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

the evidence for pre-Pacaud strata is only available indirectly as isolated inherited zircons, which, to date,
have only been found in the westernmost part of the SAGB (along with rare pre-Abitibi zircons found in
plutonic samples), and isolated Nd and Hf isotopic evidence for localized contamination by older crustal
sources (Ketchum et al. in press). The Pacaud assemblage may have been underlain by back-arc basin- or
oceanic-mantle, which was subsequently subducted into the Archean mantle, as Nd isotopes from
volcanic samples of all ages predominantly suggest magma derived from juvenile undepleted sources
(Ayer, Amelin et al. 2002; Ayer, Ketchum and Trowell 2002; Ketchum et al., in press).
The Stoughton–Roquemaure and Kidd–Munro assemblages (2724–2710 Ma) are only found north of
the PDDZ and south of the LLCDZ (Kidd–Munro assemblage also found in the Shining Tree and south
Swayze areas), but no volcanism nor any inherited zircons of this age are present in the central area in the
time interval between the Deloro and Tisdale assemblages. Truncation of these volcanic units by the
deformation zones implies that horst and graben style synvolcanic faulting predated ductile deformation
with uplift of the central area resulting in non-deposition in this area from 2724 to 2710 Ma, whereas
voluminous volcanism (predominantly rift-related mafic and ultramafic in composition) occurs in the
areas north and south of these postulated early faults. Although granitoids in the SAGB have not been
extensively dated, tonalitic synvolcanic intrusions with ages of 2723 to 2713 Ma have been found in the
Kenogamissi and Round Lake batholiths. This suggests this area may have been uplifted by intrusion of
granitic rocks underlying the central area, which may have been partially responsible for the age gap.
Figure 35. Timeline for the evolution of southern Abitibi greenstone belt including the volcanic and sedimentary assemblages,
intrusions, deformation, metamorphism and the main mineralization episodes (modified from Bateman et al. 2005, p.3).
Geochronological data are from numerous sources including the new results in this report (Corfu 1993; Mortensen 1993a, 1993b;
Powell, Carmichael and Hodgson 1995; Bleeker, Parrish and Sager-Kinsman 1999; Davis et al. 2000; Heather 2001; Ayer, Amelin et
al. 2002; Ayer, Ketchum and Trowell 2002; Ropchan et al. 2002; Ayer, Barr et al. 2003; Dubé et al. 2004; Ketchum et al., in press).
84