Geochronology in migmatites - a Sm-Nd, U-Pb and Rb-Sr study from the Proterozoic Damara belt (Namibia): implications for polyphase development of migmatites in high-grade terranes

Sm-Nd (garnet), U-Pb (monazite) and Rb Sr(biotite) ages from a composite mi gmatite sample (Damara orogen, Namibia) constrain the time of high-grade re gional metamorphism and the duration of regional metamorphic events. Sm-Nd garnet whole-rock ages for a strongly restitic melanosome and an adjacent i ntrusive leucosome yield ages of 534+/-5, 528+/-11 and 539+/-8 Ma. These re sults provide substantial evidence for pre-500 Ma Pan-African regional meta morphism and melting for this segment of the orogen. Other Darts of the mig matite yield younger Sm-Nd ages of 488+/-9 Ma for melanosome and 496+/-10, 492+/-5 and 511+/-16 Ma for the corresponding leucosomes. Garnet from one x enolith from the leucosomes yields an age of 497+/-2 Ma. Major element comp ostions of garnet are different in terms of absolute abundances of pyrope a nd spessartine components, but the hat shape of the elemental patterns sugg ests late-stage retrograde equilibrium. Rare earth element compositions of the garnet from the different layers are similar except for garnet from the intrusive leucosome suggesting that they grew in different environments. M onazite from the leucosomes is reversely discordant and records Pb-207/U-23 5 ages between 536 and 529 Ma, indicating that this monazite represents inc orporated residual material from the first melting event. Monazite from the mesosome MES 2 and the melanosome MEL 3 gives Pb-207/U-235 ages of 523 and 526 Ma, and 529 and 531 Ma, respectively, which probably indicates another thermal event. Previously published Pb-207/U-235 monazite data give ages b etween 525 and 521 Ma for composite migmatites, and 521 and 518 Ma for mona zite from neosomes. Monazite from granitic to granodioritic veins indicates another thermal event at 507-505 Ma. These ages an also recorded in Pb-207 /U-235 monazite data of 508 Ma. From the metasediment MET 1 from the migmat ite and also in the Sm-Nd garnet ages obtained in this study. Taken togethe r, these ages indicate that high-grade metamorphism started at c. 535 Ma (o r earlier) and was followed by thermal events at c. 520 Ma and c. 505 Ma. T he latter event is probably connected with the intrusion of a large igneous body (Donkerhoek granite) for which so far only imprecise Rb Sr whole-rock data of 520+/-15 Ma are available. Rb-Sr biotite ages from the different l ayers of the migmatite are 488, 469 and 473 Ma. These different ages indica te late-stage disturbance of the Rb-Sr isotopic system on the sub-sample sc ale. Nevertheless, these ages are close to the youngest Sm-Nd garnet ages, indicating rapid cooling rates between 13 and 20 degreesC Ma(-1) and fast u plift of this segment of the crust. Similar Sm-Nd garnet and U-Pb monazite ages suggest that the closure temperatures for both isotopic systems are no t very different in this case and are probably similar or higher than the p reviously estimated peak metamorphic temperatures of 730+/-30 degreesC. The preservation of restitic monazite in leucosomes indicates that dissolution of monazite in felsic water-undersaturated peraluminous melts can be slugg ish. This study shows that geochronological data from migmatites can record polymetamorphic episodes in high-grade terranes that often contain cryptic evidence for the nature and timing of early metamorphic events.