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

Abstract:

The high grade gneiss assemblage of central and southern Malawi belongs to the Neoproterozoic Mozambique belt of East Africa, and reached peak metamorphic conditions at 900 ± 70°C 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 and farther south in the region around Blantyre and Zomba. The ages document three distinct events, (1) a Kibaran-age period of intrusion of calc-alkaline granitoids around 1040-929 Ma; Nd isotope data indicate overall juvenile compositions consistent with a magmatic arc environment, or emplacement into 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 ∼ 1.0-1.5 Ga (Kibaran) protoliths or mixing of juvenile material with subordinate amounts of older crust; (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 continental margin setting. Penetrative deformation occurred prior to the thermal peak of metamorphism and also affected the youngest intrusions. It must, therefore, have been of Pan-African age as was the late, retrograde amphibolite facies overprint. Rocks with presumably rather different histories prior to the thermal peak were stacked together. An earlier Kibaran granulite metamorphism, postulated by previous authors, could not be verified. Rather uniform and widespread maximum PT-conditions of the granulite facies in central and southern Malawi, followed by near-isobaric, slow cooling indicate a transient metamorphic gradient of about 26°C/km and suggest stacking of a relatively hot crust and subsequent slow exhumation. The late Kibaran and Pan-African magmatic events in Malawi may be linked to granitoid magmatism 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 ∼ 640 and 520 Ma across the Mozambique belt from Malawi to Antarctica suggests that this belt was assembled from a number of terranes that accreted at different times over a period of ca. 100 million years. The available data, therefore, suggest that East Gondwana was not a coherent block colliding with West Gondwana but consisted of individual terranes before being amalgamated into the supercontinent Gondwana some 550-530 Ma ago. © 2001 Elsevier Science B.V. All rights reserved.