Did a proto-ocean原型盆地

Did a proto-ocean basin form along the southeastern Rae

cratonic margin? Evidence from U-Pb geochronology,

geochemistry (Sm-Nd and whole-rock), and stratigraphy of the Paleoproterozoic Piling Group, northern Canada

Natasha Wodickat, Marc R. St-Onge, David Corrigan, David J. Scott§, and Joseph B. Whalen Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario K1A 0E8, Canada

ABSTRACT fragmented into small crustal block(s) and

narrow zone(s) of incipient oceanic crust farther The Paleoproterozoic Piling Group along outboard of the Piling Group basin. Rapid the southeastern Rae margin, northern Canada, subsidence of the southeastern Rae margin is characterized by thick, deep marine turbidite ensued, leading to deposition of euxinic (Astarte deposits not observed in time- equivalent, River formation) and overlying turbiditic strata intracratonic basin units further southwest. (Longstaff Bluff formation). The post-ca. 1915 Models invoked to explain this feature include Ma northern tur- biditic sedimentary units were development of a full-ocean, back-arc, or likely derived from a thoroughly mixed, proto-ocean basin followed by turbidite two-component source with possible input from sedimentation. We present new and existing the Snowbird tectonic zone and Bravo Lake U-Pb geochronological, Nd isotope, geochemical, formation, whereas the post-ca. 1930 Ma and stratigraphic evidence that support a southern turbidite unit may have been sourced proto-ocean basin model, and we explore the from the Meta Incognita microcontinent, pres-events leading to the formation and closure of ently exposed further south. We favor a rift such a rift basin during the middle margin over a foreland basin setting for the Paleoproterozoic. Sedimentation initiated deposition of the northern turbidite deposits. largely after ca. 2160 Ma with deposition of Subsequent mantle upwelling associated with craton-derived, shallow marine siliciclastic incipient ocean formation may have triggered strata (Dewar Lakes formation). Continued melting of highly thinned continental crust extension resulted in accumulation of resulting in emplacement of late- stage, ca. 1897 south-facing carbonate beds (Flint Lake Ma, contaminated rapakivi granite and highly formation) and likely concomitant, arclike differentiated mafic sills. Our results are most tholeiitic to picritic volcanism and voluminous consistent, albeit not exclusively, with the much volcaniclastic sedimentation (lower Bravo Lake formation) farther outboard at ca. debated model of asthenospheric upwelling and

1980 Ma. Accumulation of intrabasinal incipient rifting along the Rae-Hearne

siliciclastic strata above lower Bravo Lake boundary farther southwest at ca. 1.9 Ga. Later formation rocks may mark a hiatus in mafic- accretion of the Meta Incognita microcontinent ultramafic magmatism. By ca. 1923 Ma, upper Bravo Lake formation, within-plate- type led to basin closure and development of a

alkaline sill emplacement and volcanism north-verging fold-and-thrust belt after ca. 1883 occurred within highly extended crust. The Ma. oceanic island basalt-like signatures of the Bravo Lake formation rocks (but lack of depleted, mid-ocean-ridge basalt-type INTRODUCTION

compositions) suggest that by this time the Sedimentary basins form in most tectonic thinned Rae continental lithosphere had settings: divergent, convergent, or transform.

fE-mail: Natasha.Wodicka@NRCan-RNCan.gc.ca Understanding the processes related to basin §Current address: Canadian Polar Commission, 360 formation, evolution, and destruction is therefore Albert Street, Ottawa, Ontario K1R 7X7, Canada important for deciphering the tectonic development

of associated continental or oceanic crust. In

Precambrian sedimentary succes

GSA Bulletin; November/December 2014; v. 126; no. 11/12; p. 1625-1653; doi: 10.1130/B31028.1; 16 figures; 1 table; Data Repository item 2014252; published online 16 July 2014.

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© 2014 Geological Society of America sions, deciphering the age, character, tectonic evolution, and provenance record of a sedimentary basin can be particularly challenging because of post-depositional metamorphism and deformation that commonly obscure primary sedimentary features, and the lack of chronostratigraphic control. Despite these challenges, studies integrating detrital zircon geochronology with other tools, including sequence stratigraphy, isotope information, and geochemical characteristics, have been applied effectively in identifying the sources and tectonic settings of ancient sedimentary basins (e.g., Ross et al., 2005; Schulz et al., 2008; Collo et al., 2009; Rainbird et al., 2010). During the Paleoproterozoic, sedimentary and volcanic sequences accumulated in a number of sedimentary basins across the Churchill Province (Nunavut, Canada) in response to breakup of the supercraton Kenorland and assembly of the Nuna supercontinent (e.g., Williams et al., 1991; Aspler and Chiarenzelli, 1998; Rainbird et al., 2010). The Piling Group on Baffin Island (Fig. 1) forms part of the basal Paleoproterozoic supracrustal succession that overlies the Rae craton (e.g., Rainbird et al., 2010). However, unlike time-correlative units on mainland Nunavut (e.g., Amer, Ketyet River, Chantrey, and Montresor groups), which record the development of an intracratonic basin (Rainbird et al., 2010), the Piling Group contains thicker and more extensive deep marine deposits that suggest continental separation as first proposed by Morgan et al. (1975, 1976). Although there is broad agreement that the Piling Group basin once faced an ocean along the southeastern Rae margin (e.g., Corrigan et al., 2001, 2009; Scott et al., 2003; St-Onge et al., 2005, 2009), the nature, age, and tectonic evolution of the basin have not been fully documented. In this contribution, we present new, and synthesize available, field, stratigraphic, U-Pb1625

Wodicka et al.

enozoic I Paleogen

crust with Neoarchean component

Archean I I Cratons

e basalt

Paleozoic

Figure 1. Simplified geological map of the Canadian Shield (modified from Corrigan et al. [2009]; subsurface units in the western part of the shield after Ross et al. [1991], and distribution of Hoare Bay Group on Baffin Island after Sanborn-Barrie and Young [2013] and Sanborn-Barrie et al. [2013]) and western Greenland (modified from Escher and Pulvertaft [1995]). Areas investigated and sampled in this study are denoted by the black box (see Fig. 2) and the star (LB—Longstaff Bluff formation sample LB-05). The map also shows the location of Paleoproterozoic cover sequences on the Rae craton, including the Piling (P), Penrhyn (PN), Chantrey (C), Montresor (M), Amer (A), Ketyet River (KR), Thluicho Lake (TL), and Hoare Bay (HB) groups in northern Canada and the Karrat (K) and Anap nuna (An) groups in western Greenland. Greenland is shown in a pre-drift position with respect to Baffin Island and mainland Canada, following the reconstruction of Roest and Srivastava (1989). Other abbreviations: CI—Chesterfield Inlet segment; CS—Cumberland Sound; LH—Lake Harbour Group; QMB—Queen Maud block; SL—Snowbird Lake segment; STZ—Snowbird tectonic zone; TTMZ—Taltson-Thelon magmatic zone. Inset map shows location of study area in North America.

I I Carbonate platforms

Paleo- to Mesoproterozoic

I I Grenville orogen

I I Anorthositic to granitic plutons I I Intracratonic basins

Paleoproterozoic Granitic batholiths HB Arc-related rocks / oceanic crust

An Mafic volcanic rocks

l l Ca. 2.45-1.88 Ga supracrustal sequences 2.0-1.8 Ga

magmatic belts (e.g., Taltson-Thelon, Great Bear)

■ Dominantly early Paleoproterozoic

geochronological, geochemical, and Nd isotopic data that (1) help constrain the age and provenance of a stratigraphically representative suite of samples from the Piling Group, and (2) allow an improved understanding of the tectonic evolution of the southeastern Rae craton margin during the middle Paleoproterozoic (Figs. 1 and 2). In particular, we explore questions related to the events leading to the formation and eventual closure of a proto-ocean basin, if once present, along this segment of the Rae margin. The Piling Group is ideal for a provenance study because it is widely and well exposed (>45,000 km2) at relat ively low metamorphic grades (e.g., St-Onge et al.,

2005; Gagne et al., 2009). Our results also GEOLOGICAL SETTING highlight the exceptional value of integrating

The Paleoproterozoic Piling Group on central

various observational and analytical tools in the

Baffin Island, Nunavut, Canada (Fig. 1),

study of ancient sedimentary basins.

comprises shallow-water siliciclastic-carbonate strata, mafic-ultramafic volcanic rocks, and deep-water basinal strata (e.g., Morgan et al., 1976; Henderson et al., 1979; Henderson and Tippett, 1980; Tippett, 1984a; Jackson, 2000; Corrigan et al., 2001; Scott et al., 2002, 2003). It is generally regarded as a continental margin succession (Morgan et al., 1975, 1976) originally deposited on the southeastern margin of the Archean Rae craton and subsequently deformed and metamorphosed during the ca. 1.92-1.80 Ga

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Geological Society of America Bulletin, November/December 2014

Did a proto-ocean basin form along the southeastern Rae margin?

Trans-Hudson orogen (Corrigan et al., 2001; Scott it has been correlated with the Penrhyn Group on et al., 2002; St-Onge et al., 2005, and references Melville Peninsula and the Karrat Group in therein). Traditionally,western Greenland (Fig. 1; e.g., Jackson and

Taylor, 1972; Escher and Pulvertaft, 1976; Taylor,

1982; Henderson and Pulvertaft, 1987). More

recent work suggests that it forms part of an even

more extensive cover sequence on the Rae craton,

with stratigraphic correlatives extending from the

western Churchill Province on mainland Nunavut

(e.g., Amer, Ketyet River, Chantrey, and

Montresor groups; Rainbird et al., 2010), across

Baffin Island (Hoare Bay Group; St-Onge et al.,

2009), to western Greenland (Anap nuna Group;

Garde and Steenfelt, 1999).

On Baffin Island, the Rae craton comprises ca.

2900-2775 ± 2 Ma granodioritic to monzo-

granitic orthogneiss and temporally distinct

volcano-sedimentary sequences, namely the ca.

Geological Society of America Bulletin, November/December 2014 1627

76°W 70°N

Wodicka et al.

70°W

70°N

DS37A

Figure 2. Generalized bedrock geology of central Baffin Island (modified from St-Onge et al., 2005). Locations of samples for U-Pb zircon geochronology are indicated by black circles for detrital zircon samples and stars for igneous zircon samples. Mineral abbreviations: And—andalusite; Bt—biotite; Crd—Cordierite; Grt—Garnet; Kfs—K-feldspar; Ms—Muscovite; Sil—Sillimanite.

Neoproterozoic

Franklin dikes Paleoproterozoic

Bt monzogranite Leucocratic

monzogranite Cumberland batholith Rapakivi monzogranite-granodiorite Piling Group

Longstaff Bluff fm (Grt-Crd-Kfs-melt pod) Longstaff Bluff fm (Bt-Sil-Kfs土melt pod)

Longstaff Bluff fm (Bt-Ms-Crd土And) Longstaff Bluff fm (Bt-Ms土Grt)

Astarte River formation Bravo Lake formation

Flint Lake formation I I Dewar Lakes formation

D336

|D335|

C325

Nadluard/uk

Lake

H334

Archean

Hbl-Bt土Cpx gabbro Bt土Hbl monzogranite Eqe

C323

MSO62

Wordie

Bay greenstone belt

Psammite, semipelite; amphibolite

|

| Quartzofeldspathic orthogneiss

▼ ▼ ▼ Dewar Lakes - Bravo Lake thrust fault

Say

10

Oblique-slip fault

10

20

30 40

50 km

68°N

2829 Ma Mary River Group and the younger ca.

2740-2725 Ma Eqe Bay and Isotorq greenstone belts, all of which are intruded by 2726 +3/-2-2658 +16/-14 Ma granodioritic to mon- zogranitic and rare tonalitic plutonic rocks (Figs. 1 and 2; Jackson et al., 1990; Scott and de Kemp, 1999; Jackson, 2000; Wodicka et al., 2002; Bethune and Scammell, 2003; Scott et al., 2003; Young et al., 2004, 2007; St-Onge et al., 2005; Johns and Young, 2006). Various felsicplutonic rocks, ranging in age from 1897 +7/-4 to

ca. 1805 Ma and including the northernmost components of the post-accretion Cumberland batholith, intrude the Piling Group strata and Archean Rae craton rocks (Jackson et al., 1990; Henderson and Henderson, 1994; Wodicka and Scott, 1997; Scott and Wodicka, 1998; Scott, 1999; Wodicka et al., 2002; this study; Bethune and Scammell, 2003; Gagne et al., 2009; Whalen et al., 2010). To the south lies the Meta Incognita micro-continent (Fig. 1; St-Onge et al., 2000), which comprises dominantly ca. 2680-1950 Ma orthogneiss, a heterogeneous volcanic-bearing supracrustal assemblage (Schooner Harbour sequence), a clastic-carbonate shelf succession (the < 2.01 Ga > 1.90 Ga Lake Harbour Group), a structurally overlying succession of feldspathic quartzite and pelite (Blandford Bay assemblage), and intrusive monzogranite to

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Geological Society of America Bulletin, November/December 2014

Wodicka et al.

lesser quartz diorite plutons of the 1.865-1.847 Ga Bay Group; Corrigan et al., 2009) or to the Cumberland batholith and younger 1.841.82 Ga southeast through Cumberland Sound (St-Onge et granitoid plutons (e.g., Scott, 1997; Scott and al., 2006). Wodicka, 1998; Scott et al., 2002; Wodicka et al.,

2010; St-Onge et al., 2009; Whalen et al., 2010). Stratigraphy of the Piling Group St-Onge et al. (2006) proposed that accretion of the

The Piling Group is divided into five forma-Meta Incognita microcontinent to the southeastern margin of the Rae craton occurred between 1883 ± tions (Morgan, 1983; Tippett, 1984a), which 5 and 1865 +4/-2 Ma. The trace and precise nature comprise, in ascending stratigraphic order, the of the boundary (Baffin suture; St-Onge et al., 2006) Dewar Lakes, Flint Lake and Bravo Lake, Astarte between the two crustal blocks are uncertain, River, and Longstaff Bluff formations (Figs. 2 and owing largely to emplacement of the voluminous, 3). The Dewar Lakes formation can be subdivided post-accretion Cumberland batholith. Based into three members (Scott et al., 2003), the lowest primarily on the distribution of the Piling Group of which comprises thinly bedded, muscovite-rich versus Lake Harbour Group, it is tentatively quartzite, gray- to pinkweathering psammite, and situated in the central part of Baffin Island feldspathic quartzite. The overlying, and by far the (St-Onge et al., 2006, 2009; Fig. 1). Its trajectory most widespread, middle member comprises further east is even more cryptic, extending either medium to thickly bedded, sillimanite-rich to the east-northeast across Cumberland Peninsula quartzite with rare mica and is characterized by

dominantly southward- directed cross bedding. (northwest of the Hoare

The upper member is

Paleoproterozoic monzogranite

FT7! Turbidite |

| Pelite, semipelite

Volcaniclastic rock 1=1

D336

Mafic-ultra mafic sill A Mafic volcanic rock Carbonate I 1

MSO35 Quartzite

Archean monzogranite lv^>l Archean gneiss

DS42A

? ★

detrital zircon sample

igneous zircon sample

C325

LB-05

C321 C323

H334

D537A

N706? D335

D330

Bravo Lake formation

Figure 3. Generalized stratigraphic column for the Piling Group showing approximate position of analyzed samples. In the absence of marker horizons within the turbidite sequence, the stratigraphic position of samples DS42A, MSO35, and D336 was estimated on the basis of their location within the regional synformal structural basin (Fig. 2). All three samples are from the northern Longstaff Bluff formation. The stratigraphic height of the southern Longstaff Bluff formation sample LB-05 (Fig. 1) (shown here in a lower position for illustrative purposes) is unknown. See text for further details.

composed of thin beds of quartzite to psam- mite and rusty semipelite to pelite. The overall thickness of the Dewar Lakes formation varies significantly from less than a meter to locally greater than ?1000 m. Thickness variability may reflect primary sedimentary depocenters (Tippett, 1984a; Jackson, 2000; Scott et al., 2003). Basal quartzite and rare psammite of the lower Dewar Lakes formation are in stratigraphic (locally reworked) contact with underlying Archean orthogneissic and plutonic rocks, and the formation as a whole is interpreted as a clastic sheet deposited on Rae basement. The dominance of quartz over feldspar and rock fragments throughout the formation suggests a relatively high degree of sedimentological maturity (Tippett, 1984a). Most of the Dewar Lakes formation was likely deposited in a shallow marine environment (e.g., Tippett, 1984a; de Kemp et al., 2002).

White- to gray-weathering dolostone, marble, and calc-silicate strata of the Flint Lake formation are interlayered with minor to rare semipelite, pelite, quartzite, iron formation, and chert, and stratigraphically overlie the upper member of the Dewar Lakes formation (Morgan et al., 1976; Corrigan et al., 2001; Scott et al., 2002; St-Onge et al., 2005). The exposed thickness of this formation varies considerably both along and across strike, from several meters to 500-1000 m in the thickest exposures. The overall decrease in carbonate material toward the south led Scott et al. (2002, 2003) to suggest that the Flint Lake formation originally formed a relatively narrow (75-100 km wide) southfacing carbonate shelf adjacent and parallel to the southeastern edge of the Rae craton, with significant along-strike variation in primary thickness.

The Bravo Lake formation forms a nearly continuous, 120-km-long east-west-trending mafic volcanic belt in the southern part of the Piling structural basin (Fig. 2). Like the Flint Lake formation, it conformably overlies the siliciclastic rocks of the upper Dewar Lakes formation, suggesting that the lithologically distinct carbonate and volcanic sequences occupy an equivalent stratigraphic position within the Piling Group (Fig. 3; de Kemp et al., 2002; Scott et al., 2003). Based on lithostratigraphic and structural considerations, the Bravo Lake formation, and in places the Dewar Lakes formation, are interpreted to have been tectonically juxtaposed against the younger Longstaff Bluff formation across a north- to northwest-directed thrust fault (Fig. 2; Tippett, 1984a; Corrigan et al., 2001; de Kemp et al., 2002; Stacey and Pattison, 2003). The Bravo Lake formation has an estimated thickness of 1-2.5 km and can be subdivided into two sequences (Modeland

1628 Geological Society of America Bulletin, November/December 2014