Abstract

The Tsomtsaub pluton (central Damara orogen, Namibia) consists of quartz diorites, granodiorites and granites. Intrusion ages of the undeformed and unmetamorphosed quartz diorites and granites are constrained by LA-ICP-MS U-Pb zircon analyses, which yielded ages of 541 ± 3 Ma (quartz diorites) and 506 ± 6 Ma (granodiorites), respectively. The older age predates the main phase of high-T regional metamorphism and the younger age fits to the inferred age of the main peak of regional metamorphism. Elemental and isotope variations are decoupled, indicating that combined assimilation-fractional crystallization processes were not important during evolution of these rocks. Therefore, Sr-Nd isotope compositions (quartz diorites: ɛNd(init.): −1.5 to −1.8; 87Sr/86Sr(init.): 0.7049 to 0.7058; granodiorites/granites: ɛNd(init.): −2.7 to −5.3; 87Sr/86Sr(init.): 0.7044 to 0.7096) indicate that generation of the quartz diorites and granodiorites involved distinct sources. Lead isotope ratios show some overlap between quartz diorites (206Pb/204Pb: 18.36 to 18.38; 207Pb/204Pb: 15.66 to 15.67; 208Pb/204Pb: 38.09 to 38.12) and granodiorites/granites (206Pb/204Pb: 18.39 to 18.78; 207Pb/204Pb: 15.62 to 15.67; 208Pb/204Pb: 37.90 to 38.17). However, a large variation in 207Pb/204Pb at relatively constant 206Pb/204Pb is apparent for all rock types. Based on a comparison with results from fluid-absent melting experiments using amphibolites, a lower crustal metabasalt, probably enriched in K2O, is a likely source rock for the quartz diorites. For the granodiorites, a high-K andesitic or tonalitic source is likely. The granites are interpreted as fractionation products of the granodiorites. Semi-quantitative pressure-temperature estimates using Qtz-Ab-Or systematics and Zr saturation temperatures indicate pressures in excess of 5 kbar and temperature of c. 900°C, placing the site of melting in the lower crust. The weakly evolved Sr-Nd isotope ratios and the moderate radiogenic Pb isotope ratios are consistent with juvenile mafic to intermediate sources for the quartz diorites and granodiorites. This contrasts with previous suggestions favouring an origin of quartz diorites and granodiorites by either melting of an enriched mantle source during Pan-African times followed by AFC processes, or by melting of ancient, depleted mafic crust.

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