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New Sm-Nd Data from the Zemlak Domain New Sm-Nd Data from the Zemlak Domain: Testing the Model for Tectonically Amalgamated Taltson Basement Complex and Proto-Rae Cratonic Blocks within the Rae Province of Northwestern Saskatchewan Ken Ashton 1, Robert A. Creaser 2 and Kathryn Bethune 3 Information from this publication may be used if credit is given. It is recommended that reference to this publication be made in the following form: Ashton, K., Creaser, R.A. and Bethune, K. (2016): New Sm-Nd data from the Zemlak Domain: testing the model for tectonically amalgamated Taltson basement complex and proto-Rae cratonic blocks within the Rae Province of northwestern Saskatchewan; in Summary of Investigations 2016, Volume 2, Saskatchewan Geological Survey, Saskatchewan Ministry of the Economy, Miscellaneous Report 2016-4.2, Paper A-7, 12p. Abstract Three of five new Sm-Nd analyses from the Zemlak Domain yield TDM ages in the 3.04 to 3.00 Ga range, similar to previously analyzed Zemlak samples, and slightly older than the 2.89 to 2.84 Ma range for rocks in the Nolan Domain. A 2.52 Ga gneissic granodiorite from a highly magnetic zone near the northern boundary of the Zemlak Domain yielded a TDM age of 2.96 Ga with a -6.9 ƐNd value calculated at 1900 Ma. A -0.7 ƐNd value calculated at its crystallization age, together with an unpublished arc-type geochemical signature, suggests that this rock is slightly contaminated by older crustal material and may have been emplaced above a subduction zone. A pink leucogranite, from south of the eastern end of Tazin Lake, yielded a similar TDM age of 3.01 Ga but a more evolved -11.3 ƐNd value (calculated at 1900 Ma), consistent with its inferred anatectic origin. In the southeastern zone of the Zemlak Domain, between the Black Bay and Island Bay faults, variably magnetic granodioritic orthogneisses are overlain by the Paleoproterozoic Murmac Bay group. A gneissic granodiorite from the LeBlanc Lake area of this southeastern zone yielded a 3.04 Ga TDM age and a -6.6 ƐNd value. A sample of the Cairns Island granite, also collected from this southeastern zone, yielded a similar 3.00 Ga TDM age but a -11.1 ƐNd value, indicative of a more evolved history and consistent with its derivation from crustal melting. The fifth sample, collected from variably mylonitic gneissic granodiorite in a zone central to the northern and southeastern Zemlak Domain, yielded a younger 2.87 Ga TDM age and a -6.9 ƐNd value, more similar to the ca. 2.6 Ga rocks of the Nolan and northwestern Beaverlodge domains. Keywords: Sm-Nd, Zemlak Domain, proto–Rae craton, Taltson basement complex 1. Introduction It was recently proposed that the Rae Province in northwestern Saskatchewan (Figure 1) could have resulted from the amalgamation of two cratonic blocks (Ashton et al., 2014; Bethune, 2014) during the ca. 2.5 to 2.3 Ga Arrowsmith orogeny (Berman et al., 2005, 2013). One block, termed the proto–Rae craton, was thought to include rocks of the Nolan Domain and parts of the central Beaverlodge Domain, and be dominated by ca. 2.68 to 2.58 Ma granitoid rocks (Figure 2; Van Schmus et al., 1986; Hartlaub et al., 2004, 2005; Ashton et al., 2007a; Bethune et al., 2013). The other block consisted of ca. 3.0, 2.5 and 2.3 Ga granitoid rocks (Persons, 1983; Hartlaub et al., 2004, 2007; Ashton et al., 2007a) of the ‘Taltson basement complex’ (Panӑ, 2010) that extend from northeastern Alberta and the southern Northwest Territories eastward into Saskatchewan as the Zemlak and southwestern Beaverlodge domains. The basal Murmac Bay group was interpreted as a post-orogenic extensional response to this plate amalgamation and, together with the older rocks, was subsequently overprinted by the ca. 1.98 to 1.92 Ga Taltson orogeny (Ross et al., 1991; McDonough et al., 2000; Ross, 2002). 1 Saskatchewan Ministry of the Economy, Saskatchewan Geological Survey, 1000-2103 11th Avenue, Regina, SK S4P 3Z8 2 University of Alberta, Department of Earth and Atmospheric Sciences, 1-26 Earth Sciences Building, Edmonton, AB T6G 2E3 3 University of Regina, Department of Geology, 3737 Wascana Parkway, Regina, SK S4S 0A2 Although the Saskatchewan Ministry of the Economy has exercised all reasonable care in the compilation, interpretation and production of this product, it is not possible to ensure total accuracy, and all persons who rely on the information contained herein do so at their own risk. The Saskatchewan Ministry of the Economy and the Government of Saskatchewan do not accept liability for any errors, omissions or inaccuracies that may be included in, or derived from, this product. Saskatchewan Geological Survey 1 Summary of Investigations 2016, Volume 2 Figure 1 – Orogenic map of the western Canadian Shield showing main tectonic elements both exposed and beneath Phanerozoic cover. In order to test this tectonic model, it would be useful to learn more about the age and crustal history of each of the inferred plates. One way to do this is by analyzing rocks for their Sm-Nd isotopic composition. Previous work has shown that the temporally restricted 2.68 to 2.58 Ga (i.e., ca. 2.6 Ga) granitic rocks that make up the Nolan Domain and parts of the central Beaverlodge Domain have consistent depleted mantle (TDM) ages in the 2.92 to 2.84 Ga range and slightly positive ƐNd values calculated for the time of crystallization (Figure 3). Given the more temporally heterogeneous composition of the Taltson basement complex, which contains granitic rocks of ca. 3.0, 2.5 and 2.3 Ga ages, it is not surprising that corresponding TDM ages span a broader range, from 3.24 to 2.86 Ga (Figure 3). The ca. 3.0 Ga suite yields TDM ages older than 3.0 Ga and slightly positive ƐNd values (calculated for the time of crystallization), suggestive of minor influence from older rocks. The ca. 2.5 Ga suite yields TDM ages in the 2.96 to 2.86 Ga range and slightly negative ƐNd values (calculated for the time of crystallization), suggestive of a more significant influence from older rocks. In contrast, the two youngest of the five suites are crustal melts, which is inherent in their more evolved isotopic compositions. The ca. 2.3 Ga igneous suite yields TDM ages of 3.13 to 2.88 and ƐNd values of -1.9 to -6.7 (calculated for the time of crystallization). The single analyzed ca. 1.9 Ga rock yields a TDM age of 3.24 and ƐNd value of -13.6 (calculated for the time of crystallization). Saskatchewan Geological Survey 2 Summary of Investigations 2016, Volume 2 Figure 2 – Simplified geological map of the Rae Province in northwestern Saskatchewan (NTS 74N) showing locations and U-Pb crystallization ages of previously dated rocks, and locations and calculated TDM ages from pre-existing and new Sm-Nd isotopic analyses derived from all rocks that predate the Martin Group. Approximate inferred boundary between proto–Rae craton and Taltson basement complex shown as dot-dash lines. Inset shows lithotectonic domains in NTS 74N. Saskatchewan Geological Survey 3 Summary of Investigations 2016, Volume 2 Figure 3 – Nd evolution curve for the five known suites of granitoid rocks in the western Rae Province of northwestern Saskatchewan (see text for references). ƐNd values have been plotted at the crystallization age of each sample to show the degree of evolution of each igneous suite; coloured lines have been projected back to the depleted mantle curve and represent the maximum and minimum TDM ages for each suite. Abbreviations: CHUR, chondritic uniform reservoir; DM, depleted mantle. Therefore, TDM ages at the upper end of the age range for the Taltson basement complex (i.e., ≥3.0 Ga) are useful for helping to define its extent. They can be generated directly from rocks of the ca. 3.0 Ga suite or from a younger crustal melt (i.e., the ca. 2.3 Ga or 1.9 Ga suite) incorporating variable proportions of Mesoarchean material. TDM ages at the lower limit of that range (i.e., ≤3.0 Ga) can be generated directly from the 2.5 Ga suite of the Taltson basement complex or from the 2.6 Ga suite of the proto–Rae craton, with only minimal input from Mesoarchean sources. Alternatively, TDM ages in the 2.8 to 3.0 Ga range can be derived from rocks of the more evolved 2.3 or 1.9 Ga suites by incorporating a smaller component of ancient material. Assuming that the ranges of TDM ages and ƐNd values previously obtained for the five suites in the region are representative, the new Sm-Nd analyses, together with whole-rock geochemical analyses, may also provide constraints on the crystallization ages of the analyzed rocks. Therefore, to obtain better coverage of TDM ages in the Zemlak Domain and to potentially provide constraints on crystallization ages, five additional samples of granitic rocks from the eastern part of the domain were analyzed. 2. Regional Geology and Sm-Nd Signatures of Previously Analyzed Rocks in the Tazin Lake Area The Nolan Domain (Figure 2) is dominated by weakly deformed and weakly to moderately magnetic (Ashton, 2009) 2.61 to 2.58 Ga granitic to granodioritic rocks (Van Schmus et al., 1986; Ashton et al., 2007a), with minor migmatitic to diatexitic psammopelitic rocks on its more highly metamorphosed eastern margin. Rocks of the adjacent Zemlak Domain are compositionally more heterogeneous, ranging from granite to diorite, and are much more intensely deformed. Rocks along the domain boundary are characterized by a southward-increasing strain and metamorphic gradient (Ashton et al., 2005).
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