Magma mixing in komatiitic lavas from Munro Township, Ontario

Arndt, N.T. and Nesbitt, R.W. (1984) Magma mixing in komatiitic lavas from Munro Township, Ontario. In, Kroner, A., Hanson, G. and Goodwin, A.M. (eds.) Archaean Geochemistry: The Origin and Evolution of the Archaean Continental Crust. Berlin, DE. Springer, pp. 99-114.

Abstract

Komatiites at Munro Township, northeast Ontario, show greater LREE depletion and have lower ratios of highly to moderately incompatible elements (e.g. Ti/Sc) than associated komatiitic basalts. These differences indicate that the two magma types are not related to one another by low pressure fractional crystallization: they formed either from mantle sources with slightly different compositions, or from the same source under different conditions of partial melting or high pressure fractionation.

In some situations these two magma types have mixed together to form hybrid magmas. The best example is Fred’s Flow, a thick mafic-ultramafic layered unit that has chemical characteristics intermediate between those of the komatiites and komatiitic basalts. Textural evidence for mixing is found in the flow top breccia which contains two types of fragment, one with komatiitic composition and the second with basaltic composition. Particularly significant are augite phenocrysts in the breccia, which have compositions indicating that they could not have crystallized from the liquid that formed the bulk of Fred’s Flow.

Magma mixing may also have played a role in the formation of komatiitic basaltic flows with acicular pyroxene textures and komatiites which contain anomalously Forich olivine xenocrysts or unusually high concentrations of incompatible trace elements.

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