Spatial and temporal variations in the geochemistry of komatiites and komatiitic basalts in the Abitibi greenstone belt

https://doi.org/10.1016/S0301-9268(02)00009-8Get rights and content

Abstract

Komatiites and komatiitic basalts in the Abitibi greenstone belt exhibit pronounced spatial and temporal variations in major and trace element geochemistry that provide important constraints on their petrogenesis and tectonic setting. Ti-depleted komatiites exhibit high Al2O3/TiO2 (25–35) and low [Gd/Yb]MN (∼0.6–0.8), suggesting derivation by (1) dynamic melting of a refractory source leaving a refractory harzburgite residue, similar to that which produced Gorgona picrites, or (2) relatively high degree melting of a garnet-rich source leaving a peridotite residue. Al-depleted–Ti-enriched komatiites exhibit low Al2O3/TiO2 (6–14) and high [Gd/Yb]MN (∼1.2–2.0), suggesting derivation by partial melting of a garnet peridotite source leaving a garnet-bearing residue. Al-undepleted komatiites exhibit moderate Al2O3/TiO2 (15–25) and moderate [Gd/Yb]MN (∼0.8–1.2), suggesting derivation by shallower or higher-degree partial melting of a garnet peridotite source leaving a garnet-free residue. Those in the 2750–2735 Ma Pacaud assemblage are all Ti-depleted komatiites, those in the 2725–2720 Ma Stoughton–Roquemaure assemblage are predominantly Al-depleted–Ti-enriched komatiites with lesser Al-undepleted komatiites, those in the 2718–2710 Ma Kidd–Munro assemblage are predominantly Al-undepleted komatiites with lesser Al-depleted–Ti-enriched komatiites, and those in the 2710–2703 Ma Tisdale assemblage are predominantly Al-undepleted komatiites. Some of the komatiites in the younger assemblages (Kidd–Munro and Tisdale) are enriched in highly incompatible lithophile elements, exhibit low [Nb/Th]MN, and felsic-intermediate rocks in these assemblages contain inherited zircons, suggesting contamination by upper crustal rocks or sediments derived from upper crustal rocks. The high Mg contents of the komatiites suggest that they formed in mantle-derived plumes. The number of plumes required to generate the komatiitic magmatism in the Abitibi greenstone belt is not clear; it may be possible to derive all of the different magma types from different parts of a single plume at different times, but the long duration of komatiitic magmatism (≥47 My) suggests that multiple plumes are more likely. In any case, the temporal changes in composition are consistent with a decreasing influence of garnet in the source region of the plume(s) with time. There are several possible mechanisms to account of these variations, none of which are mutually exclusive: (1) a decrease in the depth of melt extraction as the plume(s) ascended; (2) a decrease in the amount of magma derived from deeper levels in the plume relative to that derived from shallower levels in the plume, possibly representing a change in plume morphology with time, for example as it was dragged by the overlying plate; (3) a decrease in the rate of plume ascent due to interaction with subducting slabs; or (4) a decrease in the thickness of the lithosphere during plume ascent, perhaps related to lithospheric thinning by the rising plume(s). The restriction of contamination to individual flow units or individual parts of flow units in the Kidd–Munro and Tisdale assemblages suggests that crustal contamination occurred locally, via assimilation of country rocks during emplacement.

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.

Section snippets

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

  • 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.

  • Komatiites in the 2725–2720 Ma Stoughton–Roquemaure assemblage include Al-depleted–Ti-enriched komatiites and lesser Al-undepleted komatiites.

  • 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

References (124)

  • I.H. Campbell et al.

    Implications of mantle plume structure for the evolution of flood basalts

    Earth Planet. Sci. Lett.

    (1990)
  • K.D. Card

    A review of the Superior Province of the Canadian Shield, a product of Archean accretion

    Precamb. Res.

    (1990)
  • J. Fan et al.

    Geochemical characteristics of aluminum depleted and undepleted komatiites and HREE-enriched low-Ti tholeiites, western Abitibi greenstone belt: a heterogeneous mantle plume-convergent margin environment

    Geochim. Cosmochim. Acta

    (1997)
  • M.M. Ghomshei et al.

    Underplating of oceanic lithosphere in the Archean: a possible mechanism for the formation of Archean komatiites

    Tectonophysics

    (1990)
  • C.T. Herzberg et al.

    Phase equilibrium constraints of the origin of basalts, picrites and komatiites

    Earth Sci. Rev.

    (1998)
  • R.E.T. Hill et al.

    The volcanology of komatiites as deduced from field relationships in the Norseman–Wiluna greenstone belt, Western Australia

    Lithos

    (1995)
  • A.W. Hofmann

    Chemical differentiation of the earth: the relationship between mantle, continental crust, and oceanic crust

    Earth Planet. Sci. Lett.

    (1988)
  • H.E. Huppert et al.

    Cooling and contamination of mafic and ultramafic magmas during ascent through continental crust

    Earth Planet. Sci. Lett.

    (1985)
  • S.L. Jackson et al.

    Review of Archean supracrustal assemblages of the southern Abitibi greenstone belt in Ontario, Canada: products of microplate interaction within large-scale plate-tectonic setting

    Precamb. Res.

    (1994)
  • K.P. Jochum et al.

    High sensitivity Nb analysis by spark-source mass spectrometry (SSMS) and calibration of XRF Nb and Zr

    Chem. Geol.

    (1990)
  • Y. Lahaye et al.

    The influence of alteration on the trace-element and Nd isotopic compositions of komatiites

    Chem. Geol.

    (1995)
  • C.M. Lesher et al.

    REE and Nd isotope geochemistry, petrogenesis and volcanic evolution of contaminated komatiites at Kambalda, Western Australia

    Lithos

    (1995)
  • W.F. McDonough et al.

    The composition of the earth

    Chem. Geol.

    (1995)
  • R.W. Nesbitt et al.

    Geochemistry of Archean spinifex textured peridotites and magnesian and low magnesian tholeiites

    Earth Planet. Sci. Lett.

    (1976)
  • E.G. Nisbet et al.

    Constraining the potential temperature of the Archean mantle: a review of evidence from komatiites

    Lithos

    (1993)
  • E. Ohtani

    Majorite fractionation and the genesis of komatiites in the deep mantle

    Precamb. Res.

    (1990)
  • S.W. Parman et al.

    Emplacement conditions of komatiite magmas from the 3.49 Ga Komati Formation, Barberton greenstone belt, South Africa

    Earth Planet. Sci. Lett.

    (1997)
  • D.G. Pearson et al.

    Stabilisation of Archean lithospheric mantle: a Re–Os isotope study of peridotite xenoliths from the Kaapvaal craton

    Earth Planet. Sci. Lett.

    (1995)
  • C.J. Allègre

    Genesis of Archaean komatiites in a wet ultramafic subducted plate

  • Arndt, N.T., 1975. Ultramafic rocks of Munro Township and their volcanic setting. Unpublished Ph.D. Thesis. University...
  • N.T. Arndt

    Differentiation of komatiite flows

    J. Petrol.

    (1986)
  • N.T. Arndt et al.

    Geochemistry of Munro township basalts

  • N.T. Arndt et al.

    Were komatiites wet

    Geology

    (1997)
  • N.T. Arndt et al.

    Bizarre geochemistry of komatiites from the Crixás greenstone belt Brazil

    Contrib. Miner. Petrol.

    (1989)
  • J.G. Arth

    Behaviour of trace elements during magmatic processes—a summary of theoretical models and their applications

    J. Res. USGS

    (1976)
  • J.A. Ayer et al.

    Geological compilation of the Abitibi greenstone belt: toward a revised stratigraphy based on compilation and new geochronology results

    Ont. Geol. Surv. Misc. Pap.

    (1998)
  • Ayer, J.A., Trowell, N.F., Amelin, Y., Corfu, F., 1999. Geological compilation of the Abitibi greenstone belt: toward a...
  • Baird, A.M., 1999. Geochemistry of Chromites in Intrusive and Extrusive Komatiites in the Abitibi Greenstone Belt,...
  • S.J. Barnes et al.

    The distribution of Cr, Ni and chromite in komatiites, and application to exploration for komatiite-hosted nickel sulfide deposits

    Economic Geology

    (1999)
  • S.-J. Barnes et al.

    Ti-rich komatiites from northern Norway

    Contrib. Miner. Petrol.

    (1990)
  • S.-J. Barnes et al.

    A comparative study of olivine and clinopyroxene flows from Alexo, Abitibi greenstone belt, Ontario, Canada

    Contrib. Miner. Petrol.

    (1983)
  • S.J. Barnes et al.

    The perseverance ultramafic complex, Western Australia: the product of a komatiite lava river

    J. Petrol.

    (1988)
  • Barrie, C.T., 1999. The Kidd–Munro Extension Project: Year 3 Report. Unpublished Report, 263...
  • S.W. Beresford et al.

    Vesicles in thick komatiite lava flows, Kambalda, Western Australia

    J. Geol. Soc. London

    (2000)
  • A.E. Beswick

    Some geochemical aspects of alteration and genetic relations in komatiitic suites

  • W. Bleeker et al.

    Stratigraphy and U–Pb zircon geochronology of Kidd Creek: implications for the formation of giant volcanogenic sulphide deposits and tectonic history of the Abitibi Greenstone Bel

    Can. J. Earth Sci.

    (1996)
  • W. Bleeker et al.

    High-precision U–Pb geochronology of the late Archean Kidd Creek Deposit and Kidd volcanic complex

    Econ. Geol. Monograph

    (1999)
  • G.R. Byerley

    Komatiites of the Mendon formation: late-stage ultramafic volcanism in the Barberton greenstone belt

  • I.H. Campbell

    The mantle's chemical structure

  • I.H. Campbell et al.

    Melting in an Archaean mantle plume: heads it is basalts and tails it is komatiites

    Nature

    (1989)
  • Cited by (158)

    • The discovery and petrogenetic significance of komatiites

      2023, Journal of African Earth Sciences
    View all citing articles on Scopus
    View full text