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

Stratigraphic Framework
In this section of the report, we present a revised stratigraphic framework for the study area concentrating
upon changes in our understanding of Abitibi greenstone belt stratigraphy based on 1) new
geochronologic results reported above; 2) stratigraphic mapping within the subprojects summarized
below; and 3) reconnaissance mapping by the principal investigators (e.g., Ayer, Thurston et al. 2004).
The lithostratigraphic assemblage age ranges, contact relationships, dominant rock types and chemical
affinities as modified after Ayer, Amelin et al. (2002) are presented in Table 2. Figure 2 (see back
pocket) provides a map of the interpretation of assemblages, intrusions, structures and all U/Pb ages
within the study area based upon the 1:250 000 lithological compilation map of the central Abitibi
greenstone belt in Ontario (Ayer, Berger et al. 2005).
ASSEMBLAGE CONTACT RELATIONSHIPS AND THE SIGNIFICANCE
OF IRON FORMATION CONGLOMERATE AND CHERT BRECCIA
UNITS
Crystallization ages of lithotectonic assemblages within representative greenstone belts in the North
Caribou Terrane of northwestern Ontario are separated by intervals of up to approximately 100 million
years (my) (Williams, Stott and Thurston 1992), whereas 3 to 10 my intervals in greenstone belts of the
Pilbara craton (e.g., Hickman and Van Kranendonk 2004) and the western Abitibi greenstone belt (Ayer,
Amelin et al. 2002) are more common. These differing durations for accumulation of lithotectonic
assemblages bear upon models for greenstone belt development. Contacts between lithotectonic
assemblages may be tectonic, intrusive, or unconformable (Williams, Stott and Thurston 1992). Contacts
between “Timiskaming-style” assemblages in Superior Province greenstone belts are subaerial
unconformities marked by development of regoliths or paleosols e.g. (Jackson and Fyon 1991). However,
in Superior Province greenstone belts, contacts between “Keewatin” assemblages have, until recently,
been assumed to be tectonic (e.g., Williams, Stott and Thurston 1992). However, the recent recognition of
isotopic inheritance in some Superior Province greenstone belts (Ayer, Amelin et al. 2002; Thurston 2002
and references therein) and the younging of assemblage ages away from batholiths throughout the
Superior Province (Thurston 2002) requires re-examination of the character of assemblage contacts for
the “Keewatin-style” assemblages.
Shanmugam (1988) summarized the features of subaerial and submarine unconformities (Table 3).
It is useful to note that the features of submarine and subaerial unconformities are not mutually exclusive.
In deformed Precambrian orogens, such as the Pilbara craton (Blake 2001) and the Kaapvaal craton
(Cheney and Winter 1995), subtle discordances of dip and regional scale truncations mark subaerial
unconformities. Similar relationships can develop in submarine settings, however, the dominantly
volcanic environment represented by the Abitibi greenstone belt has lens-like facies distribution (Mueller
1991; Bleeker 1999; Mueller and Mortensen 2002); therefore, regional-scale marker horizons used to map
dip discordances and large-scale subtle, low-angle truncations are difficult to document. Features
indicative of submarine unconformities are a function of a model of basin development in which basins
have continuous inputs of detritus from the basin margins, resulting in continuous development of
stratigraphy over time. Horizons such as glauconitic sands or manganese nodules represent intervals of
non-deposition with or without weathering. Similarly, debris flows and slump deposits in the stratigraphy
of a given basin represent an interval of non-deposition in the central part of the basin which is taken up
by slump deposits from the basin margin. In the normal deformed metamorphosed Archean greenstone
belts, only debris flows and slump deposits have preservation potential.
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