アフリカ大陸の先カンブリア時代

その地質現象と鉱産資源

抄録

Precambrian time is the whole of geologic time from the very beginning of the earth history until the earliest fossiliferous Cambrian beds were deposited. Precambrian time covers almost 90% of the total length of time that has passed since the formation of the earth. Until recently, however, this long period of geologic time was among the least known segments of the geologic record.
The actual absence of fossils in Precambrian rocks makes it very difficult to correlate rocks of one locality to those of the others or to identify the age of geological formations from different localities. By introducing the dating methods based on radioactive decay, reliable age data on minerals and rocks have been accumulated, especially since 1950. Precambrian is now outstanding in availability of a very notable number of exact ages, among which the oldest ones are estimated at 3, 500 million years. The Precambrian rocks are exclusively found in the vast shield areas of the world. In the African continent, they occupy 57% of the whole continent in its areal distribution.
In recent years, researches on the Precambrian rocks in the African continent have made a remarkable progress, especially on the following five points:
(1) Since the improvement and spread of radiometric dating techniques, various Precambrian orogenic belts and early Palaeozoic one have been dated. Stratigraphic successions of the Precambrian system were greatly revised. Fairly well correlation of Precambrian rocks from one region to another is done throughout the African continent.
A. Holmes (1963) wrote vividly this situation of drastic revision of the stratigraphy on Precambrian system in Africa as follows: “For me, probably the most dramatic and unexpected surprise of a decade packed with surprises was the announcement of the great age of the Bushveld Complex, about 2, 000 million years, and the consequent realization that the Transvaal Group of strata must be older still. Until 1901 the Transvaal ‘System’ was correlated on lithological grounds with the Palaeozoic Cape ‘System’. Then for over half a century the Transvaal ‘System’ was confidently thought to be of late Precambrian age and, lithologically, a typical representative of the Algonkian. Yet it has turned out to be immensely older than such characteristically Archean rock sequences as the Grenville of the Canadian shield and Svecofennian of the Baltic shield.”
(2) The almost all Precambrian rocks of the African continent have been hitherto considered to represent the Precambrian Craton (Shield). Recently, time, character, and areal distribution of the Precambrian orogenic cycles in Africa have been confirmed, and the following five orogenic cycles have been recognized.
a. Upper Luanyi Cycle (more than 3, 000m. y. ago)
b. Shamvaian Cycle (2, 700-230m. y. ago)
c. Limpopo Cycle (2, 150-1, 650m. y. ago)
d. Kibaran Cycle (1, 290-850m. y. ago)
e. Katangan Cycle (620-485m. y. ago)
The Katangan belt of orogenesis, late Precambrian to early Palaeozoic in age, is shown to be extensively developed throughout Africa. Of recent years, awareness of the significance of this Katangan Cycle has been growing. From an important but essentially local feature, it has grown to the status of a “Pan-African thermo-tectonic episode.”
The Kibaran belt of east and central Africa is also probably extended to the Orange River belt and to Natal in South Africa.
These Katangan and Kibaran belts represent a distinctive regime of younger orogens consisting of mobile zones which have suffered orogenic deformation from time to time during the past ca. 1, 200m. y., and this younger tructural regime is readily differentiated from older cratons which have remained stable over the past ca. 1, 500m. y.
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