Anatexis and high-grade metamorphism in the Champira Dome, Malawi: petrological and Rb-Sr studies

Abstract

SUMMARY. Banded gneisses and migmatites in the Champira Dome contain assemblages of the biotitecordierite-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 878r/86Sr ratio of o.7o64 + 0.ooo3. 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. THE Champira Dome ( fig. IA) consists largely of banded sillimanite-cordierite garnet gneisses. It was emplaced as a tectonic wedge between ENEtrending boundary faults and appears to have a different structural and metamorphic history to that of the surrounding rocks (Peters,.i975). The purpose of this paper is to elucidate as far as possible the origins and metamorphic history of the rocks of the dome. Two suites of specimens were examined. Specimens MB 52 and 54-59 are banded gneisses. To the north-east of these ( fig. z B) is an area of migmatite . Both suites contain mineral assemblages characteristic of the

An isotropic alteration product of cordierite

“…Cordierite in banded quartz-sillimanite hornfelses in the Mchinji district of Malawi (13 ~ 47 89 S., 32 ~ 48' E.) is mostly replaced, some grains by sericitic aggregates and some by isotropic material (Haslam, 1980). Three grains replaced by the isotropic substance were analysed by electron microprobe, one pale brownish yellow, one medium brown, and one darker brown.…”

“…The similarity with the material derived from cordierite, however, both in appearance and in chemical composition (Table I), make it improbable that different parent minerals could have been involved. Some of the cordierite in this suite of rocks developed by replacement of biotite (Haslam et al, 1980), and it thus seems likely that the biotite in MB 60 was partially replaced by cordierite, and that the latter mineral was subsequently replaced by the isotropic substance. low--lower than in the associated cordierite in MB 62 (0.02 ~), (b) the CaO content, though less than 1 ~, is significantly higher than in the cordierite (< 0.01 ~o), and (c) the K20 content is significantly greater than in the cordierite (< 0.1 ~o)-An X-ray diffraction powder photograph of grains from MB 60 showed lines characteristic of a dioctahedral kaolinite group mineral, together with traces of other crystalline phases (probably illite and quartz).…”

“…Dana, 1892, p. 421;Chandler, 1975), but they show considerable variety. Examples of brown and yellow isotropic alteration products have been reported in the Cape Province of South Africa (Mathias, 1952); in the Chipata district of eastern Zambia (Vavrda and Vrfina, 1972) and nearby in the Mchinji district of Malawi (Haslam, 1980); further north in Malawi, in the Champira Dome (Haslam et al, 1980); in the Loos-Hamra region of Sweden (yon Eckermann, 1936); and in NE Scotland (Ashworth, 1976). This communication describes the Champira Dome occurrence, and gives some data for the Mchinji occurrence.…”

Proterozoic geochronology and tectonic evolution of southern Africa

“…Haslam et al 1986;Johns et al 1989). The Champira Dome within this area in Malawi (Haslam et al 1980) may contain even older, Archaean crust (Cahen et al 1984). Whether all of the Palaeoproterozoic basement in this complex, poorly understood region is directly related to the Ubendian-Usagaran-Bangweulu province is unclear because of strong overprinting during younger orogenic events.…”

Thermal and mechanical development of the East African Rift System

“…We interpret structures with a WSW trend within the rift valley as strikeslip faults in an accommodation zone between the Nkhata and northern Nkhotakota extensional segments, but the sense and amount of any horizontal displacement is unconstrained by existing data. report no evidence onshore for recent displacement along WSWtrending shear zones bounding granulitic rocks to the west of the rift (Haslam et al, 1980); hence extension appears to be restricted to the rift valley bounded by the border fault segments.…”