Geochronological constraints on the Hartbees River Thrust and Augrabies Nappe: New insights into the assembly of the Mesoproterozoic Namaqua-Natal Province of Southern Africa

Abstract

The region around Augrabies in the Namaqua sector provides insights into the processes that formed the Namaqua-Natal tectonic Province. Progressive ductile shear deformation during the long lasting Namaqua Orogeny was coeval with the global Grenvillian Orogeny. The Mesoproterozoic Namaqua-Natal Province is composed of severely deformed and metamorphosed rocks in terranes bounded by major thrust and shear zones. The interpreted research results deal with events around the Hartbees River Thrust, a suture zone where the granulite grade Grünau (Kakamas) terrane was overthrust onto the amphibolite grade Bladgrond (Bushmanland) terrane. The series of distinct deformation stages D1–D6 that have been identified during previous research in the western regions of the Namaqua sector, is also discernible in the study region around Augrabies. Microbeam geochronological techniques were used to date zircons from four samples of a granitic rock (the Karama’am Augen Gneiss) intrusive into the Hartbees River Thrust zone, and of a sample of granite (the Augrabies Granite) from the core of a large allochthonous sheath fold nappe in the Grünau terrane. The mean crystallisation age of the Karama’am intrusive is 1108±4Ma and of the Augrabies Granite 1168±6Ma. The sheath fold core of the latter granite is enclosed in the sheetlike Rooipad Granite (Riemvasmaak Granite) previously determined to be 1155±7Ma. Crustal residence times deduced from Lu–Hf isotopic compositions of zircon cores and xenocrysts, cluster around 1710Ma for the Augrabies Granite and about 200Ma older for the Karama’am Granite Gneiss (with the oldest date at 2069Ma). Collectively the available data enable reconstruction of the sequence of Namaqua Orogeny deformation stages: terrane assembly (D1) at ∼1195Ma with metamorphism at 1191±12Ma, pervasive folding and thrusting (D2) at 1168±6Ma, formation of major sheath and coaxial folds (D3) at 1155±7Ma, late open folds (D4) at 1090±16 and a late metamorphism (coeval with major shear zones D5–D6?) at 1018±11 to 1024±14Ma.