The c. 1.1 Ga Kgwebe metavolcanic rocks exposed in the
northwest of Botswana are late
Kibaran rocks. They represent a bimodal suite of Within-Plate
low titanium-phosphorus (LTP) continental
tholeiites and post-orogenic Within-Plate high-K rhyolites. The chemical
compositions of the
Kgwebe mafic rocks are characterized by low values of Ce/Pb
(<10) and high La/Nb ratios (average
c. 2, maximum 4). Mid-ocean ridge basalts (MORB)-normalized
spidergrams show marked enrichment
in mobile elements (Sr, K, Rb, Ba) and negative anomalies in Nb. These
features suggest they
may have originated in a mantle, enriched during a previous
subduction event. The Kgwebe metarhyolites
are marked by Y>60 ppm, Sr/Y<1, Rb/Th>20 and high
K-contents. They cannot therefore be
the product of melting of sediments or a subducting slab.
It is inferred that they represent felsic
magmas resulting from melting of Mesoproterozoic (Kibaran)
calcalkaline rocks underplated in the
middle and/or lower crust. The Kgwebe bimodal metavolcanic rocks pre-date
the Neoproterozoic
Ghanzi Group rocks which are correlated with the lower
part of the Damara sequence. The chemical
composition and field relations suggest that these metavolcanic
rocks were emplaced during a late orogenic
collision-associated extensional collapse. This collapse affected a crust
thickened during the
Kibaran orogeny in the Namaqua-Natal Belt of southwest Africa.