Black ultracataclasite, Tanque Verde Mountain, Tucson, Arizona


I collected this rock back in ~1976 as a part of mapping and analysis of the Rincon Mountains metamorphic core complex.  Later, one of my students found a piece of this rock across the valley in the Tucson mountains.  The piece found was like a  piece of flint, used by ancient peoples as a knife.  The blade was undoubtedly picked up in the Rincon Moutains, and eventually carried to (and discared in) the Tucson Mounains.  In fact, on Tanque Verde ridge at the site of this cataclasite there are abundant petroglyphs and lots of sharp flakes of glassy-looking cataclasite.  The conchoidal fracturing of the ultra-fine-grained cataclasite lends itself to the formation of tiny sharp blades.  

Age & Formation
Original protolith was probably Mesoproterozoic granite, ~1.42 Ma. Cataclasis took place in the Miocene, ~20 Ma.