THE AGE OF THE ROTER KAMM IMPACT CRATER, NAMIBIA - CONSTRAINTS FROM AR-40-AR-39, K-AR, RB-SR, FISSION-TRACK, AND BE-10 AL-26 STUDIES

The well-preserved 2.5 km diameter Roter Kamm impact crater is located in the Namib desert in Namibia. The impact has occurred in Precambria n granitic and granodioritic orthogneisses of the 1200-900 Ma old Nama qualand Metamorphic Complex which were partly covered by Gariep metase diments; the granites are invaded by quartz veins and quartz-feldspar- pegmatites. Previous geological field evidence suggested a crater age of about 5-10 Ma. In order to constrain this age, we selected a set of basement rocks (granites, granodiorites) exposed at the crater rim an d studied the Rb-Sr, K-Ar, Ar-40-Ar-39, and Be-10-Al-26 isotopic syste ms as well as apatite fission track ages of these samples. The Rb-Sr i sotopic systematics confirm the derivation of these samples from the N amaqualand basement (age about 1.29 Ga), which underwent Damaran oroge nesis at about 650 Ma. No basement rocks with Rb-Sr ages younger than about 410 Ma were identified. The K-Ar ages of pseudotachylite and mel t breccia samples show that these samples are dominated by incompletel y degassed fragments of basement rocks, with some retaining their orig inal metamorphic ages of about 470 Ma. The apatite fission track ages range from 20-28 Ma, which may be interpreted as an extension of the 2 5 Ma Burdigalian peneplanation event, or as incomplete resetting of th e apatite fission tracks during the impact event. The Be-10 and Al-26 exposure age of a quartz sample isolated from a quartz-pegmatite was f ound to be 150 ka; it is likely that the exposure of the sample began after material covering it had been removed by erosion 150 ka ago. Two glassy fractions extracted from a rim granite were dated by Ar-40-Ar- 39 analysis. One sample gives practically a plateau age of 3.7 +/- 0.3 Ma, while the other gives a minimum age of 3.6 Ma. The best available age estimate for the Roter Kamm crater is therefore 3.7 +/- 0.3 Ma.