Preliminary Evidence for a Second -525-545 Ma old Event of Granilate Facies Metamorphism in the Mozambique Belt of Tanzania and its Implication for a Two-Stage Model for Gondwana Assembly

Abstract

Garnets from two samples of the Kitumbi granulites in the Mozambique Belt of north eastern Tanzania yield Sm-Nd ages of 525±15 Ma and 545±15 Ma. At the 95% confidence level, these ages are younger than the 576 ±15 to 634±16 Ma (Mean 606±12 Ma) ages previously reported from the more extensive Eastern Granulites. The Kitumbi garnets are therefore interpreted as dating cooling from a granulite facies event that is distinctly younger than the 630-700 Ma event that produced the Eastern Granulites. Unlike the Eastern Granulites, the Kitumbi rocks occur as small enclaves not separated from the surrounding amphibolite facies gneisses by any obvious structural discontinuity, lack the continental margin geochemical signatures that characterise the Eastern Granulites and yield late Archean depleted mantle crustal formation ages, similar to those obtained from the surrounding gneisses. These characteristics suggest that, unlike the Eastern Granulites that are isotopically exotic slices emplaced onto the surrounding amphibolite facies gneisses, the Kitumbi granulites are products of ~550 Ma old in situ granulite facies metamorphism of the surrounding country rocks. The existence of a regional granulite facies event at ~530-550 Ma offers a way of reconciling the palaeomagnetic and petrological/geochronological evidence for Gondwana assembly. Thus, as has been previously proposed by Meert et al. (1995), the 630-700 Ma old event that produced the Eastern Granulites may record regional crustal thickening arising out of collision of India, Madagascar, parts of Easten Antarctica and the Kalahari craton (IMSLEK terranes) with the Congo craton and the Arabian Nubian Shield (ANS). The younger granulite event recorded in the Kitumbi area could then mark a younger collision between Australo-Antarctica and the combined IMSLEK-Congo-ANS collage marking the final assembly of Gondwana.


Tanzania Journal of Science Volume 26 (2000), pp. 51-66