Pseudotachylites of the Beaverhead impact structure: Geochemicaly geochronological, petrographic, and field investigations

Dikes and pods of pseudotachylite up to one meter thick have been found over an area >50 km² in the same area as shatter cones and other possible features of shock metamorphism in the Beaverhead and Tendoy Mountains in southwestern Montana, defining the allochthonous remains of the Beaverhead impact structure (see also Hargraves et al., Chapter 19, this volume). They are not associated with any tectonic feature in the area and have several features uncommon in pseudotachylites formed by tectonic processes (large size, vesicles, pseudotachylite clasts within pseudotachylite), but which have been documented in pseudotachylites from other impact structures. Rare single sets of planar deformation features (PDFs) are found in quartz grains in the pseudotachylites with crystallographic orientations similar to those found in shocked quartz from other impact structures. The major and trace element chemistry of the pseudotachylites is similar to their host rocks, but with some enrichments (Al, Mg, Fe, K, volatiles) and depletions (Si, Na) indicating low-grade metasomatic alteration. ⁴⁰Ar/³⁹Ar laser microprobe analyses of pseudotachylites from three localities show a wide spectrum of ages, from Precambrian to Tertiary. The distribution of ages suggests two isotopic signatures, one of Precambrian age (although younger than the age of the protoliths), and the other of Cretaceous age. ²⁹Si Magic angle spinning (MAS) nuclear magnetic resonance (NMR) studies of the pseudotachylite failed to detect the presence of highpressure polymorphs of quartz. The evidence suggests that these pseudotachylites were formed by the same event that formed the shatter cones. Related work (see Hargraves et al., Chapter 19, this volume) suggests that the original crater was at least 75 km in diameter and was formed in the late Proterozoic to Cambrian, 20 to 150 km to the west of the present location.