Tenoumer Crater, Mauritania: Age and petrologic evidence for origin by meteorite impact

Inclusions of granitic basement rocks, collected from the sparse ‘lava’ exposed at the 1.8-km-diameter Tenoumer crater, Mauritania, exhibit distinctive petrographic features considered indicative of shock-wave action and diagnostic for meteorite impact. The observed effects include (1) distinctive planar features in quartz, oriented parallel to ω{ 101¯3 } and to other specific planes; (2) intense selective vesiculation of quartz and felspar grains; (3) presence of lechatelierite apparently formed by fusion of quartz grains; and (4) partial fusion and decomposition of biotite grains. These effects are quite distinct from those produced from conventional fusion of granitic xenoliths in lavas. The evidence from these inclusions supports the theory of a meteorite impact origin for the Tenoumer crater. The lava, which is chemically similar to the basement rocks and which has apparently been rapidly quenched, is interpreted as an impact melt formed by fusion of the basement rocks at the time of impact and injected into fractures in the crater wall, carrying with it highly shocked inclusions of basement rock. Values of Sr87/Sr86 determined for the lava are approximately 0.720; they are almost identical with values determined for basement rock samples and are too high to represent primary melt derived from the mantle. Potassium-argon age determinations on the melt rock establish the age of the Tenoumer crater as 2.5±0.5 m.y.