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

03VOI-0422-1 Trachytic lava, Timiskaming assemblage, Gauthier Township
(NAD83, Zone 17 UTM 587842E, 5332221N; #31 on Figure 2 and Table 1)
Sample 03VOI-0422-1 was collected from a distinct unit of feldspar-phyric trachytic lava from the
volcanic-dominated section of the Timiskaming assemblage in central Gauthier Township.
A modest amount of zircon was recovered from this sample, dominated morphologically by small,
short (2-3:1), generally cylindrical and often sharply terminated prisms. For the most part, zircons were
colourless to brown and free of cracks and inclusions, but distinct cores were visible in some grains and
were avoided where possible.
Analytical data for two abraded single grains and one fraction comprising two small grains are 0.3 to
3.2% discordant and yield overlapping 207 Pb/ 206 Pb ages between 2668.3 and 2670.1 Ma (Table A1). The
strongly collinear nature of the data enable a regression line to be fit through the origin that provides an
upper intercept age of 2669.6±1.4 Ma (71% probability of fit; MSWD = 0.34; Figure 7C). Two other
fractions, consisting of a single, colourless stubby prism (A2) and two small short brown prisms (A4) are
only slightly discordant (0.6%), but have distinctly higher 207 Pb/ 206 Pb ratios than the previous fractions.
Their corresponding ages, at 2694 and 2676 Ma, respectively, suggest minor amounts of inheritance—
possibly from Porcupine and even earliest Timiskaming sources—although the ages should be regarded
as strictly minimum estimates.
The 2669.6±1.4 Ma age for the early Timiskaming assemblage alkaline volcanic rock from Gauthier
Township is younger than any previous depositional estimate based on the youngest measured detrital
zircons from the Timiskaming Group in the Timmins, Kirkland Lake or Larder Lake areas (Corfu,
Jackson and Sutcliffe 1991; Ayer et al. 2003). However, this age is in accord with maximum age
estimates of Timiskaming assemblage sandstone and conglomerate deposition from the uppermost Three
Nations formation (