Geochemical and tectonic evolution of the Damara Belt, Namibia

Abstract

Summary The Damara is a high-grade, apparently intracratonic, Pan-African Orogenic Belt which has been the subject of unusually detailed geochemical and petrological studies. Basement inliers (2.0–1.0 Ga) underlie predominantly continental sediments, deposited in the period 1.0–0.7 Ga, and these are overlain unconformably by molasse-type deposits at least 580 Ma old. Basic volcanic rocks are rare, but the centre of the Damara Belt is characterized by large volumes of syn- and post-tectonic granitic rocks intruded between 750 and 450 Ma. Many were derived from the pre-existing crust as indicated by their initial 87 Sr/ 86 Sr = 0.705–0.76, ɛ Nd = 0 to −17, δ 18 O = +7.1 to +15.2‰. Moreover, combined Nd- and Sr-isotope studies on sediments, granites and basement rocks suggest that at 500 Ma, in the centre of the belt, 2.0 Ga basement was sandwiched between rocks with younger model ages above and below. Metamorphic temperatures and pressures reached ∼620°C and 7–8 kb in the S where the Rb/Sr biotite ages are 500–480 Ma and ∼660°C and 3–4 kb in the centre of the belt where the biotite ages are 460–440 Ma. Prograde metamorphism (and partial melting) in the central part of the belt continued after the closure of the biotite ages along the margins and since the former is a zone of domal uplift structures, the crustal melting and diachronous metamorphism are attributed to differential uplift in an area of thickened continental crust. There is good evidence that the evolution of the Damara was intimately linked with that of the N-S trending belt now exposed in South America, although in detail the relationship between metamorphism and the inferred tectonic movements is still poorly understood. Nonetheless, since preliminary data indicate that the N-S belt may have formed along a destructive plate margin, it is speculated that for at least some of its history, the Damara evolved in an extensional intracontinental basin behind a zone of active subduction.