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

The larger batholithic complexes which are external to the supracrustal rocks such as the Kenogamissi
and Round Lake batholiths contain the older members of the felsic to intermediate synvolcanic intrusion
group (ca. 2745–2710 Ma). These batholiths have exerted control on the preservation of early stratigraphy
as the volcanic assemblages uniformly wrap around and young away from their margins (see Figures 1 and
2). Thus, at the scale of the greenstone belt, the batholiths represent centres of structural domes and the
intervening areas define belt-scale synclinoria such as the Blake River synclinorium.
The notion of a belt-scale synclinorium cored by the Blake River assemblage has a long history
going back at least to the work of Jensen and Langford (1985). Benn and Peschler (2005) have recently
proposed a detachment fold model for the development of the synclinorium during a single, belt-wide
folding event that deformed the whole region. The model is based upon structural analysis and general
interpretation of the previous generation of reflection seismic surveys. In our work, we are able to
postulate a more detailed version of the belt-scale folding showing deformation was protracted and
occurred in a number of distinct intervals based on geochronologic, stratigraphic, potential field
geophysical data and high-resolution reflection seismic surveys.
This pattern is interrupted by the trends of Porcupine and Timiskaming assemblage rocks which
unconformably overlie the older assemblages. These large-scale folds are visible in the reflection seismic
profiles beneath the Porcupine assemblage on the Crawchest seismic line. Younger members of the felsic
to intermediate synvolcanic intrusion group (2710–2696 Ma) appear to be relatively minor in extent and
occur within both the supracrustal belt and the batholiths. The mafic to ultramafic synvolcanic intrusions
have a pronounced affinity for the supracrustal belt indicating that they are most likely sills and/or upper
crustal magma chambers coeval with volcanism.
The older group of the syntectonic intrusions (2695–2685 Ma) may be related to the compressive
stresses that induced early folding and faulting related to the onset of continental collision between the
Abitibi and older subprovinces to the north. These intrusions occur within the external batholiths, and as
smaller batholiths, stocks and dikes internal to the supracrustal belt such as the Lake Abitibi and
Watabeag batholiths, the Adams and Clifford stocks, and numerous porphyry intrusions and dikes.
Regional deformation episodes have been identified based on overprinting relationships of folds and
faults with D1 constrained by cessation of Blake River assemblage volcanism at 2696 Ma and onset of the
deposition of the Porcupine assemblage at 2690 Ma. D1 folds are commonly refolded and transposed by
later deformation and, therefore, it is often difficult to determine the original orientation of D1 structures.
Within the study area, D1 folds do not appear to have any associated fabric and, thus, regional-scale folds
of this generation have not been specifically identified. However, where F1 fold trends are discernible in
other parts of the SAGB, they typically have a northerly orientation, such as the broad anticline cored by
the Kenogamissi batholith (Ames et al. 1997) and northerly trending F1 folds with well-developed axial
planar cleavages observed in outcrops in the Swayze belt (Heather 2001; Becker and Benn 2003).
Note, on Figure 2, specific fold generations have only been assigned in the Timmins and Kirkland-
Larder Lakes areas, where the detailed structural studies associated with this project have been
undertaken, and that the first (1 st ), second (2 nd ) and third (3 rd ) generation folds correspond with D2, D3
and D4, respectively, in the Timmins area. Folds in the remaining parts of the study area have not been
ascribed to specific generations because of a lack of detailed structural mapping. In the Timmins area, the
base of the Porcupine assemblage is interpreted to be a low-angle unconformity that cuts progressively
lower into the Tisdale assemblage stratigraphy eastward along the north limb of the Porcupine syncline
(Bateman et al. 2005). This is likely the result of uplift and erosion and suggests the unconformity was
caused by east-west oriented compression and/or extension related to D1 folding and the emplacement of
syntectonic intrusions such as the broad northerly trending F1 antiform coring the Kenogamissi batholith
(Ames et al. 1997).
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