Genesis of the melt rocks at Tenoumer Crater, Mauritania

A common problem encountered at probable meteorite impact craters is the occurrence of associated melt rocks, presumably produced by the impact event, that cannot be matched chemically to the target rocks. A similar situation is found at Tenoumer crater, Mauritania. Chemical analyses of 11 melt rocks range from felsic country rock values to much more mafic compositions in two well-defined chemical sequences. Only one of the melt rocks is essentially isochemical with the adjacent granodioritic country rock. This specimen consists of a heterogeneous glassy to cryptocrystalline groundmass commonly containing shocked country rock fragments and lechatelierite. It is clearly an impactite. Conversely, some of the more mafic melt rocks display reasonably normal igneous textures and may show no shock features whatsoever. Without the certain knowledge that all these rocks had a common genesis, some of these mafic rocks could be mistaken for products of endogenous magmatic activity. Neither meteoritic contamination nor vapor fractionation can explain the chemical sequences exhibited by these melt rocks. They most likely result from the combination of felsic country rocks with various amounts of an amphibolite vein that appears to strike through the center of the crater. The hypothesis is advanced that the amphibolite was preferentially incorporated in the impact-generated melt because of favorable kinetic and viscosity characteristics. Since mafic segregations are common in Precambrian metamorphic terrains, this mechanism may also explain other similar occurrences. The clear-cut evidence from Tenoumer makes it difficult to deny an impact origin for certain craters solely on the basis of associated melt rocks that are not isochemical with the country rocks.