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

BLAKE RIVER ASSEMBLAGE
Lower Part
The lower Blake River assemblage has an age range of 2704 to 2701 Ma. It was previously identified as
the Kinojevis assemblage in Ayer, Amelin et al. (2002). However, its difference in age from the Kinojevis
Group in Quebec that has an age of 2718±2 Ma (Zhang et al. 1993), which correlates with the Kidd–
Munro assemblage in Ontario, has led to confusion in Abitibi stratigraphic nomenclature and, thus, this
unit is herein identified as the lower part of the Blake River assemblage. This better fits its original
identification as the Garrison subgroup of the Blake River group by Goodwin (1979). The unit consists
predominantly of tholeiitic mafic volcanic rocks with isolated units of tholeiitic felsic volcanic rocks and
turbiditic sedimentary rocks.
Based on our new mapping and geochronology, we have also revised stratigraphy in the Kamiskotia
region (see “Kamiskotia Area Subprojects”) where the southern part of this area is now interpreted to be
underlain by the lower and upper parts of the Blake River assemblage (see Figure 2)—an area that was
previously assigned to the Tisdale assemblage (Ayer, Amelin et al. 2002). We now also interpret that the
lower Blake River assemblage underlies areas with poor exposure and limited geochronology in Mahaffy,
Macdiarmid and Geary townships (see Figure 2).
Figure 15. Cathodoluminescence images of inherited and magmatic zircon from upper Tisdale assemblage volcanic sample
98JAA-0011. The spot locations, numbers and ages correspond with the SHRIMP II analyses in Appendix 2 (Table A2).
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