Geochemical and Isotopic Analyses of the Ultrapotassic Paleoproterozoic Christopher Island

Geochemical and Isotopic Analyses Of the Ultrapotassic Paleoproterozoic Christopher Island Formation, Baker Lake Group, Nunavut

Brian L. Cousens

Ottawa-Carleton Geoscience Centre

Department of Earth Sciences

Carleton University

1125 Colonel By Drive

Ottawa, ON. K1S5B6 Canada

At about 1.83 Ga, the western Churchill Province of northern Canada was the site of one of the largest ultrapotassic magmatic events in Earth history. Minette dikes were injected across an area of ~240 000 km2, and voluminous minette flows and pyroclastic deposits accumulated in an extensive series of continental subbasins, collectively composing the Christopher Island Formation (CIF) of the Baker Lake Basin (Figure1). Characterized by high volatile contents, incompatible element enrichment, and distinctive isotopic compositions, ultrapotassic rocks are widely accepted as melts of metasomatized lithospheric mantle [Foley, 1992; Mitchell and Bergman, 1991]. They thus are valuable probes to assess both the evolution of the lithospheric mantle beneath cratons and post- metasomatic cratonic tectonic events [e.g., Canning et al., 1998; Cousens et al., 2001; Feldstein and Lange, 1999; Lambert et al., 1995; Peccerillo, 1999; Wannamaker et al., 2000].

Predominantly volcanic rocks of the CIF are near the base of the Baker Lake Group, the lowermost member of the Dubawnt Supergroup [Gall et al., 1992]. The Dubawnt Supergroup consists of continental siliciclastic rocks and intercalated volcanic rocks deposited between 1.84 and 1.72 Ga [Rainbird et al., submitted]. Plutonic and volcanic rocks of the CIF have proven difficult to date precisely, but recent efforts show that volcanism extended from approximately 1.84 to 1.79 Ga [Rainbird et al., submitted], coincident with collisional and post-collisional processes in the Trans-Hudson Orogen on the southern flank of the western Churchill Province (Figure 1).

As part of the Western Churchill NATMAP, this data set represents the results of a regional geochemical and isotopic survey of CIF dykes and flows, designed to evaluate: the breadth of CIF volcanism; the geochemical attributes of the CIF; if a major lithospheric discontinuity (the Snowbird tectonic zone, Figure 1) represents a Paleoproterozoic suture between the Rae and Hearne domains of the western Churchill Province; and models for the origin of CIF potassic to ultrapotassic rocks. An overview of the geochemical and Nd isotopic characteristics of our suite of CIF rocks and their application to the Snowbird tectonic zone [Cousens et al., 2001], as well as an analysis of regional differences and similarities in CIF petrography and geochemistry, the importance of a primary carbonate phase in magma chemistry, Sr and Nd isotopic compositions, and diverse petrologic models for the origin of the CIF [Cousens et al., submitted] are published elsewhere. In addition, annual reports including detailed interpretations of the geochemical data are published as Open Files with the DIAND Geology Division, Yellowknife [Cousens, 1998; Cousens, 1999; Cousens, 2000].

The data are split into four tables. The first includes sample locations, major and trace element data. The remaining three tables present Nd, Sr and Pb isotopic data.

Samples of CIF lavas and dykes were collected from the Angikuni Lake-Rack Lake, MacQuoid-Gibson Lake, Kaminak Lake, Baker Lake, Kasba Lake-Lake Athabasca (Martin Group), and Whitehills Lake areas (Figure 1). Hamish Sandeman, Rob Rainbird, Thomas Hadlari, Tony LeCheminant, Al Donaldson, Tony Peterson, Ken Ashton, Derek Smith, Eva Zaleski, Simon Hanmer and Russell Hartlaub provided samples outside of the Angikuni Lake-Rack Lake area. Major and trace element data from rocks of the Martin Group and CIF dykes from the Kaminak Lake area are courtesy of Ryan Morelli and Yannick Beaudoin, respectively.

CIF lavas and dykes from all areas range from phlogopite-bearing mafic minettes to aphyric or feldspar-bearing felsites. In the Dubawnt, Kamilukuak and Baker Lake areas, a felsic-mafic-felsic minette sequence, interstratified with clastic sedimentary rocks, is observed [Peterson, 1994; Rainbird et al., submitted; Rainbird and Peterson, 1990]. Near Baker Lake, Archean basement is usually overlain by orange to red-weathering, potassium-feldspar-bearing felsite with only minor phlogopite. Sanidine porphyries, including dusty potassium feldspar crystals up to a centimeter in size, also occur [Smith, 2001]. Overlying mafic minettes are rich in phlogopite and clinopyroxene, and include flows, sills and pyroclastic rocks. Where present, the upper felsites are grey to orange, slightly potassium feldspar-phyric flows and domes.

In the Angikuni Lake area, Baker Lake Group rocks outcrop in two northeast- trending subbasins (Fig. 1) that extend from northern Angikuni Lake [Aspler et al., 1999; Aspler et al., 1998]. Mafic units commonly include up to 30% phlogopite and clinopyroxene phenocrysts in a potassium-feldspar-rich matrix. Felsic rocks contain variable proportions of feldspar phenocrysts, commonly including minor corroded phlogopite crystals. We sampled core from three holes drilled by WMC International Ltd. near the center of the easternmost sub-basin at Rack Lake (informal name, Figure 1) [Cousens, 1999]. Hole 94-2 consists of over 170 m of poorly-phyric, phlogopite- clinopyroxene, variably carbonate-rich mafic minette flows of uniform compositions, and Hole 94-1 includes over 140 m of mafic pyroclastic minette. Hole 95-1 intersected a 500 m-thick section of siliciclastic rocks (Angikuni Formation) that intervenes between Archean basement and the CIF, yet contains CIF-like detritus [Aspler et al., 2002].

In the MacQuoid-Gibson and Kaminak Lake areas [Beaudoin, 1998; Sandeman et al., 2000], the CIF is represented exclusively by dykes. CIF dykes at MacQuoid-Gibson can be split into three types: 1) hornblende-phlogopite-plagioclase spessartites; ,2) typical CIF phlogopite minettes; and 3) rare black, poikiolitic, phlogopite-potassium feldspar dykes exemplified by the diamondiferous Akluilak dyke [Armitage, 1998; MacRae et al., 1995].

A CIF dyke was sampled from the Snowbird tectonic zone at Kasba Lake [Hanmer et al., 1995]. Farther southwest, along the north shore of Lake Athabasca, potassic to sodic dykes, sills, and flows of the Martin Formation are geochemically similar to the CIF [Ashton et al., 1999; Hartlaub, 1999; Morelli et al., 2001]. Martin Formation volcanic rocks include plagioclase and sanidine phenocrysts, accompanied by variably altered clinopyroxene, but lack phlogopite.

Analytical Procedures

All rock samples were cut into thin slabs, from which weathered rims were trimmed and discarded. The remaining slab material was wrapped in plastic and broken into 1-cm size chips with a rock hammer. The chips were further reduced to sand size in a Bico jaw crusher, then ground to a fine powder in an agate ring mill. S and CO2 were determined by infrared combustion (LECO furnace) at the Ontario Geological Survey (OGS) Geochemical Laboratories in Sudbury, Ontario. Major element oxides were determined by fused-disc X-ray fluorescence (XRF) spectrometry at either the OGS or the University of Ottawa. For samples submitted to the OGS for XRF, Nb, Zr, Y, Sr, Rb, Ba, and Cr were determined by pressed-pellet XRF spectrometry, Co, Cu, Ni, Sc, V, and Zn by inductively-coupled plasma (ICP) emission spectrometry. For samples submitted to the University of Ottawa for XRF, Nb, Zr, Y, Rb, Sr, Ba, Cr, Co, Ni, V, and Zn were analyzed on the same fused disk used for major element analysis. For all samples, the rare earth elements, Hf, Ta, Th, U, and Nb (and in some cases Sc and Pb) were analyzed by acid-dissolution ICP-mass spectrometry at the OGS. The precision of the analyses is listed in the data table, based on several blind duplicate analyses and the reproducibility of international standards.

143Nd/144Nd, 87Sr/86Sr, and Pb isotope ratios, as well as Nd and Sm concentrations, were determined at Carleton University and [for details, see Cousens,

1996; Cousens, 1997]. Whole-rock powders were spiked with a mixed 148Nd-149Sm spike prior to dissolution. The uncertainties in Sm and Nd concentrations are +/- 1-2%, but 147Sm/144Nd ratios are reproducible to better than 1%. 58 runs of the La Jolla standard average 143Nd/144Nd = 0.511877 + 18 (September 1992-December 1998). Epsilon Nd (NdT) values [DePaolo and Wasserburg, 1976] were calculated relative to a modern CHUR (Chondrite Uniform Reservoir) value of 0.512638 and 147Sm/144Nd =

0.1967, using an age of 1830 Ma. The precision of the NdT values are + 0.8 epsilon units, based on duplicate analyses of geochemical standards and other rock samples. Duplicate runs of four CIF samples from this study all agree within 0.5 epsilon units. 147 144 Depleted mantle model ages (TDM) were calculated assuming a Sm/ Nd of 0.2140 and 143Nd/144Nd of 0.513151 for modern depleted mantle. All Pb mass spectrometer runs are corrected for fractionation using NIST SRM981. The average ratios measured for SRM981 are 206Pb/204Pb = 16.890 + .006, 207Pb/204Pb = 15.429 + .007, and

208Pb/204Pb = 36.502 + .024 (1 s.d.), based on 58 runs between September 1992 and December 1998. These values are the best estimate of the precision of the CIF Pb isotope analyses. The fractionation correction is +0.13%/amu [based on accepted values of Todt et al., 1984]. Two Sr standards are run at Carleton, NIST SRM987 (87Sr/86Sr =

0.710251 + 18, n=42, September 1992 - December 1998) and the Eimer and Amend

(E&A) SrCO3 (87Sr/86Sr = 0.708037 + 30, n=18, September 1994-December 1998). Initial Sr isotope ratios were calculated using Rb and Sr abundances determined by XRF, each having a precision of approximately 1%. The precision of initial 87Sr/86Sr ratios is approximately + 0.000050.

Acknowledgments

Funding was provided through contracts with the Yellowknife Geology Division, Indian and Northern Affairs Canada, facilitated by Bill Padgham and Carolyn Relf, as part of the Western Churchill NATMAP project. Larry Aspler and Jeff Chiarenzelli were critical collaborators in this project. Rex Brommecker of WMC originally suggested we examine the Rack Lake drill core. Al Armitage and Ryan Morelli kindly provided unpublished analyses from MacQuoid-Gibson dykes and Martin Formation volcanic rocks, respectively. Thanks to the XRF facility at the University of Ottawa and the Ontario Geological Survey laboratories for major and trace element analyses. Donna Switzer, Julie Thompson, Muy Ngo, Brenda Obina and Samantha Seigel performed much of the isotopic lab work.

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