Geochemistry and petrography of the ultramafic metavolcanic rocks in the eastern portion of the Shebandowan greenstone belt, Northwestern Ontario

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.