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Palaeoproterozoic granulite-facies metamorphism and granitoid intrusions in the Ubendian-Usagaran Orogen of northern Malawi, east-central Africa

Abstract:

The Paleoproterozoic basement of northern Malawi shows evidence for granulite-facies metamorphism older than the 1930 ± 30 Ma-old Nyika Granite. Low-pressure granulite-facies metamorphism in regionally coherent and extensively exposed cordierite-garnet granulite reached 750-850°C and 5-5.5 kbar. The Chelinda Granite intruded during this event and has been dated by single zircons at 1995 ± 0.4 Ma (207Pb/206Pb age; 2σ-mean errors). Anatectic melt which formed concurrently with cordierite growth in the cordierite-garnet granulite yielded a 207Pb/206Pb zircon age of 1988 ± 0.6 Ma. These ages are interpreted to date the peak of regional low-pressure granulite-facies metamorphism. The Nyika Granite, and other smaller collisional-type two-mica granites, intruded after the peak of granulite-facies metamorphism. Their zircon ages range from 1930 ± 30 Ma to 1969 ± 0.4 Ma and constrain the end of regional high-temperature conditions. Enderbitic gneiss, which is tectonically intercalated with the cordierite-garnet granulite, records peak-metamorphic conditions of ca 850-880°C and 9-11 kbar. Metamorphic zircons from the enderbitic gneiss yielded a 207Pb/206Pb age of 2002 ± 0.3 Ma. This age for high-pressure granulite-facies metamorphism in northern Malawi is similar to the recently reported age for eclogite-facies metamorphism in the Usagaran Belt of central Tanzania and indicates that orogenic activity during the Ubendian-Usagaran Orogeny culminated at ca 2000 Ma. Pre-2000 Ma magmatic events encompass the intrusion of the enderbitic gneiss precursor at 2093 ± 0.6 Ma. Furthermore, a late granite intrusion of the Rumphi Igneous 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 affinity to volcanic-arc granites. Therefore, the age of 2048 Ma is interpreted to date a mature stage of volcanic-arc magmatism. The nature of the pre-2000 Ma magmatic events remains open, however; the granitoids might have been emplaced in a long-lived Andean-type subduction zone. Tectonic juxtaposition of both granulite-facies rocks and of the latter two with the RIC occurred after Paleoproterozoic granulite-facies metamorphism in northern Malawi. © 1997 Elsevier Science B.V.