IMPACT PSEUDOTACHYLITE
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". I donÕt accept this suggestion. The two rocks with very different
origins are here referred to as Òfault pseudotachyliteÓ and Òimpact
pseudotachyliteÓ.
The sample shown above 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 203 million years ago,
near the end of the Late Triassic.
Reference
on pseudotachylite:
Reimold, W.U. & R.L. Gibson. 2005.
"Pseudotachylites" in large impact structures. in
Impact tectonics. Impact Studies 8: 1-53.