Spatial and temporal variations in the geochemistry of komatiites and komatiitic basalts in the Abitibi greenstone belt
Introduction
Komatiites are ultramafic volcanic rocks that occur mainly in Archean greenstone belts (Arndt and Nesbitt, 1982). They are interpreted to have been derived by high degree partial melting of mantle plumes (Campbell and Griffiths, 1990), to have erupted very rapidly (Huppert and Sparks, 1985a, Huppert and Sparks, 1985b, Barnes et al., 1988, Williams et al., 1998), and to have flowed long distances and covered large areas (e.g. Lesher et al., 1984, Hill et al., 1995). As such, they provide important constraints on the composition and thermal structure of the Archean mantle and the stratigraphy and tectonic setting of Archean greenstone belts.
Komatiites have been studied worldwide, but only a few studies have explored temporal variations in komatiite geochemistry within a single greenstone belt (e.g. Xie et al., 1993). The Abitibi greenstone belt of the Superior Province of Canada contains some of the greatest abundances of well preserved komatiitic rocks in the world and there are ∼200 U–Pb radiometric age determinations available to constrain eruption ages (e.g. Davis et al., 1994, Bleeker and Parrish, 1996, Ayer et al., 1998, Ayer et al., 1999, Heather, 1998), so it is an ideal location to evaluate spatial and temporal variations in komatiite geochemistry and constrain the petrogenesis and tectonic setting of an Archean greenstone belt.
To achieve these aims, we have assembled a database of 2185 major and trace element geochemical analyses of komatiites and komatiitic basalts in the Abitibi greenstone belt from the literature (n=1192: e.g. Eakins, 1972, Fleet and MacRae, 1975, Imreh, 1975, Arndt, 1975, Arndt, 1986, Nesbitt and Sun, 1976, Coad, 1976, Gélinas et al., 1977, Arndt and Nesbitt, 1982, Ludden and Gélinas, 1982, Barnes et al., 1983, Cattell and Arndt, 1987, Johnstone, 1987, Xie et al., 1993, Lahaye et al., 1995, Lahaye and Arndt, 1996, Larson, 1996, Davis, 1997, Dostal and Mueller, 1997, Fan and Kerrich, 1997, Lesher et al., 1997, Baird, 1999, Barrie, 1999, Stone and Stone, 2000), provincial and federal government databases (n=319; e.g. Howe et al., 1987, Heather and Shore, 1999), an unpublished database compiled for Outokumpu Ltd. (n=625; Stone and Lesher, 1995), and a suite of samples collected specifically for this study (n=49). These data, when evaluated within the context of regional geological and geochronological constraints, allow us to establish spatial and temporal variations in komatiite geochemistry, to develop a model for the petrogenesis of the komatiites, and to place constraints on the tectonic development of the Abitibi greenstone belt.
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Geologic setting
The Superior Province is composed of plutonic, volcano-sedimentary, gneissic, sedimentary, and high-grade gneiss subprovinces (Thurston, 1991) that range in age between 3.1 and 2.7 Ga (Card, 1990). The volcanic and sedimentary rocks in the Abitibi greenstone belt have been metamorphosed, however, volcanic structures and igneous textures are commonly well preserved, so igneous terminology will be used to describe metamorphosed rocks for which protoliths can be unambiguously identified.
The ∼2.7
Komatiite geochemistry
The geochemistry of komatiites is controlled by (1) the compositions of the source and residue, (2) the pressure, temperature, and degree of partial melting, (3) the type of melting (e.g. equilibrium vs. fractional vs. dynamic), (4) the nature and degree of crustal contamination, (5) the degree of fractional crystallization and/or accumulation, and (6) the degree of remobilization of elements via alteration, metamorphism, and/or metasomatism (e.g. Nesbitt et al., 1979, Beswick, 1982, Arndt and
Methodology
All samples utilized in this study were carefully selected to avoid alteration. Komatiites exhibit a wide range of textures and volcanic facies, including massive aphyric, random and platy spinifex-textures, as well as crescumulate, orthocumulate, mesocumulate, and adcumulate komatiites. Random spinifex-textured rocks are considered to be closest to the lava composition. Flow tops were rarely analyzed, as they tend to act as loci for alteration and/or deformation. Hyaloclastites are poorly
Filtering for alteration and detection limits
All komatiites in the Abitibi greenstone belt have experienced varying degrees of hydration and carbonation due to seafloor alteration, hydrothermal alteration, and regional metamorphism (e.g. Beswick, 1982, Arndt, 1986, Arndt, 1994, Arndt et al., 1989, Lahaye et al., 1995, Lahaye and Arndt, 1996). The metamorphic grade in the belt is typically lower to upper greenschist facies, rarely reaching lower amphibolite grade along the margins of batholiths. Typical greenschist facies mineral
Source composition and depth of melting
The high MgO contents (up to 30%), very low highly-incompatible lithophile element contents, and uniformly low moderately incompatible lithophile element contents of non-cumulate Al-undepleted komatiites in the Abitibi greenstone belt suggest that they were generated by high degree partial melting of a depleted mantle source. Experimental data suggest that komatiites of this type separated from dunite [L+Ol] and harzburgite [L+Ol+Opx] residues. Although such residues can be stable throughout a
Spatial variations in the geochemistry of komatiites in the Abitibi greenstone belt
Al-undepleted komatiites are the most common komatiite type in the Abitibi greenstone belt and occur in all regions of the belt (Table 5). Al-depleted–Ti-enriched komatiites occur in most regions of the Abitibi greenstone belt, except in the Bartlett and Halliday Domes, the Destor Porcupine Fault Zone, and the Reaume–McCart area (Fig. 1). However, Ti-enriched–Al-depleted komatiites with high [Gd/Yb]MN ratios >1.2 are less common and appear to be restricted to the Round Lake Dome, Shining Tree
Temporal variations in the geochemistry of komatiites in the Abitibi greenstone belt
Ti-depleted komatiites in the Pacaud assemblage erupted between 2750 and 2735 Ma. It is possible that dynamic melting of a refractory component in a plume may have produced them, similar to the process proposed suggested for Gorgona picrites (Arndt et al., 1997b). However, in this case, we might expect to see other higher MgO components of the plume. These components have not been identified, but it is not clear whether they were not produced, whether they were produced but not erupted, or
Models for the evolution of plume-derived melts in the Abitibi greenstone belt
The temporal variations in komatiite geochemistry (Table 6) are consistent with successively shallower levels of melt extraction from the plume(s) during a ∼47 million year period. Earlier komatiites exhibit a high pressure garnet signature, whereas later komatiites do not. A number of models may be proposed depending on the number of plumes required. If a single plume is invoked for all of the komatiitic magmatism in the Abitibi greenstone belt, this would represent an extremely long-lived
Conclusions
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Komatiites in the 2750–2735 Ma Pacaud assemblage are exclusively Ti-depleted komatiites and appear to have been derived by (1) dynamic melting of a refractory source, similar to Gorgona picrites, or (2) high degree batch melting of a garnet-enriched source.
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Komatiites in the 2725–2720 Ma Stoughton–Roquemaure assemblage include Al-depleted–Ti-enriched komatiites and lesser Al-undepleted komatiites.
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Komatiites in the 2718–2710 Ma Kidd–Munro and 2710–2703 Ma Tisdale assemblages include Al-undepleted
Acknowledgments
This work has been supported at various stages by grants from Outokumpu Mines Ltd., the US National Science Foundation (EAR-9405994), the Ontario Geological Survey, and the Natural Sciences and Engineering Research Council of Canada (IRC 663-001/97 and OG 203171/98). The samples for the Outokumpu project were collected in collaboration with P.C. Davis, W.E. Stone, M.S. Larson, and A.M. Baird, with the very capable assistance of T. Schuster and M. Mainville. We are very grateful to them and to
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