Ion probe dating of the Achab Gneiss, a young basement to the Central Bushmanland Ore District?

Two samples representing the banded and megacrystic varieties of Achab Gneiss, long regarded as the basement of the Central Bushmanland Ore District in the Bushmanland Terrane of the Namaqua-Natal Province, were dated by ion probe. The megacrystic sample contained zircon xenocryst cores with a consistent concordia age of 1194 ± 15 Ma. This probably represents assimilated wall rocks and corresponds in age to the Little Namaqualand Suite. Concordia ages reflecting Achab magmatic emplacement are 1166 ± 13 and 1163 ± 11 Ma, respectively. The two varieties thus belong to the same magmatic suite, although they are probably not comagmatic. The previous 2020 ± 150 Ma whole-rock Pb–Pb isochron is interpreted as a mixing line involving Archean and Proterozoic source rocks. However the Achab Gneiss might still retain its status as the basement of the Bushmanland Ore District, because at least parts of the supracrustal rocks are younger than 1166 Ma. The F1–F4 regional deformations documented in the Achab Gneiss all postdate 1166 Ma. Metamorphic zircon rims dated by discordia intercepts at 1030 ± 18 and 1038 ± 14 Ma, are inferred to reflect the first regional metamorphism to affect these rocks. This corresponds to the high-T, low-P late-Namaqua metamorphism recognized throughout the Bushmanland Terrane of the Namaqua-Natal Province.