Anatexis and High-Grade Metamorphism in the Champira Dome, Malawi: Petrological and Rb-Sr Studies

H. W. Haslam, M. S. Brewer*, A. E. Davis and D. P. F. Darbyshire
Institute of Geological Sciences, 64–78 Gray's Inn Road, London WC1X 8NG
*Present address: Bermuda College, Department of Academic Studies, Prospect, Devonshire 4-04, Bermuda.

Summary: Banded gneisses and migmatites in the Champira Dome contain assemblages of the biotite-cordierite-almandine subfacies of the granulite facies. The banded gneisses represent a sequence of arkosic sediments metamorphosed to sillimanite-cordierite gneisses. The migmatites were derived from more argillaceous and potassic sediments, with lower oxidation ratios and lower Ba/Rb ratios. They consist of a quartzo-feldspathic leucosome, representing anatectic melt, with schlieren of the refractory minerals sillimanite, garnet, biotite, and oxide minerals. Cordierite developed from garnet and biotite, except in rocks of high FeO/(FeO + MgO) ratio. Both rock types contain assemblages of magnetite + hercynite + corundum + ilmenite + hematite, formed by unmixing of high-temperature solid solutions. Rb-Sr studies of the banded gneisses gave an age of 2327 ± 25 Ma, which is interpreted as the date of metamorphism, and an initial 87Sr/86Sr ratio of 0.7064 ± 0.0003. The migmatite samples plot close to this 2327 Ma regression line, but the strontium isotopes were considerably disturbed, though not homogenized on the scale of sampling, 962 ± 34 Ma ago. It is considered that the anatexis in the migmatites was contemporaneous with the metamorphism of the banded gneisses and that the 962 ± 34 Ma event may be correlated with recrystallization of the migmatites accompanying the growth of cordierite.

Mineralogical Magazine; June 1980 v. 43; no. 330; p. 701-714; DOI: 10.1180/minmag.1980.043.330.03
© 1980, The Mineralogical Society
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