A petrogenetic study of anorogenic felsic magmatism in the Cretaceous Paresis ring complex, Namibia: evidence for mixing of crust and mantle-derived components

Paresis is one of a group of Cretaceous ring complexes extending from the coast some 350 km NE across the Damara Belt. It consists of over 90% rhyolites and comendites, with subordinate intrusions of quartz syenite and alkali-feldspar syenite. These felsic units are accompanied by very minor amounts of silica-undersaturated basalt, phonolite and lamprophyre. Located near the edge of the Congo Craton, Paresis is the farthest inland of any felsic complex in the Cretaceous ring complex group. The other ring complexes in the vicinity consist of carbonatites and undersaturated alkaline rocks (e.g., Okorusu, Kalkfeld, Etaneno, Ondurakorume). Geochemical and isotopic data provide evidence for both mantle and crustal components in the sources of the Paresis magmas. The alkaline basalts, phonolite and lamprophyre have overlapping Sr and Nd isotopic initial ratios which plot within the mantle array close to bulk-earth values (epsilon Nd = -0.9 to -2.8 and Sr-87/Sr-86(i) = 0.7042-0.7054) and may indicate a mantle plume component. The rhyolite units comprise metaluminous feldspar rhyolites and peraluminous, more differentiated quartz-feldspar rhyolites. Both units show prominent negative Nb and Ta anomalies on mantle-normalized multielement diagrams and have extremely nonradiogenic Nd ratios (epsilon Nd = -21) and Sr initial ratios of 0.7117-0.7138. These isotopic values suggest a crustal origin from pre-Damara (Early Precambrian) gneisses and granitoids, which are exposed in the Congo craton and related inliers in northern Namibia. The comendites are peralkaline, highly differentiated rocks. In contrast to the rhyolites, comendites lack mantle-normalized Nh and Ta anomalies, have higher HFSE and LREE contents, extreme negative Eu anomalies and epsilon Nd values of -11. Like the comendites, alkali-feldspar syenites are peralkaline, lack Nb and Ta anomalies, have negative Eu anomalies and high HFSE concentrations. Their epsilon Nd values are - 6.5 to - 8. The quartz syenites overlap with the peralkaline units in isotopic composition but they are chemically very different. They are metaluminous, have a low degree of differentiation and have prominent negative mantle-normalized Nb and Ta anomalies like the rhyolites. The Sr-Nd isotopic composition and incompatible element ratios of the comendites, quartz syenites and alkali-feldspar syenites are intermediate between those of the rhyolites and basalts, precluding a simple origin by basalt fractionation or crustal melting. Instead, the data indicate that the comendite and syenites formed from hybrid magmas with both crustal and mantle-derived components. The metaluminous quartz syenites can be modelled by assimilation of pre-Damara crust by basaltic magma. The peralkaline nature and trace element characteristics of comendite and alkali-feldspar syenites, on the other hand, require an alkaline and HFSE-enriched basic endmember, whose fractionated equivalent may be the Paresis phonolite. (C) 2000 Elsevier Science B.V. All rights reserved.