GEOLOGY OF ZAMBIA
BANGWEULU BLOCK (BB)
Cratonic unit stable since 1750Ma covering 150000km2 in Northern Zambia. It is
surrounded by mobile belts Ubendian to the NE, W and SE, Kibaran and Irumide
belts respectively. To the South is the Zambezi belt and to the SW, the Lufilian arc.
Introduced only in 1972. Before 1978, BB was thought to be archaen because of its
configuration in relation to mobile belts.
CRYSTALLINE BASEMENT OF THE BB
This is composed of:
Schist belts
Granitoids
Volcanics
1. SCHIST BELTS: On the Eastern side and scattered, linear, elongate belts, lens
shaped. They are poorly exposed and only three larger ones are observed. Their size
range into tens of kilometres long to inclusions in granitoids.
COMPOSITION:
Various types of schists: micaceous, quartzo-feldspathic and chlorite schist, with
muscovite, quartz, chlorite and epidote (low grade). This is a bit comparable with
archaean Greenstone belts.
Structure:
Allschists in the schist belts trend E to W. Contactcts with granitoids are gradational.
2. GRANITOIDS
They occupy the largest part of the block. Various types are distingushed:
(i) Concordant Granitoid Batholiths.
COMPOSITION: Usually from granodioritic to tonalitic.
TEXTURE/STRUCTURE: Fine to coarse grained, unfoliated to foliated. Many
different phases grading each other. Toward schist belts from granitoids, there are
increasing schist inclusions in the granites. Very similar to the diapiric granites in the
archaen, thereby supporting that the granites are intrusive into the schists.
(ii) Discordant Granite Intrusions
COMPOSITION: Leucocratic granite
TEXTURE/STRUCTURE: Lath shaped, with sharp contacts as opposed to
gradational. Often porphyritic with K-feldspar crystals of up to 15cm wide and more
or less parallel.
3. VOLCANICS
(These are reffered to as Luapula-Tanganyika volcanics or porphyries in old
literature). In later literature, they are known as volcanics of the BB.
CHARACTERISTICS
They form a more or less continous belts W and N of the Mporokosso basin. To the
South and east, they become reduced to smaller units/ belts, rafts or septac.
TRENDS: NNE, unrelated to the schists. Contacts with surrounding granites
interdigitating or interfingering, implying that thevolcanics are intruded by the
granites.
TYPES OF VOLCANICS
Mainly pyroclastic rocks as opposed to lavas I.e. agglomerate, tuffs, volcanic breccias,
ignibrite ( minor lavas, more silica). Also hypabyssal rocks (rhyolite, andesite)
GEOCHEMISTRY
The geochemical signature gives indications of how thos rocks were formed,
e.g.Igneous rocks: magma origin and evolution-upper mantle, partial melting or a
combination of the two. It can also give an answer regarding the origin of rocks
whether from one parent magma or not. One means to check is by means of variation
diagrams. Smooth curves indicate good relationship between them.
CONCLUSIONS:
Volcanics and granitoids are consanguinoussuite in the BB, they originated from one
parent magma.
Modal composition also also coincide on classification, both volcanics and granitoids
plot in the high K calc alkaline series
of the active continental margin and Island arc.
AGE DATES
(87Sr/86Sr)0
1. <0.703 Source upper mantle, no contamination
2. 0.703 - 0.730 Source upper mantle contaminated with crustal material
3. > 0.730 Source Crustal origin, anatexis
AGE OF THE BB
From Mansa volcanics, SE of Mbala, all ages range between +/-1800 - 1869 Ma. This
indicates that largely the BB is of Ubendian/Eburnean age.
UBENDIAN BELT
On the NE of the block, extending 1000km with prominentlyNW-SE trend and
extends into the DRC where it is called Ruzizi. Thus it lies between the Tanzania
shield and the BB.
It is characterized by high grade gneisses, granulites, amphibolites and migmatites
It is the oldest (2055 Ma). In Zambia, the granulites have E-W trend.
Conclusions:
Precusors of the Schist belt/Ubendian gneisses are pre-Ubendian. There must have
beenpre Ubendian basement, but could it have been Archaen?
Initial 87Sr/86Sr gives 0.706-0.707. This gives cae 2: presence of crustal material in
granitoids.
1. Pre Ubendian sedimentation/Volcanic precursorsof schist belts on Pre cambrian
basement?
2. Ubendian orogeny: Metarmophism, deformation, magmatism,
3. Widespread Plutonism1860-1600Ma resulting into Mambwe granodiorite and
Luchewe granite
4. Late Ubendian Plutonism at approximately 1830 Ma.Granitoids on the Western
Side of the BB and acid volcanics that are largelypyroclastics.
The Ubendian belt is thought of being a subduction zone with the Tanzanian craton
subducting beneath with the BB representing an island arc.
5. 1838+/-86Ma Late/ Post Ubendian magmatism.
