PALAEOPROTEROZOIC GRANULITE-FACIES METAMORPHISM AND GRANITOID INTRUSIONS IN THE UBENDIAN-USAGARAN OROGEN OF NORTHERN MALAWI, EAST-CENTRAL AFRICA

The Paleoproterozoic basement of northern Malawi shows evidence for gr anulite-facies metamorphism older than the 1930 +/- 30 Ma-old Nyika Gr anite. Low-pressure granulite-facies metamorphism in regionally cohere nt and extensively exposed cordierite-garnet granulite reached 750-850 degrees C and 5-5.5 kbar. The Chelinda Granite intruded during this e vent and has been dated by single zircons at 1995 +/- 0.4 Ma (Pb-207/P b-206 age; 2 sigma-mean errors). Anatectic melt which formed concurren tly with cordierite growth in the cordierite-garnet granulite yielded a Pb-207/Pb-206 zircon age of 1988 +/- 0.6 Ma. These ages are interpre ted to date the peak of regional low-pressure granulite-facies metamor phism. The Nyika Granite, and other smaller collisional-type two-mica granites, intruded after the peak of granulite-facies metamorphism. Th eir zircon ages range from 1930 +/- 30 Ma to 1969 +/- 0.4 Ma and const rain the end of regional high-temperature conditions. Enderbitic gneis s, which is tectonically intercalated with the cordierite-garnet granu lite, records peak-metamorphic conditions of ca 850-880 degrees C and 9-11 kbar. Metamorphic zircons from the enderbitic gneiss yielded a Pb -207/Pb-206 age of 2002 +/- 0.3 Ma. This age for high-pressure granuli te-facies metamorphism in northern Malawi is similar to the recently r eported age for eclogite-facies metamorphism in the Usagaran Belt of c entral Tanzania and indicates that orogenic activity during the Ubendi an-Usagaran Orogeny culminated at ca 2000 Ma. Pre-2000Ma magmatic even ts encompass the intrusion of the enderbitic gneiss precursor at 2093 +/- 0.6 Ma. Furthermore, a late granite intrusion of the Rumphi Igneou s Complex (RIC) yielded a single-zircon age of ca 2048 +/- 0.7 Ma. The chemistry of those late-stage granites of the RIC displays an affinit y to volcanic-are granites. Therefore, the age of 2048 Ma is interpret ed to date a mature stage of volcanic-are magmatism. The nature of the pre-2000 Ma magmatic events remains open, however; the granitoids mig ht have been emplaced in a long-lived Andean-type subduction zone. Tec tonic juxtaposition of both granulite-facies rocks and of the latter t wo with the RIC occurred after Paleoproterozoic granulite-facies metam orphism in northern Malawi. (C) 1997 Elsevier Science B.V.