Geo 365: Jan. 10, Day 10: Marys Peak Sill

Dana gets a closer look at the (near) base of Marys Peak sill, near Parker Creek Falls, which is just out-of-frame to the left. The composition here is more mafic, and based on observations from a thin section I made of a sample, is mostly labradorite feldspar, An 65, at about 70% of the rock volume. I no longer have that section, nor ready access to a petrographic scope, so I'm merely recalling, but this is one weird rock. The remainder is mostly pyroxene, but there is also a small amount of alkali feldspar and quartz, in  granophyric intergrowths, a vanishingly small amount of olivine- which technically shouldn't be possible- and a fair amount of an opaque mineral I took to be magnetite. Also, as I mentioned yesterday, what seemed to me a surprising amount of apatite, a trace mineral by most definitions, but at the upper end of what would be considered "a trace," at about 2 percent. Overall, I'd call this rock a granophyric gabbro, though texturally, it might perhaps be more appropriate to refer to it as a diabase.

As I noted above, quartz and olivine should not be able to co-exist in close proximity, though it's quite possible in the same intrusive rock mass that's been fractionally segregated. That and the existence of the aplites I posted Monday are my favored explanation for this conundrum. Had the rock mass cooled slowly and evenly, that last couple percent of olivine would have back-reacted with the silica to form pyroxene. But the sudden loss of volatiles, mostly water, as the aplite dikelets were forced into the overlying rock, basically quenched the system before it could reach equilibrium. Just before stopping here, we made a stop at a spot where I have, in the past, occasionally been able to find examples of diktytaxitic texture, but we weren't able to find any on this trip. That would represent the source of the residual melt that formed the overlying aplites, and the vesicles were the regions where that felsic melt resided before it was forced out by escalating vapor pressure,

That's my guess, anyway, and I'm sticking with it until someone comes up with a better one.

Photo unprocessed. July 8, 2012. FlashEarth location.