oh man oh man i found the most beautiful seam of porphyry (some of it basically glomeroporphyritic) in the dacite-rhyolite mountains behind my parents place! i was out for only two hours and lugged a big heavy piece home with me along with some smaller ones:
look at that huge cryst on the left one.
it has some nice biotite in it as well. it was really fuckin wet (and sweaty!!) though and i just about made it back before it started pouring. looking forward to tomorrow to see how it looks when it dries in the sun
also, looks completely different from the very red dacite (im now thinking that one was more of a rhyolite?) i got from the same area last year, just maybe 100m apart.
the bedrock it came from. look at those three color shifts, left to right, dark to beige to spotted. this is a magma rich in felsic minerals that probably cooled real slow at first, then got mixed up with a mafic one via plate subduction, heated back up and had a big devastating yellowstone-like eruption ~1,9 billion years ago. add some tectonic drift, accretions, isostatic rebound, then some magma intrusions.
