Ubendian basement and its late Mesoproterozoic and early Neoproterozoic structural and metamorphic overprint in northeastern Zambia

Abstract

The Palaeoproterozoic basement in the Muyombe and Luwumbu River areas of northeastern Zambia comprises a WNW–ESE (to E–W) trending cordierite–garnet–sillimanite granulite unit with numerous enderbite bodies and an amphibolite-facies migmatite unit. Zircons from a biotite metatonalite intruding the granulites were dated at 1960.7 ± 0.4 Ma, and this is time-equivalent with the Nyika granite in adjacent Malawi. Mesoproterozoic intrusions into this basement are represented by a nepheline syenite at Mivula Hill (zircon age: 1360.1 ± 0.8 Ma) and the porphyritic Ntendele biotite metagranite (zircon age: 1329.1 ± 0.6 Ma). The Ntendele granite attains plutonic dimensions north of Muyombe. The Mesoproterozoic Mafinga Group occurs in two major belts imbricated in the basement, i.e., the Makutu Range belt and Nyika escarpment belt, both trending NE–SW, nearly at right angles to the Ubendian structures in the basement. Numerous smaller tectonic slices of the Mafinga Group, in the Ubendian basement gneisses farther south of the two major belts, document a significant areal extent of this unit. Structures and metamorphic effects (greenschist-facies, biotite ± garnet zone) imprinted in the Mafinga Group rocks correlate with those in the Irumide belt farther west near Isoka and to the north, in the Mafinga Hills. A mylonitic foliation and newly-formed mineral assemblages (biotite zone) in the Ntendele granite correspond to deformation and metamorphism of the Mafinga Group. Mylonitization affecting the basement complex near the Mafinga Group slices resulted in strongly but heterogeneously sheared domains with a corresponding Irumide-age structural and metamorphic overprint. 40Ar/ 39Ar dating of muscovite from mylonitic gneisses derived from basement rocks and from a Mafinga Group phyllite point to closure of the K–Ar system in the interval 860–890 Ma. These early Neoproterozoic ages correlate with published muscovite and biotite K–Ar ages of biotite gneisses and muscovite schists from other parts of the Irumide belt in NE Zambia. They probably indicate slow cooling rates after main Irumide tectonism and associated magmatism around 1015 Ma. The 40Ar/ 39Ar data show that the Muyombe area was not affected by Pan-African thermal overprinting on a regional scale. In view of contrasting relations between the area studied and published results from northern Malawi, where both structural and thermal Pan-African overprints are documented, it is suggested that the NW–SE trending Mugesse shear zone in northern Malawi and along the border with NE Zambia is an important structural boundary separating contrasting crustal domains, i.e. the Ubendian belt, Irumide belt, and East African Orogen.