Program, Abstracts, and Guidebooks - University of Minnesota Duluth

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A MODEL FOR TECTONIC VARIATION OF 'GRM4ITIC TERRAIN'
IN SOUTHEASTERN MANITOBA
I. F. ERMANOVICS
Geological Survey of Canada, Ottawa
A B ST RA CT
The Precambrian rocks of the Superior (Structural) Province
of southeastern Manitoba, between latitudes 51 and 54 degrees fall into
three groups: metavolcanic—sedimentary rocks (domain I); an adjacent,
hybrid mobile zone (domain II) and a siliceous (sialic) nucleus (domain III).
Domain III, situated between 510 15' and 30' N, comprises
augen-gneiss and weakly layered to stratiform layered gnefss (SO per cent
of the domain) whose compositions range from quartz monzonite to gran—
odiorite; mafic hornblende gneiss and amphibolite are abdundant locally.
Siliceous mafic—noor quartz monzonite to granodiorite intrude these
gneisses and the magnetite content of the massive rocks is correlatable
to regional magnetic 'highs'. Metavolcanic—sedimentary rocks (3 per cent
of domain III) and mafic granodioritic gneiss occupy relict keels of
folds; 'down—plunge' views of such structures show that these remnants
are underlain by siliceous gneiss and massive rocks peculiar to rocks
of domain III.
Rocks of domain II, flanking belts of uietavolcanic—sedimentary
rocks, consist of high—grade aluminoüs and inafic gneiss intruded by
diapiric mafic granodiorite to quartz gabbro; large bodies of quartz
monzonite are absent from this domain. The coarse—grained igneous rocks
may be the intrusive equivalents (cogenetic magtnas) of the lavas of
domain I and both domains constitute the total volcanic—sedimentary
tectogene.
A seismic !break!, located along the lithologic boundary between
domains II and III, indicates displacement of the Conrad discontinuity
downward beneath domains I and II with respect to domain III. Thus if
the seismic break is a fault (albeit annealed by later intrusions) and
if the volcanic—sedimentary tectogene is underlain by rocks of domain III,
then the sialic nucleus (domain iii) is exposed by virtue of erosion.
It is concluded that the volcanic—sedimentary rocks were
deposited upon a sialic (relatively siliceous) basement which is now
represented by !granitic gneiss'.