Isotope and Trace Element Geochemistry of Cretaceous Damaraland Lamprophyres and Carbonatites, Northwestern Namibia: Evidence for Plume--Lithosphere Interactions

Abstract

Major and trace element and radiogenic and stable isotope data are INTRODUCTION reported for Cretaceous lamprophyres and carbonatites associated The close association of alkaline silicate rocks (ijolites, with alkaline igneous complexes in Damaraland, northwestern nephelinites and phonolites) and carbonatites within cerNamibia. Lamprophyres, emplaced as late-stage dykes, stocks and tain intra-plate intrusive complexes is a well-recognized diatremes, include both alkaline and ultramafic varieties. Many of and geographically widespread phenomenon (Le Bas, the latter contain abundant carbonate-rich ocelli. The carbonatites 1987; Barker, 1989). However, the genetic relationship are primarily sovites and Fe-rich beforsites emplaced as plugs and between them and the processes involved in their gendykes. Both lamprophyres and carbonatites are characterized by eration are still not fully understood. Considerable debate strong, but variable, enrichment in incompatible trace elements (e.g. exists as to whether carbonatite magmas are derived via La 45–7232 ppm; Nb 89–1961 ppm; Sr 700–43 000 ppm), the direct partial melting of mantle peridotite (e.g. Gittins, and variable but coherent initial Sr/Sr (0·70351–0·70468), 1988; Green & Wallace, 1988; Wallace & Green, 1988; Nd/Nd (0·51244–0·51266) and Pb isotope ratios (e.g. Yaxley et al., 1991), or whether they represent mantlePb/Pb = 17·77–19·99). Individual lamprophyre types derived magmas which have undergone modification at show distinct, but restricted, ranges in isotopes and incompatible lower pressures by a process of silicate–carbonate liquid element ratios (Zr/Nb = 0·9–2·0; Y/Nb = 0·15–0·34; La/ immiscibility from, or extensive fractionation of, a carYbn= 20–58), indicating derivation from a trace element enriched, bonated silicate parental magma (e.g. Middlemost, 1974; heterogeneous mantle source. Lamprophyres and carbonatites found Koster van Groos, 1975; Le Bas, 1987, 1989; Kjarsgaard within the same complexes have similar isotope ratios, indicating & Hamilton, 1989; Lee & Wyllie, 1994). derivation from a common source. Isotope and trace element variA linear series of anorogenic igneous complexes extend ations, coupled with the temporal and spatial association with the through Damaraland in northwestern Namibia (Fig. 1) palaeoposition of the Tristan plume, are thought to indicate that and provide an example of the contiguous alkaline silicate the Damaraland lamprophyre and carbonatite magmatism formed and carbonatite lithologies. The composition of these as a consequence of melting of metasomatic vein material introduced complexes shows a general northeasterly spatial trend into an isotopically depleted subcontinental lithospheric mantle by from granitic associations close to the Atlantic coast, alkaline melts or fluids derived from the upwelling Tristan mantle through differentiated basic, to peralkaline and carplume at the time of continental break-up. bonatitic compositions located ~250–350 km inland (Martin et al., 1960). In addition to the silicate rock– carbonatite associations within three of the Damaraland complexes, carbonate-rich globular structures (ocelli) have also been described within the late-stage nephelinitic