CRUSTAL FORMATION AND RECYCLING DURING SUPERCONTINENT CYCLES: EVIDENCE FROM DETRITAL ZIRCONS OF THE DAMARA OROGEN

Orogenic belts formed during the amalgamation of Gondwana provide valuable clues to the evolution and growth of continental crust. U-Pb dating and Hf-isotope data reveal contributions from both older Precambrian sources and from juvenile, mantle-derived material. The Neoproterozoic Damara Sequence, found along the southern and western margins of the Congo Craton, yields distinctive age components derived from the craton and from syn-rift magmatic sources. Detrital zircons from metasedimentary rocks show major age populations at 2050-1900 Ma, 1100-950 Ma, and 800-600 Ma. In addition to the Proterozoic zircons, there are a small number of late Archean grains with minor peaks at 3000 Ma and 2700-2600 Ma. Epsilon Hf values for the various age populations range from strongly negative to positive with the positive values approaching depleted mantle values at the respective times. A majority of the zircon populations from the Damara Sequence give initial epsilon Hf values between +10 and -10, with exceptions for grains with ages of ca. 1100-1000 Ma, which give values as low as -20, and ca. 700-800 Ma, which yield dominantly positive values. Detrital zircons from the Neoproterozoic to Cambrian Nama Group, deposited in the foreland basin of the Damara Orogen, yield age populations similar to those of the Damara Sequence, with the addition of a population of ca. 500-570 Ma grains derived from the Damara Orogen. The proportion of Archean age grains is greater in the Nama Group with populations at ca. 2300 Ma, 2500-2600 Ma, 2700 Ma and 3400 Ma. Initial epsilon Hf values from the Nama display similar trends to those of the Damara Sequence with most epsilon Hf values ranging from +8 to -10. These data suggest that each of the Paleoproterozoic and Neoproterozoic cycles represented within the Damara Sequence and Nama groups in the Damara Orogen include a mixture of juvenile material in the form of detritus formed by Neoproterozoic syn-rift magmatism associated with the opening of the Khomas Ocean and recycled crust from earlier Proterozoic events.