Abstract:
The Island Lake greenstone belt is one of the major
Archean supracrustal exposures in the northwestern part of
the Superior Province of the Canadian Shield. This belt is
subdivided into two units: 1) a lower sequence characterised
by pillowed to massive, locally pyroclastic, basalt to andesite
with a thin central zone of felsic derivatives, all of which
are interbedded with and overlain by thick sequences of
turbidite facies rock; 2) the upper unit which consists of
thick stratified conglomerate overlain by thickly bedded
arkose and feldspathic greywacke.
Reconnaissance sampling traverses were completed across
both the strike of the belt and along its margins with
adjacent granitoids.
Most of the belt is within the greenschist metamorphic
f acies with amphibolite facies occurring in certain areas
near t he margins. A post-tectonic, low pressure thermal event
may be responsible for the development of a unit of cordierite
schi s t which stretches southeastwards from the east end of
Cochrane Bay.
Volcanism is cyclical in nature changing from tholeiitic
to calc-alkaline. There is a general progression in the
character of the lavas from mafic t o felsic with stratigraphic
height. Chemica l d a ta sugges t that h i gh level fractionation
of a mantle- derived ' dry' magma i s t he s ource of the thole i iti c
lavas. Contamination of this magma with 'we t' sia l and
subsequent fractionation may be r esponsi b l e for the calcalkaline
phases .Observations of stratigraphic relationships (in particular
the contact between the supracrustals and the granitoids)
coupled with the metamorphic and chemical studies, allow the
construction of a preliminary model for the evolution of the
Island Lake greenstone belt. The following sequential development
is suggested: 1) a platform stage characterised by the
subaqueous effusion of mafic to intermediate lavas of alternating
tholeiitic and calc-alkaline affinities; 2) an edifice
stage marked by the eruption of felsic calc-alkaline rocks;
3) an erosional stage characterised by the deposit~on of thick
sequences of turbidite facies rocks; 4) the impingement of
granitic masses into the margins of the greenstone belt,
which was probably related to a downward warping of the supracrustal
pilei 5) the erosion of sialic massifs surrounding
and within the greenstone belt and of early supracrustal
piles, to give the clastic upper unit.
