The geochemistry of the Karoo igneous volcanic and intrusive rocks of Botswana

Abstract

The Mesozoic basalts and dolerites of Botswana underlie an estimated area of 150 000km2 and form part of the Karoo Igneous Province of southern Africa. The distribution of Karoo basalts in Botswana is limited essentially to three main sub-basins, the Central Kalahari Subbasin, northern Botswana and the Tuli Syncline and a major dyke swarm, with a WNW strike, extends across Botswana from the Namibian to Zimbabwean borders. This dissertation is a reconnaissance study which concentrates on the recognition and definition of distinct geocheinical sub-groups within the Karoo volcanic and intrusive rocks of · Botswana. 128 new whole rock samples were analyzed for major and trace element concentrations, in addition to the 70 whole rock analyses from Botswana which were available in the UCT database.· Mineral analyses and rare earth element compositions for selected samples are also presented. The basalts and dolerites of Botswana are assigned to one of the three geochemical lineages, i.e. the low-K20, the high-K20 and the felsite lineages on the basis of Si02, MgO and K20 concentrations. A number of distinct geochemical sub-groups· are recognised within these lineages according to whole rock compositions, normative mineralogy, petrography and outcrop character. The low-K20 lineage is subdivided into two main sub-groups on the basis of the Ti02 and Zr concentrations, i.e. the LTZ- and HTZ-type basalt and dolerite sub-groups. The LTZtype basalt sub-group (with ~2% Ti02 and ~250ppm Zr) represents the bulk of the Botswana dataset where the LTZ basalts of Botswana are shown to be lateral equivalents to the Lesotho Formation basalts of the Central Karoo area, considerably expanding the known outcrop area of this basalt type. Two dolerites are the only samples of intrusive equivalents of this voluminous LTZ basalt type in Botswana.,