Distinction of Tectono-Metamorphic Structures and Mineral Chemistries in Southern Saldania Belt Rocks, at Victoria Bay.

Abstract

Pre-cambrian to cambrian rocks exposed in the southern Saldania belt at Victoria Bay, South Africa are little studied. Two metasedimentary and one igneous formations are exposed in the area of the Kaaimans inlier; 1) the metapelitic to metapsammitic Soetkraal Formation underlying, 2) the quartzo-feldspatic to calcareous Victoria Bay Formation which is overlain by 3) the megacrystic Rooiklip granite gneiss. The three formations are studied because it is unknown what tectono-metamorphic processes have affected them. The history of these rocks form part of the large scale history of the development of Gondwana and Pangea. The geodynamic regime/setting causing the intrusion of the granite into the metasediments including metamorphic reactions is thus far unknown. A literature study, structural fieldwork, petrological and chemical analyses of collected samples including thermobarometry are applied to come to answers for the stated problems. From these analyses a geological map, a structural profile and a tectono-metamorphic interpretation (including geodynamic models) are made. Results lead to the recognition of three metamorphic (M1, M2, M3) and two deformational phases (D1 and D2) in the So-and VB Fms where the Rgg shows evidence for only two metamorphic and one deformational event. All deformation events are coaxial with top to the northeast movement. The deformation phases (D1 and D2) recognised in So and VB Fms are attributed to the Saldania orogeny (540Ma) and the Cape orogeny (250Ma) both have affected the So-and VB Fms. The Rgg has only been deformed by the Cape orogeny. And a north south compression resulted in a pervasive S-C fabric with top to the northeast sense of movement. Finally the Cape orogeny is followed by a recrystallisation metamorphic overprint (M3) creating strain free quartz and calcite bands with straight twins and strong CPO. Thermobarometry could be applied to the So Fm which resulted in temperatures of 445&411°C for sample 25 and 519&490°C, during M1 and M2 for sample 26 separated by about 300m (stratigraphically).