Petrology of basement-dominated terranes: II. Contrasting isotopic (Sr, Nd, Pb and O) signatures of basement-derived granites and constraints on the source region of granite (Damara orogen, Namibia)

Abstract

Major and trace element and Nd, Sr, O and Pb isotope data from granites of the high-grade central part of the Damara orogen (Khan and Outlet gorge areas, Namibia) indicate a dominantly lower crustal origin. Based on their appearance in field, the granites can be divided into red granites, white granites and grey granodiorites. Red granites and some of the white granites from the Khan area are isotopically evolved (initial ε Nd: −12.5 to −18.9) and were likely derived from metaigneous sources with late Archaean to early Proterozoic crustal residence ages. Other white granites are less evolved (initial ε Nd: −6.5 to −8.8) and were likely derived from metasedimentary sources that are similar to the country rock metapelites. Grey granodiorites from the Khan and Outlet gorge area are also isotopically evolved (initial ε Nd: −9.9 to −13.1) but are derived from metaigneous sources with younger, late Proterozoic crustal residence ages. Major and trace element data do not support closed-system fractional crystallization processes for all samples; however, some chemical features (i.e., decreasing Rare Earth Element (REE) abundances with increasing SiO 2) underline the importance of crystal fractionation processes for each distinct magmatic pulse. Isotope data do not support mixing of different crust-derived melts or assimilation of crustal rocks by a mafic magma. Instead, highly evolved Pb isotope compositions, strongly negative ε Nd values and radiogenic Sr isotope compositions argue for an undepleted basement as a potential source. The most likely model involves mainly partial melting of different basement rocks of Archaean to Proterozoic age at different levels within the crust. Only some granites could be derived by melting of metasedimentary rocks of the Pan-African cover sequence. The consistency of the chemical data with a crustal anatectic origin and the range in radiometric ages suggests that they intruded simultaneously with crustal thickening; however, some of them may have been emplaced during extensional tectonics c. 40–50 Ma later than the main period of crustal thickening. The heating events that promoted melting of fertile deep-crustal rocks might have been caused either by the inferred high heat productivity together with crustal thickening during the main periods of orogeny, or by delamination of mantle lithosphere during the final extensional stages of the Pan-African orogeny.