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

Intrusion Framework
The plutonic rocks of the SAGB fall into three broad groups or subdivisions: synvolcanic, syntectonic and
posttectonic intrusions (Chown et al. 1992; Chown, Harrap and Moukhsil 2002; Sutcliffe et al. 1993;
Heather 1998). Detailed geochronological studies of a number of the intrusive complexes indicate that
they formed over about 90 million years by multiple intrusive events (Frarey and Krogh 1986; Mortensen
1993a, 1993b; Heather 1998; Davis et al. 2000; Ketchum et al., in press). These subdivisions are being
applied to the major batholithic complexes for the first time in the study area. They are used in a broadscale
categorization to attempt to correlate with the much more extensive database on supracrustal rocks
with synvolcanic intrusion interpreted to be pre-D1, the syntectonic intrusion interpreted to be coeval with
the Porcupine and Timiskaming assemblages and the posttectonic intrusion interpreted to postdate the
Timiskaming assemblage. In Figure 2, we have subdivided the intrusions into 4 broad categories: 1) mafic
to ultramafic synvolcanic intrusions, 2) felsic to intermediate synvolcanic intrusions, 3) syntectonic
intrusions, and (4) late-tectonic intrusions.
INTERMEDIATE TO FELSIC SYNVOLCANIC INTRUSIONS
Intermediate to felsic synvolcanic intrusions range in age from about 2745 to 2696 Ma. They are termed
synvolcanic because they are coeval with the volcanic assemblages (i.e., Pacaud to Blake River). In
addition, this intrusive group also predates significant compressional strain in the southern Abitibi
Subprovince and they are geochemically similar to the calc-alkaline members of the coeval volcanic
assemblages. The synvolcanic intrusions are typically foliated and are tonalitic to granodioritic in
composition. They are found predominantly within the larger granitic complexes such as the Ramsey–
Algoma, Kenogamissi and Round Lake batholiths (see Figure 1), often as sheets or laccoliths (Heather
2001). In the synvolcanic complex in the Rice Lake area on the southwestern margin of the Kenogamissi
batholith, strongly foliated tonalite occurs around the margins and surrounding a supracrustal enclave in
the central part of the batholith. The tonalite has crystallization ages of 2747 to 2742 Ma and contains
inherited zircons dated at ca. 2.85 Ga (Becker and Benn 2003; Ketchum et al., in press). Here, the tonalite
is intruded by weakly foliated granodiorite with an age of 2700 Ma, but also containing 2.85 Ga
xenocrysts. Foliated synvolcanic tonalite and granodiorite with ages of 2713+2/–3 Ma and 2696±3 Ma,
respectively (Heather 2001), occur in the central part of the Kenogamissi batholith and extend into the
southwestern part of the study area (see Figure 2). The oldest units within the Round Lake batholith are
gneissic to foliated tonalite with recently determined ages of 2743.6±0.9 Ma and 2713.2±1.2 Ma
(Ketchum et al., in press). These tonalitic phases of the Round Lake batholith are cut by discrete
granodiorite plutons, with ages of 2697 Ma and locally with 2715 Ma zircon xenocrysts, in the northern,
western and eastern part of the batholith (Chown, Harrap and Moukhsil 2002). Synvolcanic intrusions
within greenstones within the study area include a 2699±2.9 Ma quartz diorite on the southwestern
margin of the Watabeag batholith (Frarey and Krogh 1986) and 2695±1.5 Ma tonalite intruding the
Kamiskotia gabbroic complex, in which 2.93 Ga zircon xenocrysts were also found (Barrie and Davis
1990).
MAFIC TO ULTRAMAFIC SYNVOLCANIC INTRUSIONS
Mafic to ultramafic synvolcanic intrusions range from approximately 2740 to 2700 Ma in the SAGB
(Corfu 1993; Heather 1998; Mortensen 1993a, 1993b). They mainly occur as sills or lenticular units that
cut stratigraphy at a low angle. They are compositionally fractionated, ranging from peridotite to gabbro
and diorite, commonly with igneous layering and magma-mixing textures and vary from massive to
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