Miombo Ecoregion Vision Report - Biodiversity Foundation for Africa

Miombo Ecoregion Report, page 11
The mobile belts range from late Archaean to early Palaeozoic, with age groupings around 2700
Ma, 2000–1800 Ma, 1400–1100 Ma and 950–450 Ma. In places they show evidence of
metamorphism and deformation of more than one age. Rocks include granitic gneisses,
metasediments and metavolcanics all at high metamorphic grade. Perhaps the oldest of these, the
well-documented but still enigmatic Limpopo Belt, separates the Zambezi craton from the
Kaapvaal craton to the south. It consists of a central zone of Archaean metasediments, with a
major development of quartzites, flanked to the north and south by high grade rocks of the
adjacent cratons. Metamorphism and tectonism is late Archaean, at around 2700 Ma, followed by
a second, early Proterozoic, phase at 2000 Ma.
Emplaced at 2580 Ma, and affording a magnificent marker for the end of the Archaean and the
beginning of the Proterozoic, is the Great Dyke of Zimbabwe. Funnel-shaped in cross-section in
its present erosion plane, it is not a true dyke but a NNE-trending line of contiguous, elongate,
mafic/ultramafic, layered intrusions that almost bisects the craton. Gabbroic rocks form the upper
part and ultramafic rocks, mainly pyroxenite and dunite, the major and lower part of the layered
sequences. Weathering of the dunite has produced a surface serpentisation and nickel-rich soils
that characteristically support unique, nickel-tolerant vegetation.
Proterozoic sedimentary and volcanic rocks of various ages occur intermittently overlying the
Archaean cratons. Most of these extend into the marginal Proterozoic mobile belts to become
highly deformed and metamorphosed; in places they have also been thrust over the adjacent
cratons during the deformation. Among these is the early Proterozoic Magondi Supergroup of
basalts, quartzites, dolomitic marbles and metapelites, deposited on the NW side of the
Zimbabwe craton. Orogeny around 2000–1800 Ma produced the Magondi Mobile Belt, involving
the deformation of the Magondi rocks and underlying basement. On a larger scale, this is part of
more widespread orogenic events affecting southern, central and eastern Africa at this time. On
the northern flank of the Zimbabwe craton, the various metasediments and infolded basement of
the Zambezi Belt were extensively deformed around 850 Ma. The belt cuts across the earlier
Magondi Belt and extends west into southern and central Zambia to the Copperbelt. Eastwards it
links with the N-trending Mozambique Mobile Belt, which straddles Zimbabwe's eastern
international border with Mozambique and extends northwards as a major structural zone into
Zambia, Tanzania and Uganda. In eastern Zimbabwe, the late Proterozoic Umkondo Group
forms a cratonic cover of quartzites, shale and minor volcanics, and extends eastwards to become
an integral part of the Mozambique Belt.
Cratonic granite-greenstone terrains, mobile belts and cratonic cover rocks together form the
Precambrian Basement platform on which the Phanerozoic rocks were deposited. This basement
is host to economically important minerals – gold in the Archaean greenstone belts; chromitite
and platinum group minerals in the Great Dyke; and copper in the Proterozoic, especially in the
Copperbelt of Zambia and Katanga Province of the DRC.
The most conspicuous rocks of Phanerozoic age in south and central Africa belong to the Karoo
Supergroup. These were deposited on Gondwanaland from the late Pennsylvanian (Upper
Carboniferous) to Jurassic Periods, a span of some 100 Ma, as this giant continent moved slowly
across the South Pole and thence northwards to straddle the Equator before splitting to form the
continents that are familiar today. The sequence of sedimentation in the intracontinental basin,
from all the modern component fragments, reflects the steadily changing climate from frigid to
cool temperate to warm temperate to hot desert. Finally, cracks opened across Gondwanaland
and permitted the eruption of the vast numbers of basalt flows that constitute the uppermost part
of the Karoo Supergroup. Thereafter the modern continents moved to their present positions.