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

ocks looking east. Note the geology at the surface. The low-density 3D shell is seen in light blue. The red
element is a shell of the 3D magnetic inversion, which closely corresponds spatially to the Clifford stock.
The magnetic body appears to be plunging to the south (right), but this needs to be viewed with the full
3D imaging system to see the full three-dimensional shape. Higher density elements identifying more
intermediate volcanic rocks surrounding the felsic volcanic rocks are seen as light green in the section.
Magnetic elements (red) to the south associate with these denser elements.
The detailed report on this work (Reed 2005a) is supplemented by a separate digital products (Reed
2005b) that contains binary files that can be opened using WinDisp 3DViewer to show the full 3D
movable images; bitmap images showing many freeze-frame views of the 3D inversions (similar to
Figures 31 to 34); and bitmap images of associated surface magnetic and gravity data and geology used in
the inversions; and georeferenced tiff images of the area geology.
Discussion of Greenstone Belt Development
The evolutionary history of the SAGB extends from 2750 to 2660 Ma, a span of 90 Ma, and includes
volcanism, sedimentation and plutonism (Figure 35). The U/Pb zircon data and mapping demonstrates a
belt-wide distribution of lithostratigraphic assemblages and supports coherent, upward facing,
autochthonous stratigraphy as previously suggested (e.g., Goodwin 1977; Pyke 1982; Jensen and
Langford 1985; Heather 1998; Ayer, Amelin et al. 2002). Zircon inheritance in the Abitibi greenstone belt
was first documented by Corfu (1993) and later by Heather (1998). Inherited zircons with ages similar to
those of underlying assemblages have been detected in all but the Pacaud assemblage and most of the
plutonic suites. Zircon inheritance is present in 20% to almost 40% of samples analyzed for this study
(Ayer, Amelin et al. 2002) with inherited zircons at least 10 my older than their crystallization age, 2 of
which are pre-Abitibi in age. This pattern of isotopic inheritance together with greater amounts of crustal
contamination in the younger komatiites (Sproule et al. 2002), and field relations (Muskasenda gabbro)
are all features consistent with autochthonous stratigraphic evolution, rather than allochthonous
juxtaposition of exotic terranes.
The oldest major stratigraphic unit in the SAGB is the 2750 to 2735 Ma Pacaud assemblage.
Although not much of the Pacaud assemblage currently crops out, or has been directly dated because of
its lack of zircon-bearing volcanic units, our new inheritance data in younger volcanic assemblages
suggests the Pacaud assemblage was considerably more widespread. The zircon data also support an age
gap of about 5 my with the overlying Deloro assemblage (2730–2724 Ma; see Figure 35) in those parts of
the Pacaud assemblage intruded by Ramsey–Algoma and the Kenogamissi batholiths in the west. A more
significant gap of about 20 my occurs with the overlying Stoughton–Roquemaure assemblage where the
Pacaud assemblage is intruded by the Round Lake batholith (see Figure 2). These age gaps and localized
chert breccia units observed in the Pacaud assemblage directly underlying the Deloro assemblage could
represent submarine unconformities as a result of crustal displacement caused by tonalitic plutons
intruding lower in the crust during and postdating deposition of the Pacaud assemblage rocks. Evidence
for this are the lenticular bodies and small screens of gneissic tonalite and trondhjemite ranging in age
from 2747 to 2740 Ma occurring as remnants on the margins, or within the larger batholithic complexes
such as the Ramsey–Algoma, Kenogamissi and Round Lake batholiths (Ketchum et al., in press). The
early volcanic assemblages uniformly face away and wrap around these batholiths. However, the original
nature of these early suites has been obscured by intrusion of extensive younger synvolcanic, syntectonic
and posttectonic suites.
Thus, the Pacaud and Deloro assemblages are considered to have been regional basal supracrustal
units within the SAGB. We are uncertain what may have originally underlain the Pacaud assemblage as
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