9. REGIONAL CORRELATIONS, TECTONIC SETTINGS, AND STRATIGRAPHIC SOLUTIONS IN THE YELLOWKNIFE GREENSTONE BELT AND ADJACENT AREAS FROM GEOCHEMICAL AND SM-ND ISOTOPIC ANALYSES OF VOLCANIC AND PLUTONIC ROCKS Brian Cousens1, Hendrik Falck2, Luke Ootes2, Val Jackson2, Wulf Mueller3, Patricia Corcoran4, Craig Finnigan5, Ed van Hees6, Cathy Facey7, and Alberto Alcazar7 1. Ottawa-Carleton Geoscience Centre, Carleton University, 1125 Colonel By Drive, Ottawa, ON K1S 5B6 2. C.S. Lord Northern Geoscience Centre, PO Box 1500, Yellowknife, NT X1A 2R3 3. Dépt. des Sciences de la Terre, Université du Quebec à Chicoutimi, Chicoutimi, QC G7H 2B1 4. Dept. of Earth Sciences, University of Western Ontario, London, ON N6A 5B7 5. Dept. of Geology, University of Toronto, 22 Russell Street, Toronto, ON M5S 3B1 6. Geology Dept., Wayne State University, Detroit, MI 48202 7. Ottawa-Carleton Geoscience Centre, Carleton University, 1125 Colonel By Drive, Ottawa, ON K1S 5B6 INTRODUCTION (Padgham and Fyson, 1992; Bleeker, 2003). Thus the origin and evolution of the Slave province appears to be dramati- The Canadian Shield consists of a number of Archean cra- cally different from that of the Superior province. The small tonic fragments sutured by Proterozoic collisional belts. volume of volcanic greenstone belts limits the extent to How did these ancient cratonic fragments originate, and do which we can use the geochemistry of volcanic rocks to infer we see evidence that tectonic processes similar to modern tectonic setting(s) craton-wide, but also enhances the impor- plate tectonics existed during their creation? The best stud- tance of such studies to understanding the evolution of the ied of these cratonic fragments, the Superior province, is Slave. subdivided into long, curvilinear belts based on the relative abundances of volcanic and plutonic versus metasedimenta- The Yellowknife greenstone belt (YGB) is one of sever- ry rocks. This belt-like structure of lithotectonic assemblages al small greenstone belts exposed in the southern part of the was interpreted to signify that the Superior craton grew by Slave Province. This greenstone belt is extremely well pre- the addition of successively younger, allochthonous island served, has well-exposed contact relationships, and has suf- arc, oceanic, and forearc sedimentary terranes, punctuated fered primarily low-grade, greenschist facies metamor- by mantle plume events (e.g., Langford and Morin, 1976; phism. The YGB has been mapped in considerable detail, Thurston, 1991; Kimura et al., 1993), supporting a plate tec- has been the subject of major and trace element geochemical tonic model for Archean crustal growth that is similar to studies, and is well dated by modern U-Pb zircon techniques. modern plate tectonics. However, geological studies over the Nevertheless, only a handful of modern rare earth element past decade suggest that lithotectonic assemblages may com- and neodymium (Nd) isotopic analyses exist for volcanic monly be autochthonous in origin (as summarized in and plutonic rocks in the Yellowknife area (e.g., Dudás, Thurston, 2002). 1989; Davis and Hegner, 1992; Yamashita and Creaser, 1999). Radiogenic isotopic data, in conjunction with field Critical to models of the evolution of the Superior (and observations, and major and trace element geochemistry, other) craton are the field relationships and geochemistry of have the potential to help resolve stratigraphic and tectonic volcanic, plutonic, and sedimentary rocks in greenstone belts problems, and to distinguish between different tectonic sce- that retain a record of their tectonic setting. Based on the narios for the origin of the volcanic rocks. geochemical characteristics of modern volcanic rocks in dif- ferent plate tectonic settings, the geochemical signatures of This geochemical study began in 1996, soon after the ancient volcanic rocks have been used to distinguish completion of the first U-Pb zircon geochronological study between continental, oceanic (spreading centre, oceanic of the YGB and the recognition that the volcanic sequence plateau, seamount), back-arc, or volcanic arc origins for was underlain by older continental crust (Isachsen et al., greenstone belts in Canada and worldwide (e.g., Condie and 1991; Isachsen, 1992; MacLachlan and Helmstaedt, 1993; Baragar, 1974). Isachsen and Bowring, 1997). Several geologic problems were identified that could be addressed by a detailed geo- The geology of the Slave province contrasts strongly with chemical study. Firstly, what is the geological relationship that of the Superior province. It lacks the belt-like structure between the 2.7 Ga Kam and 2.66 Ga Banting groups of the of the Superior, is dominated by metasedimentary and plu- YGB, and what tectonic setting is inferred from their geo- tonic rocks, includes only a small volume of volcanic green- chemical systematics? Secondly, within the Kam Group, stone belts, and includes three to four rifted margins Cousens, B., Falck, H., Ootes, L., Jackson, V., Mueller, W., Corcoran, P., Finnigan, C., van Hees, E., Facey, C., and Alcazar, A. 2005: Regional correlations, tectonic settings, and stratigraphic solutions in the Yellowknife greenstone belt and adjacent areas from geo- chemical and Sm-Nd isotopic analyses of volcanic and plutonic rocks; Chapter 9 in Gold in the Yellowknife Greenstone Belt, Northwest Territories: Results of the EXTECH III Multidisciplinary Research Project, (ed.) C.D. Anglin, H. Falck, D.F. Wright and E.J. Ambrose; Geological Association of Canada, Mineral Deposits Division, Special Paper No. p. B. Cousens, H. Falck, L. Ootes, V. Jackson, W. Mueller, P. Corcoran, C. Finnigan, E. van Hees, C. Facey and A. Alcazar what magmatic evolution trends can be identified from the ed to be related to synvolcanic dykes in the overlying Kam base of the volcanic pile to its top and what processes are Group, imply that the volcanic rocks of the Kam Group were responsible? Does older basement play a role? Thirdly, with- fed through and deposited over the Bell Lake Group in the poorly studied Banting Group, are putative Banting (MacLachlan and Helmstaedt, 1995). Further evidence for rocks exposed along the western shore of Yellowknife Bay older basement rocks beneath the YGB comes from xeno- really part of the Banting Group? Are felsic dykes that cut liths of tonalitic gneiss in a diatreme at the Con mine south the Kam Group feeders to the Banting Group (Helmstaedt of Yellowknife, which have discordant U-Pb zircon ages of and Padgham, 1986; Isachsen, 1992)? What geochemical 3040 to 3300 Ma (Nikic et al., 1980). similarities exist between the Banting Group and some other To the west, the YGB is intruded by the Western Plutonic 2.66 Ga volcanic complexes in the southwestern Slave Complex, which includes from south to north the Defeat Province? Fourthly, are other small greenstone belts within Plutonic Suite, the Duckfish Granite, and the Anton the immediate Yellowknife area related to YGB volcanism? Complex (Henderson, 1985). Precise U-Pb zircon ages from these granitoids range from 2608–2641 Ma (Henderson, Regional Geological Setting 1985; Henderson et al., 1987; Dudás et al., 1990; van The surface geology of the YGB is shown in Figure 9-1, and Breemen et al., 1992). Much of the Anton Complex is intru- the regional stratigraphy (with U-Pb zircon ages) is dis- sive into the YGB and the underlying gneissic played in Figure X-X in Falck et al. (2005). The belt has basement/cover group in the Dwyer Lake area, and may be been subdivided into two groups, the dominantly mafic Kam an early deformed part of the Defeat Plutonic Suite Group and the unconformably overlying, more felsic-domi- (Helmstaedt and Padgham, 1986; MacLachlan and nated Banting Group (Helmstaedt and Padgham, 1986). Helmstaedt, 1995). To the east, the YGB is conformably Precise (analytical uncertainties of +/- 1 to 4 Ma) U-Pb zir- overlain by the Duncan Lake Group, including the Walsh con crystallization ages from Kam Group felsic volcanic Lake and Burwash formations, a thick pile of greywacke and rocks range from 2722 to 2701 Ma, although some cherty mudstone turbidites thought to be basin-fill sediments felsic tuffs of the lower Kam Group include inherited zircons (Henderson, 1985; Helmstaedt and Padgham, 1986). The as old as 2820 Ma (Isachsen, 1992). Banting Group crystal- Kam and Banting groups are unconformably overlain by the lization ages are ~2660 Ma (Isachsen, 1992). The metavol- conglomerates and sandstones of the Jackson Lake canic supracrustal package forms a steeply dipping homo- Formation (Henderson and Brown, 1966; Helmstaedt and cline, such that the stratigraphic “way-up” is to the southeast. Padgham, 1986). Recent study of the Jackson Lake con- The rocks are metamorphosed to greenschist grade, but glomerates shows that they occur along the trace of the metamorphic grade increases to amphibolite in proximity to Yellowknife River Fault Zone (YRFZ, Falck et al., 2005, younger intrusions. The belt was subsequently dismembered Fig. X-X) that separates the Kam Group from the by Proterozoic faulting into four major blocks (Helmstaedt Banting/Duncan Lake groups, and represent post-faulting and Padgham, 1986). deposition of coarse sediments within low topography in the More abundantly felsic volcanic complexes of ca. 2.66 Ga fault zone (Martel et al., 2001; 2002). age are common in the southwestern Slave Province, includ- The YGB stratigraphic sequence of Mesoarchean gneissic ing the Banting Group, the Clan Lake complex north of basement, a cover group consisting of