Computer Science
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27q.244k&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 244
Computer Science
2
Scientific paper
INTRODUCTION. The Roter Kamm impact crater is located in the Namib Desert in Namibia. The impact occurred in Precambrian granitic-granodioritic orthogneisses of the 1200-900-Ma-old Namaqualand Metamorphic Complex. The granites are invaded by quartz veins and quartz-feldspar-pegmatites. Gariep metasediments probably overlaid the Namaqualand complex at the time of the impact (Reimold and Miller, 1989). Previous estimates for the crater age are not well constrained: regional geology suggests an age of 5-10 Ma, while the only available ^40Ar-^39Ar age (Hartung et al., 1991) is 3.7 Ma. Fission tracks measured in apatites from granites found on or near the crater rim were not completely reset by the impact and suggest an uplift event around 20 Ma ago (Storzer et al., 1990). We are using several approaches to bracket the age of the crater: we have measured melt breccia and pseudotachylite K-Ar ages, and apatite fission track ages in several rim granites. We are comparing Rb-Sr isotope data for rim granites with known ages of regional resetting events (Allsopp et al., 1979). Finally, we are using ^10Be-^26Al measured by accelerator mass spectrometry to determine surface exposure ages for quartz excavated during the impact event. RESULTS AND DISCUSSION. The target rock composition and stratigraphy at Roter Kamm is relatively complicated. Melt breccias formed from pegmatites, gneisses, or schists, while pseudotachylites probably formed from gneissic basement or quartz-feldspar-pegmatites (Reimold and Miller, 1989). Whole rock Rb-Sr data for several granites yield 1498 Ma, and mineral separates from sample URK-M indicate an "age" of 466 Ma; these ages are similar to those of country rocks from the general area of the northwestern Cape/southern Namibia (Allsopp et al., 1979) which indicate two widespread regional resetting events at ca. 700 Ma (related to the Pan-African orogenic deformation), and ca. 500 Ma, related to a subsequent metamorphic event. For K-Ar ages, we analyzed samples URK-87A (quartzitic melt breccia), URK-87D and URK-92A (schistose melt breccias), and URK- 38 (pseudotachylite). The ages are (in Ma): 87A: 251+-12; 87D: 464+-21; 92A: 479+-22; 38: 255+-12. Thus the "melt" breccias are dominated by incompletely degassed fragments and at least partly retain their original metamorphic ages, which are similar to the 500 Ma ages obtained from Rb-Sr isotopes. Apatite separates from 5 rim granites were measured and yielded the following fission track ages (in Ma): RK-88: 19.8+-1.8; RK- 89: 20.9+-1.4; URK-8: 23.7+-1.9; URK-17: 26.5+-1.9; URK-18: 27.7+-2.4. About 60% of the apatites show dislocations and were not used for the measurements. The ages indicate a regional uplift event, in agreement with earlier data (Storzer et al., 1990). The cosmic-ray-induced in situ production of ^10Be and ^26Al has been used to determine erosion rates and exposure ages of terrestrial rocks and sediments. It provides an intriguing independent method for the age determination of younger meteorite craters (e.g., Nishiizumi et al., 1991). Be and Al were chemically separated from a pure quartz phase, and ^10Be and ^26Al contents are measured by accelerator mass spectrometry (in progress). We are studying quartz from quartz pegmatite from the crater rim that was most probably exposed during the impact. References: Allsopp, H.L., Kostlin, E.O., Welke, H.J., Burger, A.J., Kroner, A., and Blignault, H.J. (1979) Trans. Geol. Soc. S. Afr. 82, 185- 204. Hartung, J.B., Kunk, M., Reimold, W.U., Miller, R., and Grieve, R.A.F. (1991) Meteoritics 26, 342-343. Nishiizumi, K., Kohl, C.P., Shoemaker, E.M., Arnold, J.R., Klein, J., Fink, D., and Middleton, R. (1991) Geochim. Cosmochim. Acta 55, 2699-2703. Reimold W.U. and Miller R. (1989) Proc. 19th Lunar Planet. Sci. Conf., 711-732. Storzer, D., Koeberl, C., and Reimold, W.U. (1990) Meteoritics 25, 411-412.
Klein Jeff
Koeberl Christian
Matsuda J.
Nagao Keiichi
Reimold Wolf Uwe
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