2.67 Ga high-Mg andesites from the Musoma-Mara greenstone belt, northern Tanzania
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Date
2008-07-01
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Abstract
Neoarchaean (2.67 Ga) High-Mg andesites occur in the
Musoma-Mara greenstone belt, northern Tanzania. They are
associated with dacites and Na-granitoids both of which are
adakitic in composition. The high-Mg andesites are
characterized by higher contents of MgO (2.42 – 9.47 wt %),
Cr (41-797 ppm) and Ni (11-254 ppm) than those of normal
island arc andesites. Their La/Yb ratios are 9.87 – 22.5
whereas their Sr/Y ratios are 20 – 131.These characteristics
are similar to those shown by Cenozoic Setouchi High
Magnesian Andesites (HMA). These rocks are characterized
by 143Nd/144Nd ratios that range from 0.511062 ± 7 to
0.511308 ± 12 with corresponding εNd (t) of +0.44 and +1.81.
The high contents of Mg, Cr and Ni argue in favour of
equilibration of their parental magma with mantle peridotite
whereas their relatively low La/Yb ratios argue against the
involvement of garnet and amphibole as residual phases
during partial melting. Thus, the high-Mg andesites are
interpreted to have formed by partial melting of the mantle
peridotite that has been fluxed by slab-derived fluids. Overall,
the geochemical features of high-Mg andesites, their close
association with adakitic rocks in the MMGB together with
the short time interval taken for their emplacement are
interpreted in terms of a ridge-subduction model. It is
considered that such a model was important for generation of
late Archaean continental crust.
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Greenstone Belt
Citation
Manya, S. and MABOKO, M.A.H., 2008. 2.67 Ga high-Mg andesites from the Musoma-Mara greenstone belt, northern Tanzania. Geochimica et Cosmochimica Acta Supplement, 72, p.591.