A stratigraphic and structural reappraisal of Central Malawi:Results of a geotraverse

Abstract

The Precambrian Malawi Basement Complex, metasediments and orthogneisses of central Malawi are discussed. Three metasedimentary associations, intruded by a suite of low TiO 2, olivine tholeiite gabbros, are recognised and interpreted as a continental shelf-rise assemblage. A fourth unit, the psammitic Mchinji Group, is reinterpreted as a proximal facies equivalent of the other paragneisses. Widespread, early granitoid and alkaline plutonism is recognised and interpreted as an anorogenic magmatic province. Metamorphic transformations and deformational styles are locally controlled, probably by variable H 20. The first deformation produced migmatitic fabrics in aweu> paragneisses while water-deficient orthogneisses retained their igneous textures orrecrystall ised to granoblastic aggregates, locally with granulite-facies mineralogy. Anhydrous, kelyphitic rims formed around olivine in the metagabbros. Later deformation produced tectonte fabrics with lower- amphibolite, locally greenschist, mineralogy. Fold vergence and stretching lineations suggest N or NW tectonic transport directions and the overall structure is interpreted as a foreland fold and thrust zone with steep structures produced by imbricatlon. Previous isotope studies bracket deformation between c.1100Ma and c.450Ma, contemporary with deformation of the Muva Supergroup in the Irumide Belt of Zambia which may be the foreland-ward extention of the studv area.