THE MOZAMBIQUE BELT, EASTERN AFRICA – TECTONIC EVOLUTION OF THE MOZAMBIQUE OCEAN AND GONDWANA AMALGAMATION
The MB extends north into the Arabian Nubian Shield (ANS) and to the south into east Antarctica and contains elements of both opening and closure of the MO. The nature of this closure has provided a unique cross section of the orogen, from upper crustal to mid and lower crustal levels, along the north to south extent of the MB. The ANS is dominated by upper crustal, juvenile, Neoproterozoic rocks of low metamorphic grade formed by the assembly of several arc and back arc basin systems. It contains ophiolites occurring as dismembered nappes having a supra-subduction zone chemistry. Further south, metamorphic grade increases to amphibolite and granulite facies. Passive margin metasediments and rift sediments of the early Neoproterozoic continental margin and reworked, older, crustal material of the Tanzanian (Congo) craton are exposed in Kenya and Tanzania. Closure of the MO was previously considered a result of collision of east and west Gondwana, but it is now recognized that Gondwana formed from the accretion of smaller, independent terranes over an extended period. The Eastern Granulites in eastern Tanzania, emplaced on west verging thrusts, were previously considered metamorphosed during continent-continent collision. However, Appel et al. (1998) determined a composite anti-clockwise P/T path for granulite metamorphism consistent with magmatic underplating, residence in the mid-lower crust, subsequent cooling (640 Ma 620 Ma), and exhumation prior to ca. 500 Ma. The final closure of the MO elevated the deeper crustal levels of the orogen and erosion has removed the upper crustal arc remnants, probably very similar to those presently exposed in the ANS.
Rift and passive margin sediments from opening of MO are undated and poorly age constrained. The identity of the continent which rifted from the Tanzanian craton is unknown but recent, unusual 3.0 Ga dates (Johnson et al. in press) from reworked basement in western MB of Tanzania, may provide some clue.