1990). including 2.7 Ga tonalites, granodiorites, and ash beds in strata correlative with the Epworth The Medial zone of the orogen occurs along 2.58 Ga potassic granites.