Impact pseudotachylite (cut surface; field of view 16.6 cm across) - impact-fractured granite (orangish areas - K-feldspar & quartz) with grayish- to blackish-colored impact pseudotachylite (impact melt) vein fillings.

The rock name "pseudotachylite" has long been applied to vein-filling impact melts in impact-fractured rocks - the impact melt has a glassy to cryptocrystalline texture. Fault zone movement can also generate melt, which cools down to very similar-looking material. Fault zone melt rocks have also been called pseudotachylites.

The term “pseudotachylite” was originally defined based on melt rocks of impact origin. Despite this, Reimold & Gibson in 2005 published a 53-page paper that basically says “you shouldn't call impact melts pseudotachylites anymore” and “only fault zone melt rocks should be called pseudotachylites”. A simpler, more logical solution is two have two terms: "impact pseudotachylite" & "fault zone pseudotachylite".

The sample shown here is impact-fractured basement rock from well below the original crater floor of the Rochechouart Impact Crater in west-central France. The impact event, basement rock fracturing event, and pseudotachylite formation event all occurred 214 million years ago, during the Norian Stage of the mid-Late Triassic.

------------------ 

Reference on pseudotachylite: