PETROLOGICAL and GEOCHEMICAL CHARACTERISATION of the KGWEBE VOLCANIC FORMATION in the ghanzi-chobe BELT PORTION of the KALAHARI COPPER BELT, western Botswana.

Abstract

A combination of petrography and geochemistry was utilised to investigate the magma sources, magma evolution and petrogenesis of the Kgwebe Volcanic Formation (KVF) rock units. Based on their texture, the KVF rock units comprise amygdaloidal rocks, porphyritic mafic flows, pyroclastic rocks, porphyritic subvolcanic bodies, quartz-feldspar porphyries and clastic sedimentary rocks, that are intercalated with amygdaloidal rocks. The amygdaloidal rocks are commonly crosscut by calcite veins and characterised by amygdales (filled vesicles) set in the hyalopilitic groundmass. The porphyritic mafic flows are composed of plagioclase laths set in an aphanitic groundmass. The pyroclastic rocks are generally characterised by up to 75% of both mineral and lithic fragments originating from explosive volcanic activities. Porphyritic subvolcanic bodies are characterised by high content of subrounded epidote (after plagioclase) set in a microcrystalline groundmass. The quartz-feldspar porphyries are characterised by quartz and feldspar phenocrysts in variable proportions set in a fine-grained groundmass of variable colour. Rock units of the KVF represent two geochemically contrasting series that are not co-genetic and are derived from different geotectonic settings. The amygdaloidal rocks, porphyritic mafic flows, pyroclastic rocks and porphyritic subvolcanic bodies are sub-alkaline and basaltic in composition, whereas the quartz-feldspar porphyries span from the rhyolite/dacite (mostly) to the andesite/andesite basalt (accessorily) field compositions. Likewise, while quartz-feldspar porphyries are consistently high-K calc-alkaline bodies, their spatially associated amygdaloidal rocks, porphyritic mafic flows, pyroclastic rocks and porphyritic subvolcanic bodies counterparts are calc-alkaline. Similarly, quartz-feldspar porphyries are within plate bodies, whereas the more mafic KVF units are arc-related. Furthermore, the ferroan quartz-feldspar porphyries generated from the crust and the magnesian amygdaloidal rocks, porphyritic mafic flows, pyroclastic rocks and porphyritic subvolcanic bodies are mantle-derived, with however, significant crustal contamination fingerprints. Textural evidence (resorbed, embayed and rounded phenocrysts) and the inferred high-water content (as supported by the abundance of plagioclase phenocrysts) also suggest a rapid magma ascent and magma mingling occurred during the KVF eruption, which was both effusive and explosive. Concomitant volcanism and sedimentary processes likely occurred as supported by the intercalation of clastic sedimentary rocks within volcanics of the KVF.