Alkaline igneous rocks of the coastal belt, south of Luderitz, South West Africa

Abstract

Abstract provided by author:

The Outer Foyaite interfingers repeatedly with thinly bedded sediments at the contact zones. Assimilation - fractional crystallization processes operating at these sandstone-foyaite contacts have generated a suite of pulaskites, peralkaline nordmarkites, and peralkaline granites, which intrude the sedimentary rocks as thin sheets. Similarly, at dolomite-foyaite contacts, melteigite-ijolite-urtite and shonkinite-pulaskite suites have been generated

Metasomatic effects accompanying the foyaite emplacement are also evident. The Roof Zone sediments have undergone fenitization, giving rise to aegirine- and arfvedsonite-bearing fenites. At the West Ridge contact, Ca-Mg metasomatism has dominated over alkali metasomatism, resulting in the production of diopside-quartz-perthite and diopside-perthite metasomatites. In the NE contact zone the dolomitic sediments have been metamorphosed under conditions of high fH20/fC02 and at T =450° to 525°C, to produce forsterite- and diopside-bearing assemblages

The Pomona complex is a well developed ring complex, with a distinct radial dyke pattern. Successive intrusion of the Outer Syenite, the Inner Syenite; the Hub Syenite, and the Outer Ring Dyke define a silica-oversaturated, peeralkaline trend which culminates in the widespread development of quartz-bostonite dykes. Quartz-feldspar porphyry dykes are associated with explosively emplaced igneous breccias at the NE edge of the complex. Tinguaite dykes and a small plug of nepheline syenite intrude the Inner Syenite in the core of the complex. Exposures of Biotite-rich Monzonite occur within the Inner Syenite outcrop area, and possibly represent a cumulate developed during the evolution of the suite of syenites