Fluid-induced charnockite formation post-dating prograde granulite facies anatexis in southern Natal metamorphic province, South Africa

• Main Author: Saunders, Brenton Mark
• Published: 2014
• Subjects: Stratigraphic geology - Proterozoic; Geology - Kwazulu-Natal - Port Shepstone; Sedimentary rocks - Namaqua-Natal mobile belt; Igneous rocks - Namaqua-Natal mobile belt; Charnokite - Namaqua-Natal mobile belt
• Online Access: http://hdl.handle.net/10210/10370

M.Sc. (Geology) === The Proterozoic Namaqua-Natal Mobile Belt of southern Africa represents a succession of high grade metamorphosed igneous and sedimentary rocks rimming the southern and western extents of the Kaapvaal craton. Different petrological processes associated with the onset of granulite facies metamorphism were investigated with the emphasis on the processes on anatexis and migmatitization and the influence of the fluid phase on these processes. The investigation took place in the Margate Terrane of the Southern Natal Metamorphic Province. The Umzimkulu and Louisiana Quarries near Port Shepstone formed the bulk of the field area for this investigation. The Umzimkulu and Louisiana quarries expose two s-type granitic lithologies, namely, the Glenmore Biotite Gneiss (GBGn) and the slightly younger, intrusive Margate Leucogranite (MLGn), both of which have been metamorphosed to granulite facies. Geothermobarometric calculations on the metamorphism of the Margate Terrane all indicate temperatures and pressures of peak metamorphism be 850oC+50oC at +6 kbar. The high grade metamorphic history of the lithologies is recorded by the formation of concordant, lens-shaped, prograde anatectic leucosomes. The leucosomes are concentrically surrounded by biotite selvage zones, followed by melanosomes, and lastly undisturbed host rock material. This investigation revealed that the formation of both the charnockite veins and the prograde leucosomes occurred through processes of anatexis. Field relations suggest that the charnockitic veins formed in situ, and were structurally controlled, which is evident from their linearity. Petrographic and geochemical data provide evidence for melt involvement. This is in strong contrast to theories of charnockite formation ascribed to subsolidus granulite formation by the flushing of host lithologies by a pervasive, carbonic fluid, as has been suggested to have occurred in Southern India. The so-called "incipient charnockites" of Southern India are both morphologically and geochemically similar to the charnockite veins described in the SNMP, suggesting that an anatectic origin may be common to both.