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Komatiites of the Onverwacht Group, S. Africa: REE geochemistry, Sm/Nd age and mantle evolution

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Abstract

Komatiites of the Tjakastad Subgroup of the Onverwacht Group (S. Africa) were dated by the Sm/Nd method. A whole-rock isochron yields an age of 3.56±0.24 (2σ) AE, with initial 143Nd/144Nd ratio of 0.50818±23 (2σ), corresponding to ɛ Nd(T)= + 1.9±4.5. This age is interpreted as the time of initial Onverwacht volcanism. This result agrees with earlier Sm/Nd data of Hamilton et al. (1979) and is consistent with the Rb-Sr result of Jahn and Shih (1974).

Komatiites may be divided into 3 groups based on the typology of heavy REE distributions (Jahn and Gruau 1981). According to this scheme, the Onverwacht komatiites of the present study belong to two groups: the predominant Group II rocks showing (Gd/Yb)N≃1.4, CaO/Al2O3 = 1.33, Al2O3/TiO2≃10.6; and the subordinate Group III rocks with (Gd/Yb)N<1.0; CaO/Al2O3≃0.6 and A12O3/ TiO2≃40. This contrasting feature is best explained by garnet fractionation within the mantle sources.

Younger komatiites (∼2.7 AE) from Finland, Canada, Rhodesia, and Australia have (Gd/Yb)N≃1.0, CaO/ Al2O3<1.1 and Al2O3/TiO2≃21 based on 58 analyses. These ratios are nearly chondritic or of the bulk earth value (Anders 1977). It appears that some late Archean komatiites are different in chemistry from many early Archean komatiites. This may imply that the upper mantle chemistry has evolved through Archean times. However, the age connotation of the chemical parameters, such as CaO/Al2O3, (Gd/Yb)N or Al2O3/TiO2 ratio has not been firmly established. The characteristic “high” CaO/Al2O3 or (Gd/Yb)N ratios in many Onverwacht Group rocks can also be explained as a result of local short-term mantle heterogeneity.

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Authors and Affiliations

  1. C.A.E.S.S., Université de Rennes, Institut de Géologie, 35042, Rennes, France

    Bor-ming Jahn & G. Gruau

  2. Bureau of Mineral Resources, Geology and Geophysics, Canberra, A.C.T., Australia

    A. Y. Glikson

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  1. Bor-ming Jahn
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  2. G. Gruau
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  3. A. Y. Glikson
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Jahn, Bm., Gruau, G. & Glikson, A.Y. Komatiites of the Onverwacht Group, S. Africa: REE geochemistry, Sm/Nd age and mantle evolution. Contr. Mineral. and Petrol. 80, 25–40 (1982). https://doi.org/10.1007/BF00376732

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