Tectonic elements of the southwestern SECP

veloped primarily in juvenile B 1.95 Ga Pale- oproterozoic sediments and inferred to represent an accretionary complex along the suture between the Core Zone and the North Atlantic craton Nain Province Fig. 2. Dextral west and sinis- tral east transcurrent shear zones, which are synchronous-to post-tectonic with respect to thrusting in the New Que´bec and Torngat oro- gens, respectively, separate the bordering ‘fore- land’ orogens from the Core Zone. The Core Zone is itself a mosaic of variably reworked Archean crustal blocks Van der Leeden et al., 1990; Wardle et al., 1990; Nunn et al., 1990; James et al., 1996; Isnard et al., 1998, ca. 2.3 Ga and B 1.95 Ga supracrustal rocks e.g. Van der Leeden et al., 1990; Girard, 1990; Scott and Gau- thier, 1996, and 1.84 – 1.81 Ga granitoid rocks belonging to the De Pas and Kuujjuaq batholiths Perreault and Hynes, 1990; Dunphy and Skulski, 1996; James et al., 1996. Subsequent to Core Zone amalgamation, the Core Zone and border- ing orogens were overprinted by transcurrent shearing, which persisted locally to 1.74 Ga Wardle and Van Kranendonk, 1996. Affinity of Archean crust in the Core Zone is an outstanding problem having significant impli- cations for developing Paleoproterozoic tectonic models for the region. Addressing this problem in a comprehensive way is outside of the scope of this paper, although one possible model proposes that the majority of Archean Core Zone crust is exotic with respect to the Superior and North Atlantic cratons. In contrast, other models sug- gest that Archean rocks in the Core Zone were part of the Superior craton prior to 2.2 Ga James et al., 1998; Scott and St-Onge, 1998. Available geochronological data from the Core Zone indi- cates that Archean Core Zone rocks have broadly similar intrusive ages as rocks in the northeastern Superior craton, although this provides only cir- cumstantial evidence of their parentage. The Archean geochronological data from Core Zone rocks is non-unique and could be used to support either of the models. However, if a significant component of the Archean crustal blocks in the Core Zone did belong to the Superior Craton prior to 2.2 Ga, there is general consensus that these blocks acted as independent crustal units, relative to the bounding Archean cratons, during Paleoproterozoic construction of the SECP Wardle, 1998. There are no compelling geologi- cal or geophysical data to suggest the Core Zone includes a significant amount of North Atlantic craton crust, although minor amounts or variably reworked North Atlantic craton rocks may occur in regions adjacent to the boundary between the Core Zone and North Atlantic craton e.g. Ryan, 1990.

3. Tectonic elements of the southwestern SECP

3 . 1 . Superior craton The Archean Superior craton contiguous with the southwestern SECP Fig. 3 consists of high- grade Archean gneisses, part of the 90 000 km 2 Ashuanipi Complex Percival, 1991; James, 1997. The southeastern Ashuanipi Complex, contained in Fig. 3, mainly consists of \ 2.7 Ga metapelitic gneiss, orthogneiss, metamorphosed mafic intru- sions, diatexite plutons of orthopyroxene-bearing monzogranite to granodiorite, and granite intru- sions. The sedimentary precursors of the metapelitic gneiss were intruded by plutons and related dykes of tonalite, gabbro and granite at ca. 2.7 Ga prior to a regional tectonothermal event at 2.68 – 2.65 Ga Mortensen and Percival, 1987. Deformation was accompanied by gran- ulite – facies metamorphism and emplacement of the diatexite plutons, and followed by intrusion of late syn- to posttectonic plutons of biotite 9 mus- covite granite. Of relevance to our discussion of the evolution of the Core Zone which follows, it should be noted that rocks belonging to the Ashuanipi Complex do not occur in the Core Zone. 3 . 2 . New Que´bec Orogen NQO The Superior craton is unconformably overlain by a succession of Paleoproterozoic sedimentary and volcanic rocks, previously described as the Labrador Trough see Dimroth, 1972; Wares and Goutier, 1990. These rocks, as well as variably reworked Superior Province rocks and ‘exotic’ Paleoproterozoic supracrustal rocks, are con- tained in low-grade, thrust bound slices that make up a west-verging, Paleoproterozoic fold-and- thrust belt defined as the NQO. An understanding of the stratigraphic and structural relationships in the NQO is critical to understanding the Core Zone because stratigraphic e.g. rifting and struc- tural e.g. initiation of thrusting events in the foreland will be reflected or reflect events in the hinterland i.e. Core Zone. The Schefferville Zone, comprising the struc- turally lowest tectonostratigraphic unit in the southern NQO, consists of two sedimentary and volcanic cycles of the Knob Lake Group. In general, the two cycles record a transition from continental sedimentation and local alkaline vol- canism to progressively deeper water sedimenta- tion and tholeiitic basaltic volcanism see summaries in Skulski et al., 1993; Wardle and Van Kranendonk, 1996. The older sequence Cy- cle one of arkose, shale and dolomite records initiation of rifting and development of a passive margin sequence on extended Superior craton crust. Cycle one rocks are overlain discon- formably by a westwards-overstepping younger sequence Cycle two of quartzite, iron formation, sandstone – shale turbidites and arkose. The Schefferville Zone is structurally overlain by the Howse Zone, which is dominated by thick sequences of siltstone – shale turbidites interbed- ded with tholeiitic basalt and gabbro – sill com- plexes. However, it also contains rocks equivalent to cycles one and two described in the preceding paragraph. A possible interpretation for the tur- Fig. 3. General geology of a transect across the southern SECP. The study area this paper is located in the southwestern part of the transect see Fig. 4. Mg, Mesoproterozoic granitoid rocks; ARSZ, Ashuanipi River shear zone; LTSZ, Lac Tudor shear zone; GRSZ, George River shear zone. Paleoproterozoic thrust faults are decorated with widely spaced teeth. Grenvillian thrusts southwest corner of Fig. 3 are decorated with closely spaced teeth. bidites, basalts and related sills, is that they may have been deposited in a narrow, dextral transten- sional basin, perhaps analogous to the Gulf of California Wardle et al., 1990; Skulski et al., 1993. Cycle one and two volcanic rocks and sills in the Howse Zone are dated at ca. 2.17 – 2.14 Ga and ca. 1.89 – 1.87 Ga Birkett et al., 1991; Skulski et al., 1993; Rohon et al., 1993, respectively. These ages are interpreted to correspond with time of deposition for cycles one and two. If the narrow, transtensional basin model for Cycle two rocks is apropos, it would imply that rifted Supe- rior craton crust occurred to the east of Cycle two rocks at 1.87 Ga. To the east, the Howse Zone is structurally overlain by the Doublet terrane composed of Doublet Group mafic pyroclastic rocks, turbiditic siltstones, mafic and ultramafic sill complexes. The rocks in the Doublet terrane lack unequivocal stratigraphic links with Cycle one or Cycle two rocks in the Schefferville and Howse zones, and on this basis they are inferred to be exotic with respect to the Superior craton Wardle et al., 1995. Doublet terrane rocks may represent tran- sitional crust formed at the boundary between extended Superior continental crust and Pale- oproterozoic oceanic crust. The Laporte terrane Van der Leeden et al., 1990 is the eastern-most unit of the NQO. It mainly consists of presumed Archean gneisses and metamorphosed Paleoproterozoic pelitic, arkosic and mafic volcanic rocks, and lesser amounts of quartzite, marble and gabbro. There are no obvi- ous correlations between supracrustal rocks in the Laporte terrane and Paleoproterozoic rocks oc- curring to the west, although the Archean rocks may represent reworked Superior craton crust. The supracrustal rocks may be the relics of an accretionary prism Wardle, 1998. 3 . 3 . The Core Zone The Core Zone James and Dunning, 1996; James et al., 1996 is informally defined as a composite Paleoproterozoic terrane consisting of lithotectonic domains separated by Paleoprotero- zoic ductile high-strain zones Fig. 3. The do- mains include Archean and Paleoproterozoic \ 1.8 Ga rocks that cannot be unequivocally linked with the Superior or North Atlantic cra- tons, or with Paleoproterozoic supracrustal se- quences which occur in the New Que´bec or Torngat orogens. The western boundary of the Core Zone is marked by the Ashuanipi River shear zone James et al., 1996 in the south and the Lac Turcotte fault see Perreault and Hynes, 1990 in the north. The western margin of the Abloviak shear zone defines the eastern boundary. This definition includes the Kuujjuaq domain and the Lac Lomier complex as part of the Core Zone; this is perhaps noteworthy as these are not included in Wardle’s definition Wardle, 1998. In particular, tectonic affinity of the Kuujjuaq do- main Perreault and Hynes, 1990 is somewhat controversial; some workers e.g. Bardoux et al., 1998 consider the domain to be part of the NQO. The boundary between the NQO and the Core Zone is approximated by the inflection between a paired, negative NQO side and positive Core Zone side Bouguer gravity anomaly. On the basis of the gravity signature, which is consistent with a model involving west-directed transport of Core Zone rocks over the NQO, Thomas and Kearey 1980 proposed that this boundary is a Pale- oproterozoic suture marking a relic Andean-type margin. The offshore extension of the Core Zone underlying Ungava Bay has been traversed by the LITHOPROBE ECSOOT seismic reflection sur- vey, which showed the Core Zone to be domi- nated by 30° east-dipping reflectors that appear to penetrate the whole crust Hall et al., 1995. The southwestern Core Zone includes, from west to east, McKenzie River, Crossroads, Orma and Mistinibi-Raude domains see Van der Lee- den et al., 1990; Nunn et al., 1990; Girard, 1990; Nunn, 1994; James et al., 1996. The McKenzie River domain discussed in detail below, and see James et al., 1996 consists mainly of Archean tonalite gneiss and lesser amounts of inferred Paleoproterozoic supracrustal rocks, which are metamorphosed to upper amphibolite facies. The McKenzie River domain does not appear to share many Archean or Paleoproterozoic \ 1815 Ma features with domains to the east. Crossroads domain also discussed in detail be- low, and see James et al., 1996 contains relicts of high-grade Archean granite – greenstone terrane crust and Paleoproterozoic granitoid intrusions, which are part of the \ 500 km long De Pas batholith. The intrusions are variably deformed and recrystallized, demonstrating the domain has been overprinted by a Paleoproterozoic tec- tonothermal event which partially overlapped and postdated emplacement of the De Pas batholith. The western boundary with the McKenzie River domain is the Lac Tudor shear zone Van der Leeden et al., 1990, which deforms Archean and Paleoproterozoic rocks in both domains, and has components of dextral transcurrent and reverse displacement Van der Leeden et al., 1990; Bourque, 1991; James et al., 1996. The George River shear zone see Van der Leeden et al., 1990; Girard, 1990, which forms the eastern boundary with the Orma domain, is a wide several km, but very diffuse and poorly defined zone of heteroge- neous strain containing porphyroclastic protomy- lonitic and mylonitic rocks having dextral transcurrent kinematic indicators James et al., 1996. Mylonitization in the Lac Tudor and George River shear zones was attendant with Paleoproterozoic amphibolite – facies metamorph- ism. Archean geology of the Orma domain is similar to that of the Crossroads domain. The Orma domain contains relicts of Archean greenstone belt rocks, which are intruded by tonalite or- thogneisses having igneous crystallization ages be- tween 2682 and 2675 Ma, as determined by U – Pb age dating of zircons Nunn et al., 1990. Titanite data from the same rocks suggest they were meta- morphosed to amphibolite – facies in the Late Archean. Notably, the U – Pb data show no evi- dence that the Archean rocks were overprinted by Paleoproterozoic thermal events prior to the ca. 1720 – 1600 Ma Labradorian Orogeny; all Pb-loss in the titanites is younger than ca. 1640 Ma Nunn et al., 1990. Based on this data, the Orma domain has been considered to have mainly es- caped the ca. 1820 – 1775 Ma Paleoproterozoic tectonothermal event discussed in section four which overprinted the Crossroads and McKenzie River domains. However, at possible variance with the U – Pb data, the domain also includes foliated granitoid rocks, which are undated, but suspected to be Paleoproterozoic Nunn, 1994 on the basis of lithologic correlations with dated intrusions in the Crossroads domain. The signifi- cance of this correlation will be discussed later. The Orma domain also includes a sequence of wacke, quartz wacke, quartzite, tuffaceous rocks and metamorphosed basalt, named the Petscapiskau Group Emslie, 1970. Petscapiskau Group rocks were not metamorphosed prior to being intruded by the Mesoproterozoic ca. 1460 Ma Michikamau Intrusion Emslie, 1970. Age of the Petscapiskau Group is unknown, but there are two possible scenarios worthy of some discussion. If the Petscapiskau Group is older than the Pale- oproterozoic tectonothermal event that over- printed the Crossroads and McKenzie River domains i.e. \ 1820 Ma, it would support exist- ing U – Pb data from Archean rocks and confirm that the Orma domain escaped penetrative Pale- oproterozoic tectonothermal overprinting. If the Petscapiskau Group is younger than the Pale- oproterozoic event, it would leave open the possi- bility that the Orma domain has been overprinted by hitherto unrecognized Paleoproterozoic meta- morphism. It is remotely possible that the Petscapiskau Group is coeval with ca. 1650 Ma Labradorian supracrustal sequences e.g. the Blueberry Lake group James and Connelly, 1996, or MacKenzie Lake Group Nunn, 1993, which unconformably overly pre-1650 Ma rocks along the southeastern margin of pre-Labradorian Laurentia. The Labradorian volcanic rocks pre- dominately have felsic and intermediate composi- tions whereas Petscapiskau Group volcanic rocks are mafic. This may suggest that a Labradorian age for the Petscapiskau Group is unlikely, but it does not discount the possibility. There is an obvious need for more geochronological data from the Orma domain. The Mistinibi-Raude domain Van der Leeden et al., 1990 is in tectonic contact with the Orma domain along a northwest-striking, subvertical high-strain zone containing a strong, horizontal mineral elongation lineation Nunn, 1994. Defor- mation and attendant amphibolite-facies meta- morphism in the zone are undated but presumed to be Paleoproterozoic. The domain includes metasedimentary gneiss, granitoid migmatite, and metabasic rocks that were intruded by pre- to syn-tectonic granitic sheets; these rock units and the high-grade metamorphism that overprints them are provisionally assigned to the Archean. The Archean rocks are intruded by the enigmatic ca. 2.3 Ga Pallatin Intrusive Suite see Van der Leeden et al., 1990; Krogh, 1992 and overlain by presumed related sedimentary and volcanic rocks of the Ntshuku Complex Girard, 1990. In the northern part of the domain, Ntshuku rocks are unconformably overlain by deformed sedimentary rocks of the Paleoproterozoic Hutte Sauvage Group Van der Leeden et al., 1990. The Ntshuku and Hutte Sauvage rocks are metamor- phosed from greenschist to lower-amphibolite facies. Mesoproterozoic ca. 1460 Ma intrusions of anorthosite and related granitoid rocks intrude the Orma and Mistinibi-Raude domains, and in the southeastern Core Zone, these obscure the Paleoproterozoic Abloviak shear zone Wardle et al., 1990 and the boundary with the North At- lantic craton. The Abloviak shear zone marks the approximate eastern limit of penetrative Pale- oproterozoic deformation and defines the tectonic boundary with the North Atlantic craton. How- ever, in the area between the Mesoproterozoic Harp Lake Intrusive Suite and the Nain Plutonic Suite, the eastern limit of Paleoproterozoic defor- mation is marked by deformed Ingrid Group rocks, a sequence of conglomerates and volcanic rocks. Age of Ingrid Group volcanism is loosely constrained; U – Pb zircon and titanite ages deter- mined from two samples of felsic volcanic rocks give ages of ca. 1895 Ma and 1805 Ma Wasteneys et al., 1996. Clasts in Ingrid Group conglomerate are derived from a variety of sources including \ 3.7 Ga North Atlantic craton presumed Saglek Block crust and from ca. 1900 Ma granitoid rocks that may have been part of a Torngat magmatic arc Wasteneys et al., 1996 constructed on the North Atlantic craton. The Early Archean clasts demonstrate that the sedimentary rocks were autochthonous with respect to North At- lantic craton crust. The Ingrid Group is overthrust to the west by granulite – facies granitoid migmatite determined by U – Pb zircon geochronology to have an em- placement age of 2870 9 22 Ma Wasteneys et al., 1996. The affinity of these Archean gneisses is uncertain; they resemble North Atlantic Nain craton rocks, although Ermanovics and Ryan 1990 note that west of the Ingrid Group, Archean gneisses lack Paleoproterozoic Kikker- tavak dykes, diagnostic of adjacent North At- lantic craton crust. Farther west, the Archean gneisses are tectonically interleaved with Tasiuyak gneiss, a distinctive metasedimentary gneiss, which can be traced for at least 500 km along the entire eastern boundary with the North Atlantic craton, although it’s total strike length may ex- ceed 1300 km Scott, 1998. The high-strain zone containing deformed Tasiuyak gneiss and Archean gneisses may be related to and coeval with the sinistral transcurrent Abloviak shear zone. Detrital zircon populations and field rela- tions, which bracket the depositional age of Tasi- uyak sediments between 1940 and 1895 Ma Scott and Machado, 1993; Scott and Gauthier, 1996, and Nd-isotopic data Kerr et al., 1993; The´riault and Ermanovics, 1993 suggest that sedimentary precursors of the Tasiuyak gneiss were exotic with respect to the North Atlantic craton Wardle and Van Kranendonk, 1996. Thus, Archean rocks that occur to the west of the Tasiuyak gneiss might never have been part of the North Atlantic craton. In conflict with this interpretation, Ryan 1990 has suggested that Archean gneisses and anorthositic rocks occurring more than 50 km west of Tasiuyak gneiss and Paleoproterozoic shear zones, in the area north of the Mistastin batholith north of the area shown in Fig. 3, correlate with North Atlantic Nain craton rocks. It may be possible that there is a wide 50 km? region contiguous with the North Atlantic craton which contains tectonically interleaved North At- lantic craton rocks and Archean rocks which are exotic with respect to the North Atlantic craton e.g. Van Kranendonk et al., 1993. Locally, the Mesoproterozoic intrusions and Core Zone rocks are unconformably overlain by clastic sedimentary rocks and gabbro sills of the 1275 – 1225 Ma Seal Lake Group Romer et al., 1995. Seal Lake Group rocks obscure the boundary between the Core Zone and the North Atlantic craton in the region south of the Harp Lake Intrusive Suite. Fig. 4. Geology of the southwestern core zone in the area around the Smallwood Reservoir, Labrador southern part of NTS map area 23I. Geochronological data summarized from James et al. 1996zr, zircon; ti, titanite; ig, igneous crystallization age; mt, metamorphic age. Ellipses show sample locations this study. Most samples were collected from island outcrops in the Smallwood Reservoir. ARSZ, Ashuanipi River shear zone; LTSZ, Lac Tudor shear zone; GRSZ, George River shear zone.

4. U – Pb geochronological studies in the southwestern Core Zone