Single zircon ages, PT evolution and Nd isotopic systematics of high-gradegneisses in southern Malawi and their bearing on the evolution of the Mozambique belt in southeastern Africa

The high grade gneiss assemblage of central and southern Malawi belongs to the Neoproterozoic Mozambique belt of East Africa. and reached peak metamor phic conditions at 900 +/- 70 degreesC and 9.5 +/- 1.5 kbar, followed by an isobaric cooling path. We report single zircon U-Pb and Pb-Pb ages and Nd isotopic data for orthogneisses and metapelites collected around Lilongwe a nd farther south in the region around Blantyre and Zomba. The ages document three distinct events. (1) a Kibaran-age period of intrusion of calc-alkal ine granitoids around 1040-929 Ma: Nd isotope data indicate overall juvenil e compositions consistent with a magmatic are environment, or emplacement i nto thinned continental crust with little involvement of older basement: (2 ) a Pan-African period of intrusion of calc-alkaline granitoids around 710- 555 Ma; Nd isotopes for most samples indicate crustal residence ages of 1.0 -1.5 Ga and suggest either remelting of similar to 1.0-1.5 Ga (Kibaran) pro toliths or mixing of juvenile material with subordinate amounts of older cr ust, (3) a long-lasting thermal peak of Pan-African high grade metamorphism around 571-549 Ma. The chemical composition of the dated orthogneisses is compatible with an origin of their protoliths in an Andean-type active cont inental margin setting. Penetrative deformation occurred prior to the therm al peak of metamorphism and also affected the youngest intrusions. It must, therefore, have been of Pan-African age as was the late. retrograde amphib olite facies overprint. Rocks with presumably rather different histories pr ior to the thermal peak were stacked together. An earlier Kibaran granulite metamorphism, postulated by previous authors, could not be verified. Rathe r uniform and widespread maximum PT-conditions of the granulite facies in c entral and southern Malawi. followed by near-isobaric, slow cooling indicat e a transient metamorphic gradient of about 26 degreesC/km and suggest stac king of a relatively hot crust and subsequent slow exhumation. The late Kib aran and Pan-African magmatic events in Malawi may be linked to granitoid m agmatism of similar ages in northwestern Mozambique and the central Zambezi belt of northern Zimbabwe. but the geodynamic relationships between these terrains remain obscure. The variation in age of peak metamorphism between similar to 640 and 520 Ma across the Mozambique belt from Malawi to Antarct ica suggests that this belt was assembled from a number of terranes that ac creted at different times over a period of ca. 100 million years. The avail able data, therefore, suggest that East Gondwana was not a coherent block c olliding with West Gondwana but consisted of individual terranes before bei ng amalgamated into the supercontinent Gondwana some 550-530 Ma ago. (C) 20 01 Elsevier Science B.V. All rights reserved.