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

clast conglomerates at the top of the underlying Deloro assemblage, which, in conjunction with an age
gap of about 15 my, indicates the presence of a widespread submarine unconformity (see “Deloro
Assemblage”). Felsic to intermediate volcanic rocks intercalated with ultramafic volcanic rocks and iron
formation occupy the lowermost part of the assemblage exposed around the Shaw Dome, where Houlé
and Guilmette (2004) have identified it as “Package B” (see “Shaw Dome Area”), and south of the study
area where felsic volcanic rocks in the core of the Halliday anticline have an age of 2710.5±1.6 Ma and
are surrounded by mafic volcanic rocks and komatiite with an age of 2707.8±1.2 Ma (Ayer, Ketchum and
Trowell 2002). The Adam iron mine is also interpreted to occur in the lower Tisdale assemblage as
indicated by an age of 2712±4 Ma from heterolithic volcanic rocks intimately intercalated with komatiite
in the footwall of the iron formation, which are part of the Larder Lake group, south of Kirkland Lake
(Ayer, Amelin et al. 2002).
SHRIMP analyses of zircons from a felsic volcanic unit with an age of 2708.2±2.1 Ma (Barrie and
Corfu 1999) from the southeastern margin of the Shaw Dome in the “Package C” sequence indicates that
both Pacaud- and Deloro-age zircon xenocrysts (Figure 14) are present in this sample (#25: Figure 2
(sample 96TB-079). This implies that, at the time of deposition of this unit, both the Deloro and the
Pacaud assemblages underlay the Tisdale assemblage, thus, indicating that the Pacaud assemblage was a
more widespread basal assemblage in the SAGB than its present outcrop distribution indicates (see
Figures 1 and 2).
Figure 14. Backscattered electron images of the cores of inherited zircon and a zircon overgrowth from lower Tisdale
assemblage volcanic sample 96TB-079. The spot locations, numbers and ages correspond with the SHRIMP II analyses in
Appendix 2 (Table A2).
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