Geochemistry and tectonic significance of the Mesoproterozoic Kgwebe metavolcanic rocks in northwest Botswana: implications for the evolution of the Kibaran Namaqua-Natal Belt

Abstract

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.

A review of the Mesoproterozoic to early Palaeozoic magmatic and tectonothermal history of south–central Africa: implications for Rodinia and Gondwana

“…1). The rift stratigraphy includes tholeiitic Kgwebe Volcanic series and rhyolite and granitic stocks (Kampunzu et al 1998). The presence of Palaeoproterozoic zircon xenocrysts in volcanic units (Singletary et al 2003) Fig.…”

U–Pb zircon geochronology and petrology of granitoids from Mitwaba (Katanga, Congo): implications for the evolution of the Mesoproterozoic Kibaran belt

“…Kibaran metasedimentary and igneous rocks were affected by two main compressional events (e.g. Kampunzu et al, 1986Kampunzu et al, , 1998Rumvegeri, 1991). The timing of these contractional events was inferred to be in the 1400-1000 Ma bracket (e.g.…”

“…Cahen et al, 1984). Kampunzu et al (1998) consider that the first event marks the development of an active continental margin, between 1.4 and 1.25 Ga. The second event is taken to mark a continental collision (1.25-1.0 Ga), with production of late-to post-kinematic "fertile" granites.…”

“…Early Kibaran granitoids and related orthogneisses were emplaced during a protracted period between 1.4 and 1.25 Ga (e.g. Cahen et al, 1984;Kampunzu et al, 1998). This suggestion was mainly based on: (1) lowprecision whole-rock Rb-Sr isochron and K-Ar dates (e.g.…”

The internal structure and geotectonic setting of the Xade and Tsetseng complexes in the western most part of the Kaapvaal Craton.

“…In this westernmost area the Umkondo magmatism is bimodal with felsic and mafic lavas and volcaniclastic sediments (Modie, 1996;Schwartz et al, 1996;Kampunzu et al, 2000;Singletary et al, 2003). This off-craton magmatism is interpreted as A-type, derived by partial melting of relatively juvenile crust (Kampunzu et al, 1998;Hanson et al, 2006). By contrast, the Xade and Tsetseng complexes are more typically wholly mafic which tends to confirm their cratonic setting.…”