Evidence for a high Mg andesitic parental magma to the East and West satellite dykes of the Great Dyke, Zimbabwe: a comparison with the continental tholeiitic Mashonaland sills

One of the most significant mafic intrusive events in the Zimbabwe Craton w as the emplacement of the Great Dyke layered ultramafic-mafic complex and i ts two parallel 'satellite' dykes at the end of the Archaean (similar to 2. 6 Ga). The two satellite dykes, the East Dyke and the West (Umvimeela) Dyke , were far less affected by crystal accumulation and layering processes tha n the Great Dyke, and therefore may provide a clearer picture of parental m agma compositions of the Great Dyke event. The geochemical character of thi s major episode of mafic magmatism is markedly different to that of more ty pical continental tholeiites, such as the similar to 1.9 Ga Mashonaland sil ls, and reflects significant contrasts in petrogenetic influences. Despite its mid-continental setting, the parental magma of the satellite dykes had geochemical characteristics more often associated with magmas generated at destructive plate margins, suggesting that the nature of this magma was per haps more similar to high Mg andesitic, rather than continental tholeiitic magmatism. Fine-grained samples with similar to 14% MgO perhaps most closel y approximate to the parental magma composition. Certain major and trace el ement concentrations (high MgO, SiO2, LREE and LILE, and low Nb, Ta and Ti) and the lack of an Fe enrichment trend, suggest that the mafic magma had e ither suffered severe crustal contamination or had been derived from a meta somatised harzburgitic mantle source. (C) 1999 Elsevier Science Limited. Al l rights reserved.