Kimberlites and related rocks of the Nama plateau of south west Africa

The eruptive rocks of the Nama Plateau are divided into : (i) kimberlites, (ii) carbonatitic dykes, (iii) monticellite peridotite and (iv) rocks belonging to the
Brukkaros complex. The Gibeon kimberlite province contains at least 46 pipes and 16 major dykes of mainly basaltic kimberlite.

The inclusions in kimberlite are divided into : (i) cognate nonomineralic nodules consisting of kelyphitized titanium-bearing pyrope, coated diopside, serpentinized olivine, coated ilmenite, and composite ilmenite-encased olivines; and (ii) accidental polymineralic nodules consisting of garnet peridotite, garnet pyroxenite, retrograde eclogite and granulite.

The large phreatic volcano - Gross Brukkaros- is formedby a dome of Nama sediments. The centre of the dome contains depression, 1 1/2 miles in diameter, underlain by fine grained clastic rocks (microbreccias) derived from brecciat –tion of the Nama sediments. It is surrounded by 45 satellite vents and numerous radial dykes, both filled with coarse breccias 1n a carbonate-rich matrix. An outcrop of a monticellite peridotite occurs in a vent 3 miles south of Mt. Brukkaros.

It is concluded that: (1) kimberlites are formed by zone refining of a magna derived from a very deep source in the mantle in places beneath very stable areas of the crust (cratons and platforms), and (ii) kimberlite may develop a volatile-rich carbonatitic top fraction, which under certain circumstances causes phreatic explosions (as happened in the case of Mt. Brukkaros). The top fraction, depleted of volatiles, is represented by a monticellite peridotite.