IGCP418

"Evolution of the Kibaran belt system in southwestern Africa: Comparison with equatorial and southern Africa"

R. KEY(1) and A. B. KAMPUNZU (2)

1 Principal Regional Geologist/Technical Adviser, Geological survey Department, Private Bag 14, Lobatse Botswana. Tel +267-330-327; Fax. +267-332-013

2 Department of Geology, University of Botswana, Private Bag 0022 Gaborone, Botswana. Tel +267 355-2536; Fax +267-356-591; Email:kampunzu@noka.ub.bw

During the last decade, several works have concentrated on the dispersion of the Gondwana Supercontinent and the present reconstruction of Gondwana is relatively well constrained, partly because the geological evolution of the Pan African belt system and correlative terranes is relatively well known. Recently, some authors have explored the reconstruction of older Proterozoic belts and, in the specific case of the Kibaran belt system, it has been proposed that Mesoproterozoic continental accretion led to the building of a Supercontinent named Rodinia. In some of these reconstructions of Rodinia, the Kibaran belt system of Africa is not taken into account.

The Kibaran belt system was first defined around 1930 in Central Africa and represents a Mesoproterozoic belt system the evolution of which encompass about 400 Ma, i.e. approximately from 1400 up to 1000 Ma (from the rifting stage up to the post-collision stage). In the Kibaran-type area (Shaba, Zaire), the Kibaran is a coherent orogen including two main orogenic phases (D1 at 1350 - 1250 Ma and D2 at 1100 - 1000 Ma) which cannot be dissociated into two distinct orogens. These two orogenic phases are also known in the Grenvillian orogenic cycle with the older (Elzevirian) developed between 1350-1250 Ma and the younger (Ottawan) bracketed between ca. 1100 - 1000 Ma. This project will document in detail the geotectonic implications of these two main Mesoproterozoic orogenic phases which probably mark the tectonic assembly of the Rodinia Supercontinent. The Namaqua - Natal segments of the Kibaran belt were recently correlated with the Lurio belt (Mozambique) and with the Heimefrontjella in East Antarctica. Paradoxically, the correlation with the type area of the Kibaran belt in Central Africa is not constrained. This lack of correlation reflects principally the poor investigation of the Kibaran events in Southwestern Africa. The Kibaran belt system with its two main segments in SE Zaire (Kibaran s.s.) and in Zambia (Irumides) extends up to the borders of Angola - Botswana - Zimbabwe. Across the boundaries, they are no longer identified in Botswana and we assume that this is mainly due to the lack of cross - border geological correlation. Geological units with unquestionable Kibaran age are known in Namibia and the Sinclair and Koras basins are part of the Kibaran "network" in Africa (specifically, they are part of the late Kibaran period). In Angola, Kibaran rocks are known in the Alto Zambezi area (eastern part of Angola; ca. 11o - 12oS; 23o - 24oE) but correlation with the Kibaran s.s located ca 200 km to the northeast of this area is poorly constrained. Similarly Kibaran ages in the range ca 1400 - 1100 Ma were documented in Southwestern Angola close to the border with Namibia.

Despite the existence of several bench marks of the Kibaran belt system in Southwestern Africa, no link and no correlation of these separate "occurrences" have been achieved, partly because there are some linguistic barriers between French, English and Portuguese countries but also because some areas in the Kalahari desert are covered by sand and/or are remote and reliable geochronological data are still limited. However, indirect techniques such as geophysical interpretation, remote sensing as well as the study of mantle xenoliths can help during the correlation exercise especially as some critical areas can be after re-evaluation of existing data, classified as unquestionable Kibaran units. The correlation of the Kibaran segments from Central and Southern Africa will represent an important stage for the integration of the Kibaran belt system of Africa in the reconstruction of the Rodinia Supercontinent, although acquisition of new paleomagnetic data in Central and Southern Africa will be required. In Northeastern Zaire the Kibaran belt swings around the Congo craton and then is covered by younger sediments of the "Cuvette Centrale" in central part of Zaire. In western Zaire and Congo, Mesoproterozoic granites were identified but the classical supracrustal sequences deposited in the pre-orogenic Kibaran rift are not known (according to various recent works conducted in Congo and Gabon). However, it is important to note the possible existence of "Kibaran" metasediments and (meta) igneous rocks in Nigeria which probably represent the western prolongation of the Maniema (Zaire) segment of the Kibaran belt system. However, recent U-Pb Zircon dating of rocks from Kaduna in Nigeria gives Archaean ages and do not show imprint of the Kibaran orogeny. The Rb-Sr Kibaran ages are thought to have no geological significance. The Kibaran event in Nigeria is therefore a still largely debated topic. There is an interesting problem over Kibaran in Nigeria and there must be a possibility that the "Pan-African" Sn - Ta pegmatite belt which trends NE - SW oblique to the "Pan - African" trend is somehow linked to Kibaran structures. These areas could represent the connection needed for correlation with Brazil where several features of the Mesoproterozoic belt system match perfectly the characteristics of the Kibaran belt in eastern Zaire. Among the common features should be stressed the widespread tin-group ore deposits (i.e. Sn, W, Nb-Ta, Be, Li as well as Au) mined in Central Africa since the post-world war 1 period. Their possible correlative in South America (Brazil) were developed only more recently.

The main objectives of this project will be:

This research proposal, prepared jointly by the Department of Geology, University of Botswana and the Geological Survey of Botswana, has recently been approved by UNESCO-IUGS as IGCP418 with Dr Roger Key as International Project Leader.