Irumide and post-Mozambiquian plutonism in Malawi

Abstract

Summary: Rb-Sr whole-rock isochron ages of 443 ± 30 Ma, 449 ± 42 Ma and 489 ± 14 Ma with initial ⁸⁷Sr/⁸⁶Sr ratios of 0.70669 ± 0.00068, 0.70599 ± 0.00035 and 0.70658 ± 0.00025 were obtained, respectively, for the Cape Maclear granite, the surrounding Cape Maclear syenite and the Senga Bay granitic body. The results confirm previous estimates of the age of the post-Mozambiquian Lake Malawi Granitic Province, of which these plutons are part. Data from the Dzalanyama granite have excess scatter, but indicate an Irumide age of about 1000 Ma. This batholith is therefore much older than the Lake Malawi Granitic Province but much younger than the Nyika granite of northern Malawi, contrary to earlier suggestions. The porphyritic foliated granite of Mcezi Hill gave an age of 1016 ± 55 Ma and an initial ratio of 0.70522 ± 0.00084. Irumide plutonism in the region includes several dissimilar groups of intrusions, all with low initial⁸⁷Sr/⁸⁶Sr ratios. The Irumide age of the Dzalanyama granite makes it possible that the metasediments of the Mchinji Group are wholly post-Ubendian.