Lower crustal melting and the role of open-system processes in the genesis of syn-orogenic quartz diorite-granite-leucogranite associations: constraints from Sr-Nd-O isotopes from the Bandombaai Complex, Namibia

The Bandombaai Complex (southern Kaoko Belt, Namibia) consists of three main intrusive rock types including metaluminous homblende- and sphene-bearing quartz diorites, allanite-bearing granodiorites and granites, and peraluminous garnet- and muscovite-bearing leucogranites. Intrusion of the quartz diorites is constrained by a U-Pb zircon age of 540 +/- 3 Ma. Quartz diorites, granodiorites and granites display heterogeneous initial Nd- and O isotope compositions (epsilon(Nd) ((540) (Ma)) = -6.3 to - 19.8; delta(18)O = 9.0-11.6parts per thousand) but rather low and uniform initial Sr isotope compositions (Sr-87/Sr-86(initial) = 0.70794-0.70982). Two leucogranites and one aplite have higher initial Sr-87/Sr-86 ratios (0.70828-0.71559), but similar initial epsilon(Nd) (-11-9 to -15.8) and oxygen isotope values (10.5-12.9parts per thousand). The geochemical and isotopic characteristics of the Bandombaai Complex are distinct from other granitoids of the Kaoko Belt and the Central Zone of the Damara orogen. Our study suggests that the quartz diorites of the Bandombaai Complex are generated by melting of heterogeneous mafic lower crust. Based on a comparison with results from amphibolite-dehydration melting experiments, a lower crustal garnet- and amphibole-bearing metabasalt, probably enriched in K2O, is a likely source rock for the quartz diorites. The granodiorites/granites show low Rb/Sr (< 0.6) ratios and are probably generated by partial melting of meta-igneous (intermediate) lower crustal sources by amphibole-dehydration melting. Most of the leucogranites display higher Rb/Sr ratios (>1) and are most likely generated by biotite-dehydration melting of heterogeneous felsic lower crust. All segments of the lower crust underwent partial melting during the Pan-African orogeny at a time (540 Ma) when the middle crust of the central Damara orogen also underwent high T, medium P regional metamorphism and melting. Geochemical and isotope data from the Bandombaai Complex suggest that the Pan-African orogeny in this part of the orogen was not a major crust-forming episode. Instead, even the most primitive rock types of the region, the quartz diorites, represent recycled lower crustal material. (C) 2003 Elsevier Science B.V. All rights reserved.