Abstract:
330 km 2 of the easter-n part of the Archean Manitou Lakes -
Stormy Lake metavolcanic - metasedimentary belt have been mapped and
sampled. A large number of rocks ~.vere analyzed for the major and trace
constituents including the rare-earth elements (REE).
The Stormy Lake - Kawashegamuk Lake area may be subdivided
into four major lithological groups of supracrustal rocks
1) A north-facing mafic assemblage, consisting of pillowed tholeiitic
basalts and gabbro sills characterized by flat REE profiles, is exposed
in the south part of the map area and belongs to a 8000 m thick
homoclinal assemblage outside the map area. Felsic pyroclastic rocks
believed to have been issued from a large central vent conformably
overlie the tholeiites.
2) A dominantly epiclastic group facing to the north consists of terrestrial
deposits interpreted to be an alluvial fan deposit ; a submarine
facies is represented by turbiditic sediments.
3) The northeastern part of the study area consists of volcanic rocks
belonging to two mafic - felsic cycles facing to the southuest ; andesitic
flows with fractionated REE patterns make up a large part of the
upper cycle, whereas the lower cycle has a stronger chemical polarity
being represented by tholeiitic flows, with flat REE, which a r e succeeded
by dacitic and rhyolitic pyroclasti cs.
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4) A thick monotonous succession of tholeiitic pillmled basalt f lows
and gabbro sills with flat REE represent the youngest supracrustal rocks.
TIle entire belt underwent folding, faulting and granitic plutonism
during a tectono-thermal event around 2700 Ma ago. Rocks exposed
in the map area were subjected to regional greenschist facies metamorphism,
but higher metamorphic grades are present near late granitic
intrusions.
Geochemical studies have been useful in 1) distinguishing
the various rock units ; 2) relating volcanic and intrusive rocks
3) studying the significance of chemical changes due to post magmatic
processes 4) determining the petrogenesis of the major volcanic rock
types. In doing so, two major volcanic suites have been recognized :
a) a tholeiitic suite, mostly represented by mafic rocks, was derived
from partial melting of upper mantle material depleted in Ti, K and the
light REE ; b) a calc-alkalic suite which evolved from partial melting
of amphibolite in the lower crust. The more differentiated magma types
have been produced by a multistage process involving partial melting
and fractional crystallization to yield a continuum of compos i t i ons
ranging from basaltic andesite to rhyolite.
A model for the development of the eastern part of the Manitou
Lakes - Stormy Lake belt has been proposed.
