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

In contrast, the volcanic rocks of the Tisdale and Blake River assemblages (2710–2696 Ma) are
thickest and most extensive, have complete stratigraphic sections and are also locally truncated by the
PDDZ and LLCDZ in the central area (see Figure 2). This suggests that dip-slip displacement on the
regional faults changed with the central area of the greenstone belt experiencing greater subsidence during
this period. Although there are numerous sill-like mafic to ultramafic intrusions of this age within the
supracrustal assemblages, it is assumed that these were subvolcanic intrusions conformable with
stratigraphy and were unlikely to have caused major amounts of uplift. However, synvolcanic
granodiorite and tonalite ranging in age from 2700 to 2695 Ma have been found in the Nat River,
Kenogamissi and Round Lake batholiths and may be responsible for localized uplift and the age gaps.
Significant age gaps, which are interpreted as submarine unconformities, occur outside the central area:
between the Stoughton–Roquemaure and Tisdale assemblage in the northern part of the study area;
between the Kidd–Munro and Blake River assemblages in the Kamiskotia area; and between the
Stoughton–Roquemaure and Blake River assemblages east of the Round Lake batholith.
After 2696 Ma, the tectonic regime shifted from volcanic construction to one dominated by
deformation, plutonism and erosion accompanied by development of localized basins infilled by
sedimentary and volcanic rocks. The Porcupine assemblage was deposited with angular unconformity on
older volcanic assemblages from about 2690 to 2685 Ma. Porcupine assemblage volcanic centres
occurred at both Timmins and in the northeastern part of the Shining Tree area, regions which are also
marked by extensive porphyry intrusions of similar adakitic chemistry and age to the volcanic rocks. The
volcanic accumulations grade upwards and laterally into turbiditic sediments and localized iron formation
deposited below wave base in east-west trending basins typically bound by ductile deformation zones
postdating deposition. The early phases of syntectonic plutonism ranges from approximately 2695 to
2685 Ma, during which time, an early episode of regional D1 deformation predated the Porcupine angular
unconformity at 2690 Ma and locally (in the Swayze area) resulted in F1 folds with well-developed axial
planar cleavage (Heather 2001; Becker and Benn 2003).
SHRIMP and TIMS geochronological investigations on the Porcupine wackes in the Timmins area
indicate that zircons were derived from a terrane with similar ages to the underlying volcanic and plutonic
groups (see Figs 4F, 5A, 5D and 6A). Based on the larger zircon populations analyzed by the SHRIMP,
the Hoyle formation sample in Hoyle Township exhibits a major narrow peak at 2695 Ma and a minor
second one at 2750 Ma (see Figure 11B). The Beatty formation sample, immediately underlying the
Timiskaming unconformity in Tisdale Township, has a broader single peak at 2698 Ma (see Figure 11A),
whereas the Whitney Township sample, south of the PDDZ, has a narrow peak at 2693 Ma and a second
smaller one at 2730 Ma (see Figure 11C). These data suggest that while the Porcupine formations in the
Timmins area had the majority of their zircons supplied from the syntectonic intrusive group and/or the
Blake River assemblage, there are some differences in the secondary zircon supply. This ranges from a
minor but distinguishable secondary supply of Pacaud-age zircons in the Hoyle formation in Hoyle
Township; significant secondary supply of Deloro-age zircons in the Whitney formation, south of the
PDDZ; and predominantly syntectonic intrusive group and/or Blake River-age zircons for the Beatty
formation in Tisdale Township.
The Duparquet, Lac Caste, Kewagama, Cadillac and the Pontiac groups in Quebec have similar
maximum depositional ages (Davis 2002) to the Porcupine assemblage units in Ontario and, thus, are
thought to be correlative. However, basins hosting the Duparquet Group seem to be most similar to the
Porcupine formations in the Timmins area and the Porcupine unit south of Larder Lake, in that their
provenance appears to be only from sources within the Abitibi Subprovince. The other groups in Quebec
are similar to the Scapa sediments in that they all contain pre-Abitibi zircons (i.e., >2.8 Ga) (Ayer,
Amelin et al. 2002; Davis 2002). The Scapa sediments are a very widespread unit, extending northwest of
the study area (see Figure 2), and over 150 km to the east into the Quebec portion of the Abitibi
Subprovince. The presence of zircons from older more distal terranes, in conjunction with D1 folding and
85