Overview of Results from the Greenstone ... - Geology Ontario

contrastingly have been dated near 2700 Ma (Barrie and Davis 1990; this study). Further
geochronological study of sample 96JAA-0094 was carried out to test whether the true age for the felsic
volcanic unit is closer to a Tisdale (ca. 2710–2704 Ma) or Kamiskotia Volcanic Complex–Blake River
(ca. 2703–2696) age, and resolve whether the published 2713 Ma age accurately dates the time of
volcanism or possibly inheritance.
Three additional good-quality zircon grains were selected from sample 96JAA-0094 for air-abrasion
and U/Pb analysis. These included two very fine-grained short, colourless zircon prisms from a
subpopulation not analyzed by Ayer et al. (2002). The new isotopic data are presented in Table A1 and
are shown graphically, together with the earlier data, in Figure 4B. Data for two of the new analyses (Z1,
Z2) are concordant and have overlapping 207 Pb/ 206 Pb ages. Moreover, these are entirely within error of the
two most concordant data of Ayer et al. (2002), shown as shaded ellipses in Figure 4B. A recalculated
weighted mean age for only these four analyses is 2712.2±0.9 Ma, which confirms a Kidd–Munro age for
this unit. A third single-grain analysis (Z3) is 4% discordant but has a 207 Pb/ 206 Pb age of 2853.6±1.4 Ma,
attesting to the presence of a much older, pre-Abitibi inheritance in this area.
04JAA-0004 Deformed conglomerate, Timiskaming assemblage, northeast
Thorneloe Township
(NAD83, Zone 17 UTM 463598E, 5354205N; #9 on Figure 2 and Table 1)
Sample 04JAA-0004 represents a strongly deformed conglomerate collected from the south shore of
Kenogamissi Lake in northeastern Thorneloe Township. Dating of zircon in this metasediment was
carried out in order to test for the presence of Timiskaming conglomerates west of the Mattagami River
fault. The sample contained an abundant assortment of zircon morphological types, ranging from large,
rounded grains to sharply euhedral, occasionally elongate (up to ~4:1) whole prisms and fragments, and
rare, small, distinctly brown prismatic grains. Results from the analysis of the U/Pb isotopic
compositions of seven air-abraded single detrital zircon grains are presented in Figure 4C and Table A1.
The oldest zircon population measured is represented by two concordant analyses (rounded grains Z1, Z2)
whose 207 Pb/ 206 Pb ages cluster at 2726.1 and 2727.5 Ma. The youngest igneous grains are represented by
two fractions (euhedral grains Z4, Z5) overlapping each other and concordia, having 207 Pb/ 206 Pb ages of
2684.4 Ma and 2685.0 Ma. Two intermediate-age grains (brownish fragments Z7, Z8) are 0.7 and 1.7%
discordant with ages at 2716.1 and 2705.0 Ma, respectively. Fraction Z6 represents a single, discrete
short brown prism with high uranium concentration (524 ppm) and very low Th/U (0.07). This zircon
yielded a concordant 207 Pb/ 206 Pb age of 2618.1±2.0 Ma (2σ), and is best interpreted as a grain crystallized
during metamorphic heating of this unit (accompanying deformation?). The most rigorous constraint on
the age of sedimentation is provided by the precise analysis for grain Z4, implying that deposition of the
conglomerate was younger than 2684.4±1.7 Ma (2σ). The older age populations in this sample imply that
detritus was shed into the basin from older Abitibi crustal sources such as felsic volcanic rocks of the
Deloro, Kidd–Munro and Tisdale assemblages.
The results support the hypothesis that this deformed band of sediments represents the continuation
of Timiskaming assemblage and the Porcupine–Destor fault to the west of the Mattagami River fault.
04PCT-0062 Feldspar-phyric lapilli tuff, Deloro Township
(NAD83, Zone 17 UTM 478510E, 5364034N; #10 on Figure 2 and Table 1)
Sample 04PCT-0062 was collected from a sequence of feldspar-phyric lapilli tuffs intercalated with
ultramafic volcanic rocks and iron formation exposed along the Deloro Township power line in
northwestern Deloro Township. This sample contained very sparse and generally low-quality zircon,
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