High Sierra Topix

tlsharb wrote: ↑Thu Sep 26, 2019 11:15 am So what does it take for an unnamed peak to get named? Tiramisu is nice, but I'm still voting for Swarm Dike Pk.
ts

Here you go -
https://www.usgs.gov/core-science-syste ... s/how-do-i

tlsharb writes:

that is a "way cool" looking peak. And though I have a minor in geology from a million years ago, I have no idea what that is. Any geologists want to weigh in???

Mount Tom.jpg (4896×4342)
geomaps.geosci.unc.edu

*Damn! I am hopeless at attaching maps and such in an easily accessible manner- sorry.

Above, I've tried to include the best of the geologic maps that I found on the internet. What I learned is that if our "Swarm Dike Peak" is found in formation member: Kd, then it is described as "Diorite, quartz diorite, and hornblende gabbro." Further: "includes some hybrid rocks of granodiorite composition." On the map the member is shown with a network of pink dashes throughout, and on the same map these are described as follows:

"Felsic dikes and masses, chiefly aplite, pegmatite, and alaskite. Part of dike swarm along Pine Cr. that is marginal to quartz monzonite..."

"Mafic dikes" are also mentioned in this geologic formation.

Now if one of our real geologists would interpret that for us, we might become wiser. I'd like to know the relative ages of the various members /formations. Ie., is the dark rock (the hornblende-gabbro) a remnant of the much older ancestral Sierra, which has been shot through with the lighter rock, dike material during subsequent uplift of younger granitic plutons, or what? I didn't find the ages of the different fms. yet; I'll keep looking.

Nice picture Dave. Thx for sharing

"Generally the most important policy is local use and acceptance" (fr. Usgs). I wonder if the folks in Bishop have a special name for this?

A few geologic notes. Based on my memory as well as what has been posted of the USGS map of the area the darker rocks are gabbro or diorite (these two technically indistinguishable to naked eye because difference is based on chemical composition of the plagioclase feldspar in the rock), which is an intrusive rock (ie crystallized from melt beneath surface) in the manner of granite but with a different mix of minerals (no potassium feldspar, little or no quartz (usually) more "mafic" minerals such as hornblende or pyroxenes which are usually black to the naked eye), so that most gabbros are darker than granites/granodiorites. The lighter-colored dikes and sills are the aplites and pegmatites. Note that the geological term for something with a lot of dikes (these would be vertical sheetlike intrusions; sills are horizontal) is "dike swarm" rather than "swarm dike". Whereas these are mostly sills I have not heard of the term "sill swarm" . The pegmatites are usually composed of quartz and potassium feldspar with biotite (black flaky stuff), but commonly have black tourmaline (but sadly not the beautifully colored gem stuff). However, somewhere way off trail , I found one that had really nice sapphire (could not be extracted, though).

P.S. Just returned from one of the major geoscience conferences, the Geological Society of America Annual Meeting in Phoenix (Sept 22-25) then went straight out into the field yesterday to look at rocks on the sea cliffs from the Golden Gate southward. Had intended to do a bit of fishing but the surf was too high.