Three repetitive sequences of northward youngIng, east striking, linear,
volcano-sedimentary units are found in the late Archaean BeardmoreGeraldton
greenstone belt, situated within the Wabigoon subprovince of
the Superior Province of northwestern Ontario. The volcanic components
are characterised by basaltic flows that are pillowed at the top and
underlain by variably deformed massive flows which may In part be
intrusive. Petrographic examination of the volcanic units indicates regional
metamorphism up to greenschist facies (T=3250 C - 4500 C, P=2kbars)
overprinted by a lower amphibolite facies thermal event (T=5750 C,
P=2kbars) confined to the south-eastern portion of the belt.
Chemical element results suggest olivine, plagioclase and pyroxene are
the main fractionating mineral phases. Mobility studies on the varIOUS
chemical elements indicate that K, Ca, Na and Sr are relatively mobile,
while P, Zr, Ti, Fet (total iron = Fe203) and Mg are relatively immobile.
Discriminant diagrams employing immobile element suggests that the
majority of the samples are of oceanic affinity with a minor proportion
displaying an island arc affinity.
Such a transitional tectonic setting IS also refle.cted in REE data where
two groups of volcanic samples are recognised. Oceanic tholeiites are LREE
depleted with [La/Sm] N = 0.65 and a relatively flat HREE profile with
[Sm/Yb] N = 1.2. Island arc type basalts (calc-alkaline) are LREE enriched,
with a [La/Sm] N = 1.6, and a relatively higher fractionated HREE profile
with [Sm/Yb] N = 1.9.
Petrogenetic modelling performed on oceanIC tholeiites suggests
derivation from a depleted spinel lherzolite source which undergoes 20%
partial melting. Island arc type basalts can be derived by 10% partial
melting of a hypothetical amphibolitised oceanic tholeiite source.
The majority of the volcanic rocks in the Beardmore-Geraldton Belt are
interpreted to represent fragments of oceanic crust trapped at a consuming
plate margin. Subsequent post accretionary intrusion of gabbroic rocks
(sensu lato) with calc-alkaline affinity is considered to result in the
apparent hybrid tectonic setting recognized for the BGB
