| dc.description.abstract | The 2.7 Ga Shebandowan greenstone belt in the Wawa-Abitibi terrane
contains unusual ultramafic rocks. The two main assemblages present within the
study area are the Greenwater and Shebandowan assemblages. The 2719.7 ± 1.0 Ma
Greenwater assemblage is characterized by tholeiitic magmatism whereas the 2690-
2680 Ma Shebandowan assemblage is characterized by calc-alkalic magmatism.
Mapping of a 16 km2 area and 7 trenches has identified the following lithologies in
the field area: orthocumulate ultramafic rocks, komatiites, pyroxenite, vesicular
komatiites, serpentinite, ultramafic breccia, variolitic ultramafic rocks, mafic
intrusive rocks, intermediate intrusive and extrusive rocks, felsic volcaniclastic
rocks, monzonite, conglomerate, banded iron formation, argillite and chert.
The ultramafic rocks are dark black in colour, but light green on surface as a
product of serpentine alteration of olivine. They are highly magnetic, with magnetic
susceptibility readings from 20 to 150 x10-3 SI. In some areas they occur as finegrained,
massive rocks with no distinct textures, and they also occur as mediumgrained
dark black rocks with a cumulate texture. The komatiites occur as finegrained,
light grey, highly silicified rocks. Several types of spinifex texture were
observed throughout the field area, including: thin chaotic spinifex, thick chaotic
spinifex, oriented spinifex, and pyroxene spinifex. Mafic intrusive rocks are massive,
medium-grained, equigranular grey-blue gabbro with weakly disseminated pyrite.
The intermediate intrusive rocks consist of a fine-grained, green-grey matrix with
hornblende phenocrysts and red-pink autoliths. The intermediate extrusive rocks
are fine-grained, light grey-blue metavolcanic rocks with medium-grained
plagioclase phenocrysts. Felsic rocks in the field area are very fine-grained, light
grey, siliceous rocks. They are massive with no flow textures observed in any of the
outcrops and the weathered surfaces range in colour from beige to blue-grey. A
monzonite dyke occurs alongside an interpreted fault through the field area. The
distinctly magnetic rock consists of a fine-grained, green-grey matrix with
hornblende phenocrysts and red-pink autoliths. The conglomerate is a heterolithic
pebble to boulder conglomerate consisting of a fine-grained matrix with clasts of basalt, monzonite, and jasper ranging in size from ~2 cm to 40 cm. The argillite is a
very fine-grained dark black mudstone. It typically showed no bedding and locally
contains graphite and abundant radial pyrite concretions up to 5cm in diameter. It is
seen in the trenches intercalated with the hypabyssal komatiitic intrusions.
The komatiitic rocks have SiO2 values of 47-67 wt. % and MgO values of 2-17
wt. %. Despite being outside the accepted values for komatiites, these rocks were
identified as komatiites based on spinifex texture and spatial association with other
ultramafic units. The abnormally high silica content in the ultramafic rocks was
determined to be caused by silica alteration, and was not a primary feature of the
rocks when they were emplaced. The rocks in the field area have undergone silica
and carbonate alteration as well as greenschist facies metamorphism. The
Al2O3/TiO2 ratios of the ultramafic rocks are 17-29, and the CaO/Al2O3 ratios of the
ultramafic rocks are 2-2.5. These ratios, combined with a flat pattern on a primitive
mantle normalized multi-element diagram, identify the rocks as aluminiumundepleted
komatiites. A distinct negative Nb anomaly is present in all samples,
which could be caused by either the melt being derived from a heterogeneous deepmantle
source, where perovskite fractionation depleted the Nb, or crustal
contamination. Sm-Nd isotope analysis was completed on six ultramafic samples
from the field area. The εNd of all samples ranged from +2.34 to +2.83, which is not
consistent with contamination by older continental crust. Consequently, melting of a
heterogeneous deep mantle source is the favoured model for petrogenesis of the
ultramafic rocks.
Based on the close spatial association of tholeiitic and calc-alkalic rocks as
well as the presence of thick sequences of deep-ocean argillites, the geological
environment was determined to be an oceanic-rifted arc through which a plume of
heterogeneous deep-mantle melts ascended, that subsequently closed and then
collided with the Superior Province. | en_US |
