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

Abstract

Sm±Nd (garnet), U±Pb (monazite) and Rb±Sr (biotite) ages from a composite migmatite sample (Damara orogen, Namibia) constrain the time of high-grade regional metamorphism and the duration of regional metamorphic events. Sm±Nd garnet whole-rock ages for a strongly restitic melanosome and an adjacent intrusive leucosome yield ages of 534¡5, 528¡11 and 539¡8 Ma. These results provide substantial evidence for pre-500 Ma Pan-African regional metamorphism and melting for this segment of the orogen. Pb/ 235 U monazite data of 508 Ma from the metasediment MET 1 from the migmatite and also in the Sm±Nd garnet ages obtained in this study. Taken together, these ages indicate that high-grade metamorphism started at c. 535 Ma (or earlier) and was followed by thermal events at c. 520 Ma and c. 505 Ma. The 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 layers of the migmatite are 488, 469 and 473 Ma. These different ages indicate late-stage disturbance of the Rb± Sr isotopic system on the sub-sample scale. Nevertheless, these ages are close to the youngest Sm±Nd garnet ages, indicating rapid cooling rates between 13 and 20uC Ma x1 and fast uplift 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 not very different in this case and are probably similar or higher than the previously estimated peak metamorphic temperatures of 730¡30uC. The preservation of restitic monazite in leucosomes indicates that dissolution of monazite in felsic water-undersaturated peraluminous melts can be sluggish. 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.

Evolution of migmatitic granulite complexes: Implications from Lapland Granulite Belt, Part II: Isotopic dating

“…Definitively, the effect of fluid on isotope systems would be seen first in leucosomes. It has been observed that isotope systems of different rocks obviously have been closed or reset in different temperatures, which may vary according to the geo-logical history, rock type, grain size and fluid activity (Mezger et al, 1992;Burton et al, 1995;Jung & Mezger, 2001;Thöni, 2003). The garnet-WR SmNd age in the meso-and melanosome is quite similar to those in leucosomes (Table 5).…”

Models of garnet differential geochronology

“…These discrepancies must result in part from variable contamination of garnet separates by inclusions (e.g., see Amato et al, 1999;Jung and Mezger, 2001, 2003a, 2003b, and possibly also from differences in Sm partitioning depending on mineral assemblage (e.g., Hickmott, 1988;Hickmott and Spear, 1992;Hermann, 2002;Thöni, 2002), much as is observed for Lu in metapelites vs. metabasites. Values in Table 1 are based on chronologic studies of leached garnets (Amato et al, 1999;Jung and Mezger, 2001, 2003a, 2003bAnczkiewicz et al, 2007), adjusted for trace element studies demonstrating somewhat stronger partitioning of Sm/Nd (Hickmott et al, 1987;Hickmott and Spear, 1992). These values and resulting Sm/Nd ratios are similar to those used by Lapen et al (2003) and at the upper end of values for metabasites and metapelites summarized by Thöni (2002), but 40 times lower than the value assumed by Skora et al (2006).…”

The Timing and Duration of the Delamerian Orogeny: Correlation with the Ross Orogen and Implications for Gondwana Assembly

“…522 ‫ע‬ 5 As illustrated in figure 10, the peak of the PanAfrican events in Antarctica-Africa and India occurred between 630 and 540 Ma but had commenced as long ago as 900 Ma (Pinna et al 1993). A number of recent geochronological studies in the Namibian Damara Orogen in southern Africa have also revealed Middle to Late Cambrian static metamorphism and posttectonic felsic magmatism (510-480 Ma;Jung and Mezger 2001;Jung et al 2001;Meert 2003), the ages of which are like those of the late Ross and Delamerian orogens. These are imposed on a mobile belt whose main synkinematic activity was earlier (570-550 Ma;Jung and Mezger 2001) and associated with active plate convergence, subduction, and accretion.…”

“…A number of recent geochronological studies in the Namibian Damara Orogen in southern Africa have also revealed Middle to Late Cambrian static metamorphism and posttectonic felsic magmatism (510-480 Ma;Jung and Mezger 2001;Jung et al 2001;Meert 2003), the ages of which are like those of the late Ross and Delamerian orogens. These are imposed on a mobile belt whose main synkinematic activity was earlier (570-550 Ma;Jung and Mezger 2001) and associated with active plate convergence, subduction, and accretion. Interestingly, like the model we proposed earlier for the Delamerian, the occurrence of late A-type granite in the Damara Orogen has also been ascribed to mantle influx following lithospheric delamination (Jung et al 1998(Jung et al , 2001.…”

“…These are imposed on a mobile belt whose main synkinematic activity was earlier (570-550 Ma;Jung and Mezger 2001) and associated with active plate convergence, subduction, and accretion. Interestingly, like the model we proposed earlier for the Delamerian, the occurrence of late A-type granite in the Damara Orogen has also been ascribed to mantle influx following lithospheric delamination (Jung et al 1998(Jung et al , 2001.…”