The genesis of the Gaborone rapakivi granite complex in southern Africa

Abstract

The Gaborone Granite is a mushroom-shaped intrusion with a surface area of over 5000 km 2 . The intrusion is layered, consisting of a central core of rapakivi granite (Thamaga Granite) surrounded by successive shells of an equigranular leucocratic granite (Kgale Granite), a porphyritic granophyre or microgranite (Ntlhantlhe Microgranite) and an outermost zone of massive felsite (Kanye Volcanics). The whole lithological sequence is deduced to have been derived from a single, highly viscous magma body emplaced into the crust at a high level. The genesis is proposed as follows. The outer felsites represent quenched primary magma with the underlying porphyritic granophyres having formed during a subsequent tranquil period after emplacement. The rapakivi granite core was also of early crystallization above the floor but with significant textural characters impressed during updoming in the late crystallization stage. Residual liquid rich in SiO 2 , K 2 O and volatiles migrated upwards to form the Kgale Granite. The Gaborone Granite was emplaced in the Kaapvaal Craton at about 2400 Ma and its morphology was controlled by pre-existing structures in the crust. The country rock consists of Archaean gneisses, Lobatse Volcanic Group supracrustals and locally Transvaal Supergroup strata. Chemically the Gaborone Granite is identical to other early Proterozoic non-orogenic granites of the Kaapvaal Craton. It is also similar in many respects to the Fennoscandian rapakivi granites