Geochronological and Petrogenetic Studies of High-Grade Metamorphic Rocks and Intrusives in Namaqualand, South Africa

The metamorphic sequence in the Nababeep district of Namaqualand is considered in terms of major recumbent folding accompanied by hornblende granulite subfacies metamorphism. Rb-Sr isotopic data are presented for thirty-one whole-rock samples of metamorphic rocks and associated intrusives, together with U-Pb isotopic data for nineteen fractions of the constituent zircon and apatite. The metamorphic suite has yielded a Rb-Sr whole-rock isochron age of 1213±22 m.y., with an initial Sr87/Sr88 ratio (Ro) of 0·7191±0·0021 suggesting reworking of pre-existing crustal rocks; data from certain distinctive rock-types indicate that Sr-isotopic homogenization was incomplete during this event. Cross-cutting granites and related rocks give a Rb-Sr isochron age of 1166±26 m.y. (Ro, 0·708±0·001). These events were post-dated by the intrusion of largely dioritic bodies of the cupriferous Noritoid Suite; zircon recrystallization 1070 m.y. ago is interpreted as the age of emplacement of these bodies. On Sr-isotopic and chemical grounds, a source-bed model is tentatively suggested for the Noritoid Suite, involving derivation from gneisses and granulites of intermediate-basic composition within the metamorphic succession. Isotopic events at 1000 and 900 m.y. ago, recorded by zircon and apatite, are believed to reflect the imprint of regional pegmatite emplacement.