Peraluminous metamorphic rocks from the Namaqualand Metamorphic Complex (South Africa): Geochemical evidence for an exhalation-related, sedimentary origin in a Mid-Proterozoic rift system

Abstract

Stratiform peraluminous rocks in western Namaqualand — mainly sillimanite rocks and sillimanite-corundum rocks — are intercalated into metapelites of the Mid-Proterozoic Bushmanland Sequence. Exhalative concentrations of siliceous, B-rich, zincian, calcareous, ferruginous and phosphatic composition coprecipitated locally during deposition of the peraluminous rocks. As fluorine is a conspicuous component in most peraluminous rocks, hydrothermal alteration was a primary process prior to transport. Premetamorphic Al enrichment in these rocks was also partly due to transport as aluminous clays and hydroxides and to selective re-deposition. The most probable precursor rocks before Al enrichment were pelites, as shown with the aid of discrimination diagrams. Formation and distribution of the peraluminous rocks is thought to have occurred near hydrothermal vents during an intermediate stage of rifting that characterized the deposition of the Bushmanland Sequence. This event preceded formation of a major stratiform base-metal sulphide horizon.