Objectives Introduction Directory UMM :Data Elmu:jurnal:P:Precambrian Research:Vol103.Issue1-2.2000:

1. Objectives

There are few geochronological constraints on the intrusive and tectonothermal history of the southwestern Core Zone James and Dunning, 1996; James et al., 1996 of the Southeastern Churchill Province SECP, northeastern Lauren- tia Fig. 1. A better understanding of the Core Zone Fig. 2, a fundamental Paleoproterozoic tectonic division consisting of a tectonic collage of Archean and Paleoproterozoic rocks, is criti- cal because it contains not only a record of its internal construction but also of its assembly with bordering Archean cratons and their at- tached Paleoproterozoic supracrustal sequences. In this paper, we address some of the outstand- ing problems relating to Paleoproterozoic con- struction of three domains, McKenzie River, Crossroads and Orma domains, that make up the southwestern Core Zone. In particular, we investigate the timing and significance of granitic and mafic intrusions, their relationship to meta- morphic events and the development of major structures. The timing relationships are estab- lished as the result of regional geological studies James et al., 1993; James and Mahoney, 1994, and detailed field and geochronological analysis of critical exposures containing unequivocal field relationships James and Dunning, 1996. The results, when combined with available data from other areas in the southern SECP, which we summarize in section three of this paper, help to constrain a model for Paleoproterozoic evolution of the Core Zone. This model has broader im- plications for the overall development of north- eastern Laurentia. Fig. 1. Principal tectonic elements of northeastern Laurentia including areas in northeastern North America and western Greenland. Fig. 2. Tectonic elements of Labrador and northeastern Que´bec. Mrd, McKenzie River domain; Crd, Crossroads domain; Od, Orma domain; M-Rd, Mistinibi-Raude domain; Kd, Kuujjuaq domain. Mesoproterozoic intrusions are indicated by the open triangle pattern.

2. Introduction

The SECP is a 300 km wide, north-trending composite tectonic belt of Archean and Pale- oproterozoic rocks that is one segment of a sys- tem of Paleoproterozoic orogens linking Archean cratons in northeastern Laurentia. It is principally a continuation of the Trans-Hudson Orogen, which can be traced around the western, northern and eastern margins of the Superior craton, but it also shares common elements with the Nagssug- toqidian Orogen of Greenland Fig. 1. The SECP is exposed from Ungava Bay, where it disappears under Hudson Strait, to southern Labrador, where it is truncated by east – northeast-trending structures and tectonostratigraphic units that make up the Grenville Province. The SECP reap- pears north of Hudson Strait, on southern Baffin Island, although the precise correlation of major structures and tectonostratigraphic units between the two regions is speculative and the focus of ongoing studies see St-Onge et al., 1997; Scott and St-Onge, 1998; St-Onge et al., 1998, 1999. The SECP formed as a result of relative north- ward movement and sequential collision of Archean North Atlantic Nain and Superior cra- tons, and attached Paleoproterozoic supracrustal sequences, with an Archean cratons which resided to the north see Hoffman, 1990; Van Kranendonk et al., 1993. Also involved in the collisions were Archean crustal blocks of suspect parentage that are now confined to the interven- ing regions between the intact Archean cratons. The SECP has a broadly tripartite character con- sisting of from west to east: 1 a west-verging fold-and-thrust belt New Que´bec Orogen devel- oped in 2.17 – 1.86 Ga sedimentary and volcanic cover rocks, and involving Superior craton base- ment, 2 a medial hinterland or composite ter- rane the Core Zone having Archean and Paleoproterozoic components, and 3 a doubly- verging, fan-shaped wedge Torngat Orogen de- 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