Etendeka volcanism of the Goboboseb Mountains and Messum Igneous Complex, Namibia. Part II: Voluminous quartz latite volcanism of the Awahab Magma System

The Goboboseb-Messum volcanic centre is the source of two voluminous silicic eruptive sequences, the Goboboseb quartz latites (Units I-III), and the Springbok quartz latite unit (both within the Awahab Formation). Intrusive equivalents exist as plugs and a laccolith peripheral to the Messum Complex. The recognition of correlatives of these quartz latite units in the southeastern Parana suggests eruptive volumes of similar to 2320 km(3) (Goboboseb units) and 6340 km(3) (Springbok unit). The latter is thought to be a single eruptive event. Phenocryst assemblages are plagioclase (An(51-63)), pyroxene, titanomagnetite and apatite. Pyroxene assemblages range from augite, to augite + pigeonite, to pigeonite, to pigeonite +/- hypersthene, the assemblages changing progressively from the Goboboseb unit through to the Springbok unit. Although pyroxene phenocrysts from individual samples are compositionally very uniform there is a small increase in Fe through the sequence, attributed to decreasing temperature (+/- pressure). Thermometry suggests melt temperatures >1000 degrees C. Many plagioclases contain abundant glass inclusions of three compositional types, thought to result from active disequilibrium melting at magma chamber walls. Relatively small, but systematic changes in whole-rock composition occur stratigraphically from the lowest Goboboseb unit through to the Springbok unit and to their correlatives in the Parana, best shown by SiO2 (67-71%) and FeO* (5.4-7.4%) which increase and decrease, respectively, in the progressively younger eruptive phases. P, Ti, Y, Zr, Nb and Cu are positively correlated with FeO*. Crust-normalized spidergram plots indicate strong negative Sr anomalies, accompanied by significant Eu/Eu* anomalies (0.62-0.67). The quartz latite melts can be interpreted in terms of large-scale assimilation-fractional crystallization (AFC)-style processes, involving high degrees of lower- and upper-crustal melting, with thermal and material input from hybridized LTZ.L-type basaltic magmas (Part I). Thermal source is inferred to be the Tristan plume. The crustal end-member is thought to be the mid-Proterozoic restite source of the Damara granites, although some shallower crustal input is also likely. Modelling suggests the source may be similar to A-type granites and charnockites (i.e. relatively REE and HFSE enriched). Available seismic data suggest a simple velocity crustal profile, possibly the result of the massive crustal and uppermost mantle melting that accompanied the evolution of the Awahab magma system.