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 don’t accept this suggestion.  The two rocks with very different origins are here referred to as “fault pseudotachylite” and “impact pseudotachylite”.

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.




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