Petrology of basement-dominated terranes: I. Regional metamorphic T– t path from U–Pb monazite and Sm–Nd garnet geochronology (Central Damara orogen, Namibia)

Abstract

Mineral chemistry combined with Sm–Nd garnet–whole rock and U–Pb monazite ages provides important constraints on the geological history of metapelites, migmatites and granites from the high-grade basement-dominated part of the Damara orogen (Namibia). Based on thermobarometric data, the peak of regional metamorphism reached c. 700–750 °C and c. 5 kbar. U–Pb monazite and Sm–Nd garnet mineral ages obtained from unmigmatized garnet- and cordierite-bearing metapelites indicate that these peak metamorphic conditions were reached between 525±2 and 504±3 Ma. Some migmatites with restitic melanosomes record Sm–Nd garnet ages of 518±3 Ma indicating that high-grade regional metamorphism also culminated in partial melting. Some other migmatites and granites give older Sm–Nd garnet–whole rock ages between 530±3 and 540±4 Ma. Different initial Nd isotope compositions of granites, leucosomes and melanosomes indicate that in this case, migmatization is intrusion-related. Crust-derived granites intruded at 515±2, 512±3 and 509±5 Ma and are probably related to the second peak of regional metamorphism. The youngest granites intruded at 469±3 Ma indicating that intrusion of melts is superimposed upon the regional metamorphic history. Age constraints for the period between c. 540 and 510 Ma provide substantial evidence for crustal thickening together with igneous activity, whereas the high-temperature event, which occurred at c. 470 Ma, must be related to extensional tectonics. The concordance between the monazite and garnet ages suggest that monazite grew at high-grade metamorphic conditions. It is further demonstrated that combined monazite and garnet chronometry are useful in identifying the timing and nature of particular tectonometamorphic and intrusive events in polymetamorphic high-grade terranes.