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 postmetasomatic 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 northeasttrending 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.
Depleted mantle model ages (TDM) were calculated assuming a 147Sm/144Nd 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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