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

plutonism suggests the onset of regional compression and collision with an older Superior Province
craton to the north (Davis et al. 1995).
The D2 event postdates the Porcupine assemblage as it is deformed by it, but D2 structures are
unconformably truncated by, and thus predate, the Timiskaming. D2 resulted in localized folding and
thrusting and early south-side up, dip-slip, ductile deformation on regional deformation zones.
The Timiskaming assemblage ranges from 2676 to 2670 Ma and consists of conglomerate and
sandstone, locally with volcanic rocks. It was deposited with angular unconformity on the older volcanic
and sedimentary assemblages, typically in long linear basins with unconformable contacts along the north
margin and in faulted contact with the PDDZ and LLCDZ to the south. Facies studies indicate the basal
portions were deposited subaerially and include alluvial–fluvial environments, whereas the upper portions
are more typically deeper water sediments that include turbidites and iron-poor iron formation. The
supracrustal rocks are broadly coeval with small syntectonic alkalic intrusions, ranging from 2680 to 2672
Ma, also found in close proximity to the major deformation zones. The period during which the
Timiskaming assemblage formed is thought to represent extensional to transpressional tectonism
associated with continental island arc magmatism and sedimentation in which the supracrustal units were
unconformably deposited in subaerial alluvial fan and fluvial environments closely associated to regionalscale
faults (Ayer, Amelin et al. 2002).
Broadly synchronous with the syntectonic opening of Timiskaming basins in dilatational jogs, was
D3 folding, only the late stages of which affected the assemblage. Left-lateral strike-slip movement along
the PDDZ accompanied D3 and is interpreted to be associated with up to 13 km of sinistral offset of
markers across the PDDZ in the Timmins area.
The D4 folding created synclines within the Timiskaming assemblage rocks and right-lateral strikeslip
displacement along the PDDZ. This event may be correlative with oblique reverse-dextral slip along
the LLCDZ which has been identified as a post-Timiskaming “D2” event in the Kirkland Lake–Larder
Lake area (see “Timmins Subproject” and “Kirkland Lake–Larder Lake Subproject”). A late tectonic
magmatic event consists of Algoman granites and S-type granitic plutons, ranging in age from 2670 to
2660 Ma, postdates the Timiskaming assemblage. These plutons occur both within the batholiths and the
supracrustal rocks. They appear to be temporally associated with D4 folding (D2 deformation along the
LLCDZ), and dextral strike-slip displacement on the major deformation zones. The D4–D5 event
represents the final stage in transpressional deformation along the PDDZ in Timmins and are thought to
be similar to D3 and D4 documented in the Kirkland Lake–Larder Lake area. These deformation events
are associated with the generation of crenulation cleavages, minor folds and brittle faulting, but do not
appear to be associated with regional scale folding.
The irregular metamorphic patterns in the study area (see Figure 27) are attributed to superposition
of subgreenschist- and greenschist-grade regional metamorphism on narrow higher grade contact
metamorphic aureoles that formed at different times adjacent to the various felsic to intermediate
intrusions. The metamorphosed plutonic, volcanic and sedimentary rocks now at the surface reached
maximum pressures (depths of 8–10 km) and temperatures (350 to 450°C) during the main phase of
ductile deformation (D4 to D6) that occurred after deposition of the Timiskaming assemblage and are
estimated to have reached peak conditions in the range of 2665 to 2655 Ma (see Figure 30). Direct
evidence for the timing of metamorphism may be evident in some of the SHRIMP U/Pb zircon analyses
as indicated by ages of approximately 2650 Ma. A possible metamorphic zircon with a low Th/U ratio
from a conglomerate in Thorneloe Township yielded an age of 2618±2 Ma (see Figure 4C). SHRIMP
analyses of a number of monazite crystals with low Th/U ratios in the vicinity of the Dome Mine yielded
ages of 2640±5 Ma which could be hydrothermal or metamorphic (Ayer, Barr et al. 2003) and are
synchronous with the minimum age of 2643 Ma for regional metamorphism in the Noranda area (Powell,
86