Radiometric dating geology
None of the obtained isochron "ages" corresponds to the "date" for any recognized event, neither the original lava eruptions nor the subsequent metamorphism.
To the contrary, the rocks could still only be a few thousand years old.
These discordant results could easily be dismissed as an isolated aberration, perhaps due to the uncertain effects of metamorphism and any subsequent alteration, especially during erosion and weathering.
However, they are confirmation of the repeated failure of all the radioisotope "dating" methods to successfully date Grand Canyon rocks.
All 27 samples were sent to two well-credentialed internationally-recognized, commercial laboratories for radioisotope analyses—potassium-argon (K-Ar) at a Canadian laboratory, and rubidium-strontium (Rb-Sr), samarium-neodymium (Sm-Nd), and lead-lead (Pb-Pb), at an Australian laboratory.
Both laboratories use standard, best-practice procedures on state-of-the-art equipment.