Tectonic evolution of western central Moçambique.

Abstract

This thesis examines the geological history of western central Mocambique, with a particular emphasis on the enigmatic relationships between the Umkondo Group and the Frontier Formation. In the region the geological units can be broadly subdivided, from west to east, into the Archaean Basement Complex, the Umkondo Group, Gairezi Group and Proterozoic Barue Complex. The Basement Complex represents the eastern edge of the Archaean Zimbabwe craton that, near its margin, has been reworked by Kibaran and Pan-African events. The Umkondo Group comprises mainly sedimentary rocks that are weakly metamorphosed and mildly deformed. These rocks unconformably overlie the Basement Complex and are overthrust by the Gairezi Group in the east. The Gairezi Group is a highly metamorphosed and strongly deformed sequence subdivided into the quartziteschist Frontier Formation and the gneissschist Nhazonia Formation. This group tectonically overlies the reworked eastern margin of the Zimbabwe craton. The Barue Complex comprises all the geological units in the MQ9ambique Belt that was accreted to the Zimbabwe craton during the Kibaran event and reworked during the Pan-African event. Geothermobaromtery and structural signatures show an increase up to granulites facies and an increase in deformation from west to east across the Zimbabwe craton to the Barue Complex. No evidence could be found for a proposed sinistral strike-slip zone between the Zimbabwe craton and the Mocambique Belt. Structural and AMS data indicate the contact is an 8 Km wide duplex zone formed by the thrusting of the Mocambique Belt over the Zimbabwe craton. Evidence from single zircon SHRIMP ages indicates that the Gairezi Group sediments were derived from the Zimbabwe craton and Limpopo Belt. They were deposited either between 1800 Ma and 1430 Ma or between 1430 Ma and 1300 Ma, depending on the interpretation of the age of one discordant zircon. This event is interpreted as rifting of the Zimbabwe craton and formation of a passive continental margin. Geological and geochronological evidence show that this sequence was deformed and metamorphosed, and underwent uplift and erosion before deposition of the Umkondo Group. The sediments of the Umkondo Group were deposited in braided stream and shallow marine environments in a peripheral foreland basin. Before lithification they were intruded by dolerites sills dated at 1105 ± 5 Ma. Palaeocurrent data in previous studies indicate a source region to the west and north-west. The geochemistry presented here indicates a cratonic source and a recycled orogeny source while SHRIMP ages reveal that these sources were the Zimbawe craton and Limpopo Belt. The Umkondo Group -and the Ritschersflya Supergroup of western Dronning Maud Land, Antarctica, have been considered sequences that were originally deposited in the same sedimentary basin. However, geochemical evidence reveals that the Ritschersflya sediments were derived from an active arc that must have separated the two sequences. Consequently two separate basins must have formed during Kibaran times, one on the Zimbabwe craton and another on the Archaean Grunehogna Province in Antarctica. This needs to be taken into consideration in tectonic models for the Kibaran and in any reconstruction of Rodinia and Gondwana.