Neoproterozoic crustal growth in southern Malawi: New insights from petrology, geochemistry, and U–Pb zircon geochronology, and implications for the Kalahari Craton–Congo Craton amalgamation

Abstract

The southern part of the Malawi Basement Complex is regarded as being part of the Mozambique Belt, which was formed by a series of collisions between Archean cratons and/or Proterozoic magmatic arcs during Neoproterozoic to Cambrian time. The region is particularly important because of its location near the junction between the East-African (ca. 600–550 Ma) and Kuunga (ca. 560–530 Ma) orogens, which means that complex magmatic or metamorphic episodes may have been recorded in high-grade metamorphic rocks in the region. Here we report on new petrological, geochemical, and zircon U–Pb age data on felsic to mafic orthogneisses as well as on a syn-tectonic syenite from the Lilongwe–Zomba–Blantyre area in southern Malawi, with a view to evaluating the timing and pressure–temperature (P–T) conditions of the collision events as well as the protolith formation. The geochemical data on the felsic orthogneisses suggested magmatic-arc signatures, whereas the mafic orthogneisses had both magmatic-arc and non-arc signatures. Metamorphic P–T estimates for the orthogneisses based on conventional geothermometry and phase-equilibria modeling indicated high-pressure granulite-facies conditions of 9.1–11.6 kbar and 840 °C–930 °C. Zircons obtained from charnockite, biotite gneiss, and garnet-bearing mafic granulite yielded magmatic ages of 1011–808 Ma, 1013 Ma, and 1076 Ma, respectively, suggesting the presence of Stenian–Tonian crustal growth related to arc magmatism. Ediacaran (573 Ma) intrusion of granitic rock was also inferred from magmatic zircons found within a granitoid gneiss. The timing of the high-grade metamorphism was constrained by the ages of the zircon rims and found to be 569–557 Ma, thus coinciding with the intrusion of syn-tectonic syenite at 562 Ma. The results of this study suggest that the southern part of Malawi is composed of remnants of the Stenian–Tonian magmatic arc, which were metamorphosed together with surrounding metasediments around 560 Ma. The Ediacaran high-grade metamorphism is consistent with the age of the Zambezi Belt located immediately southwest of the Malawi Basement Complex. Both the Lilongwe–Zomba–Blantyre area and the Zambezi Belt may correspond to a single suture zone formed by the latest Neoproterozoic collision between the Kalahari and Congo Cratons.