Syn-orogenic high-temperature crustal melting: Geochronological and Nd–Sr–Pb isotope constraints from basement-derived granites (Central Damara Orogen, Namibia)

Abstract

Major and trace element and Nd, Sr and Pb isotope data from c. 550Ma-old gray granites and c. 510Ma-old red leucogranites of the high-grade central part of the Damara orogen (Namibia) indicate a dominantly deep crustal origin. Moderately peraluminous gray granites are isotopically evolved (initial εNd: c. −17) and were likely derived from meta-igneous sources with late Archean to Paleoproterozoic crustal residence ages. Based on a comparison with experimental results, the granites were derived by partial melting of a granodioritic biotite gneiss at c. 900–950°C and less than 10kbar. Slightly peraluminous red leucogranites are also isotopically evolved (initial εNd: −15 to −18) but have undergone extensive crystal fractionation coupled with minor contamination of mid crustal meta-pelitic material. Major and trace element data do not support closed-system fractional crystallization processes for all samples, however, some chemical features underline the importance of crystal fractionation processes especially for the leucogranites. Isotope data do not support mixing of different crust-derived melts or assimilation of crustal rocks by a mafic magma on a large scale. For the gray granites, unradiogenic Pb isotope compositions with substantial variation in 207Pb/204Pb at almost constant 206Pb/204Pb, strongly negative εNd values and moderately radiogenic Sr isotope compositions argue for an undepleted nature of the source. High Rb/Sr ratios of the red leucogranites permit a comparison with the gray granites but similar initial εNd values indicate that the source of these granites is not fundamentally different to the source of the gray granites. The most acceptable model for both granite types involves partial melting of meta-igneous basement rocks of Archean to Proterozoic age. The consistency of the chemical data with a crustal anatectic origin and the observation that the gray granites intruded before the first peak of high-grade regional metamorphism suggests that they intruded simultaneously with crustal thickening. The red leucogranites are interpreted to be a result of crustal melting during the main peak of regional metamorphism. The heating events that promoted melting of fertile deep-crustal rocks might have been caused by the inferred high heat productivity of heat-producing radioactive elements (Th, U, K) together with crustal thickening during the main periods of orogeny.