Constraining the timing of anatexis in the Palaeoproterozoic Steinkopf Gneiss, western Namaqua Sector of the Namaqua-Natal Metamorphic Province, through U-Pb zircon and monazite dating of migmatite leucosomes

The Steinkopf Gneiss represents remnant Palaeoproterozoic crust in the western part of the predominantly Mesoproterozoic Namaqua Sector of the Namaqua-Natal Metamorphic Province. The Steinkopf Gneiss is typically migmatitic with the development of leucosomes, which are seemingly of different ages, based on field observations such as the intensity of deformation and cross-cutting contacts. In the study area, the Steinkopf Gneiss is associated with major Mesoproterozoic granitic units such as the pre-to syntectonic Little Namaqualand Suite and Concordia Granite, the late-to posttectonic Spektakel Suite, as well as some pegmatites in the Orange River Pegmatite Belt. Previous studies have suggested a significant contribution of crustal-derived melts in the formation of all these units, however, potential sources of such crustal-derived melts have not been identified. The current study investigates the possibility that the migmatites in the Steinkopf Gneiss represent potential source materials to these melts. In order to do so, U-Pb zircon and monazite age data are provided for two leucosomes which, based on field evidence, represent some of the oldest and youngest leucosomes, respectively, in the Steinkopf Gneiss. The zircons display two distinctly different textural sectors, which each renders unique ages and Th/U values. Brightly zoned areas are associated with a weighted mean age of 1 799 ± 3 Ma, which is considered to represent the crystallisation age of the Steinkopf Gneiss. Transgressive unzoned dark areas are associated with a weighted mean age of 1 146 ± 8 Ma, which is considered to represent the timing of anatexis in the Steinkopf Gneiss during which new felsic melts were generated. Weighted mean monazite ages of 1 028 ± 3 Ma and 1 035 ± 3 Ma in the two leucosomes respectively are considered to represent their crystallisation ages. These results suggest that the Steinkopf Gneiss might have acted as a source to new anatectic melts between about 1 146 Ma and 1 030 Ma; these melts might have migrated unknown distances into the surrounding areas and contributed to the formation of new granitic crust including the Little Namaqualand and Spektakel suites, the Concordia Granite, and some of the pegmatites in the Orange River Pegmatite Belt. Such anatectic events might have occurred incrementally throughout this period but it is not possible to determine how many times, since each incremental event will partly or completely destroy evidence of earlier events.