Supercontinents and Earth Evolution Symposium 2005

GONDWANA FORMATION: A METAMORPHIC VIEW FROM THE MARGINS OF THE CONGO CRATON

Volker Schenk, Peter Appel, Niels Jöns, Timm John, Denny Loose

Institut für Geowissenschaften, Universität Kiel, Germany ([email protected])

INTRODUCTION Similarly, the Archean crust of the craton The Congo craton has a central position in reworked in the EAO of Tanzania experienced reconstructions of the Gondwana super- an anticlockwise P-T path between 650-610 Ma continent and is surrounded by transcontinental pointing to a strong crustal heating prior to orogenic belts thought to have formed during collision. In contrast, whiteschists near the Pan-African orogenic events between 650 to western border of the EAO (Mautia Hill) close to 500 Ma ago: The Central African Fold Belt the Tanzania craton were formed during crustal (CAFB) in the north, the East African Orogen thickening at about 540 Ma, i.e. about 70 m.y. (EAO) in the east, the Zambezi-Lufilian-Damara after the peak temperatures had been attained orogenic system (ZB) in the south and the in most of the EAO of Tanzania. Brasiliano Belt in the west. To decipher the The Bemarivo belt of northern Madagascar timing of subduction processes and collisional experienced crustal thickening at about 530 Ma events around the Congo craton, we studied when it became attached to the Antananarivo the metamorphism of eclogites (ZB and CAFB), block. So far, the only MORB-type chemistry of granulites (EAO, CAFB), and whiteschists (ZB, mafic rocks in the EAO of Madagascar was EAO). We consider the continental reworking of found in the Vohibory block. However, these the margins of the craton as well as the burial of rocks are of amphibolite grade and do not show Neoproterozoic cover series, in addition to traces of HP metamorphism. subduction metamorphism of oceanic crust. THE SOUTHERN MARGIN (ZB) THE NORTHERN MARGIN (CAFB) Eclogites of the central Zambezi Belt display a Western end of CAFB: The Paleoproterozoic MORB-type geochemistry and were metamor- basement and Neoproterozoic cover series in phosed at ca. 650-600 Ma. In contrast, the Cameroon experienced a common granulite- whiteschists within the Lufilian Arc–Zambezi facies metamorphism at about 602-631 Ma, due Belt formed under high-pressure amphibolite to substantial crustal thickening. Retrogressed facies conditions at ca. 530 Ma. Their eclogites with MORB-type chemistry occurring metamorphism was linked to the collision rather near the NW-edge of the Congo craton may than to subduction zone processes. The crustal indicate the so far unknown site of a suture in thickening affected the entire Lufilian Arc– the CAFB. Zambezi Belt almost simultaneously ca. 70 m.y. Eastern end of CAFB: The West Nile area after the subduction of the oceanic basin. forms part of the northern margin of the Congo craton in Uganda, and is overlain by CONCLUSION Neoproterozoic cover series. Crustal thickening The collison events along the northern margin is evident by burial of these cover series to of the Congo craton (CAFB in Cameroon and depths of 25-30 km (at 630 Ma) and by uplift of Uganda) happened nearly synchroneously at 2.4 Ga old granulites subsequent to a Pan- 630 Ma and were preceded by eclogite African granulite-facies overprint. formation of so far unknown age. Eclogite metamorphism at the southern margin (ZB) was THE EASTERN MARGIN (EAO) at 650-600 Ma and pre-dated collision and The Basement Complex of Uganda and the whiteschist formation at 530 Ma for about 70 Tanzania craton can be seen as the eastern m.y. The eastern margin of the Congo craton margin of the Congo craton along the EAO. The (EAO) in Uganda and Tanzania experienced a cratonic crust in NE Uganda experienced a anticlockwise P-T evolution and very strong strong Pan-African reheating (633±11Ma) heating (650-610 Ma) prior to crustal thickening resulting in UHT-metamorphism (>1000°C) over and whiteschist formation at 540 Ma. A slightly a large area (>2000 km2) that was followed by younger collision (530 Ma) is only known from near-isobaric cooling prior to uplift. the Bemarivo Belt in northern Madagascar.

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