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

Corfu, Jackson and Sutcliffe (1991) sampled a number of the non-marine association facies
sedimentary rocks in the Kirkland Lake and Larder Lake areas and determined their maximum age of
deposition to be about 2679 Ma. More recent geochronology from a turbidite sample interbedded with
heterolithic conglomerate beds and minor oxide-iron formation from McVittie Township, east of the town
of Larder Lake, yielded detrital zircon ages of 2674.3±3.7 Ma and 2684.9±1.9 Ma (Ayer, Ketchum and
Trowell 2002) indicating that at least this part of the re-sedimented association belongs to the
Timiskaming assemblage as was originally interpreted by Hyde (1980). New geochronology of a wacke
sample from Gauthier Township yielded detrital zircons ranging from the youngest at 2677.7±3.1 Ma, to
2682 Ma and 2770 Ma (#32: Table 1, Figure 7D)and a sample of trachyte flow from this area yielded an
even younger zircon age of 2669.6±1.4 Ma with inheritance zircons having ages of 2676.6±4.3 Ma and
2694.2±2.8 Ma (#31: Table 1, Figure 7C). These ages indicate that the upper part of the Timiskaming
assemblage in this area is as young as 2670 Ma, similar to the age of the Three Nations formation east of
Timmins (see above).
Matachewan Area
A unit of conglomerate and cross-bedded sandstone, locally associated with turbiditic sedimentary rocks,
extends west from Cairo Township to south of Hutt Township (see Figure 2). A number of geochronology
samples were analyzed from this unit in order to determine its age and provenance (Ayer, Ketchum and
Trowell 2002). The youngest detrital zircon from a sandstone in Powell Township has an age of
2685.8±1.5 Ma, as well as older detrital zircons with ages of 2692.3±2.7 Ma, 2695.2±2.4 Ma and
2757±12 Ma. A second sample was collected from fine-grained wacke interbedded with conglomerate
south of Montrose Township. The youngest zircon indicates a maximum deposition age of 2688.5±1.0
Ma, whereas a number of older detrital zircons range from 2690 to 2723 Ma. The youngest detrital
zircons, from sandstone interbedded with conglomerate south of Hutt Township, indicate a maximum
depositional age of 2690±3.0 Ma also with detrital zircons with ages of 2706 and 2715 Ma (Ayer, Amelin
et al. 2002). The above U/Pb zircon data indicate that these sedimentary units were deposited sometime
after 2685.8±1.5 Ma and that only Abitibi-age zircons have been detected to date. Thus, they could
belong to either the Porcupine or the Timiskaming assemblages given the time limits established for these
assemblages as discussed above. However, because of the facies association of localized fluvial
conglomerates and cross-bedded sandstones and the strong spatial association with the Larder Lake–
Cadillac deformation zone (LLCDZ), the unit is still considered to be part of the Timiskaming
assemblage.
In contrast with the Porcupine, the Timiskaming assemblage samples (see below) commonly contain
detrital zircons with pre-Abitibi ages (i.e., 2860–2790 Ma). These ages indicate a more widespread
provenance that included source terrains older than the Abitibi. These sources could have included
detritus from the Opatica Subprovince to the north, or unknown older terrains to the south or east which
would have been subsequently removed by collision with the Grenville Province during the Proterozoic.
However, new evidence of pre-Abitibi U/Pb ages of zircon xenocrysts, and ancient crustally contaminated
Nd and Hf isotopes (Ketchum et al., in press) in some volcanic and plutonic samples only found to date in
the westernmost part of the SAGB, suggests that pre-Abitibi crust, similar in age to the lowermost part of
the Wawa supracrustal sequences, may have underlain the AGB in a wedge extending up to 80 km east of
the Kapuskasing Structural Zone (Ketchum et al., in press). This suggests the strong possibility that the
Wawa Subprovince may have contributed at least some of this pre-Abitibi detritus to the Timiskaming.
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