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

SHRIMP U/Pb dating was carried out on detrital zircons from Porcupine assemblage units from
three localities. Sample 04JAA-0003 represents a fine-grained wacke of the Beatty formation occurring
immediately below an angular unconformity with overlying Timiskaming assemblage sandstones in
eastern Tisdale Township. Conventional TIMS analysis of six air-abraded single detrital zircon grains
from this sample demonstrated the presence of a range of ages spanning ca. 2687 to 2726 Ma, with the
youngest, reliable near-concordant analysis suggesting that sedimentary deposition postdated 2687.2±1.6
Ma (2σ). The sample comprises a variety of zircon grain shapes and sizes, from 4-5:1 elongate prisms to
equant crystals, with slightly subrounded varieties representing approximately 20 to 25% of the entire
population, the rest being sharply faceted. SEM imaging of approximately 125 zircons reveals a variety
of concentric, oscillatory zoned and straight- or weakly zoned igneous grain types. SHRIMP analyses for
54 zircons from this sample are shown in Appendix 2 and presented graphically in Figures 11A-1 and
11A-2. Most of the reliable concordant or near concordant ion probe spot age determinations cluster
between ca. 2680 and 2750 Ma, with no consistent correlation between grain morphology or degree of
rounding, except that sedimentary abrasion is not evident in the very youngest detrital grains analyzed.
The slightly skewed but essentially unimodal age distribution has a relative probability peak near 2700
Ma. The SHRIMP data reveal that a slightly older age population is probably present in this sample than
was suggested by the TIMS results. Because of the comparatively larger SHRIMP analytical errors,
particularly associated with analyses of low-U grains or domains within grains, the significance of this
should be regarded with some caution. The ion probe data also suggest that the depositional age for this
sample might be near 2680 Ma, though the most robust and precise estimate remains the 120) from this sample shows simple short prismatic and equant crystal
forms dominated by magmatic, oscillatory internal zoning and minor subrounding of exterior prism faces.
Initial, in situ SHRIMP analysis was carried out on 19grains and the data are presented in Appendix 2 and
shown graphically in Figures 11B-1 and 11B-2. The data show a distinct clustering on or near concordia,
ranging mostly between ca. 2680 and 2705 Ma, and have a prominent age mode at ca. 2690 Ma. Although
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