Friday, March 14, 2008

Tiara rocks

Maria at Green Gabbro nominates the Bishop Tuff as the Tiara of the Sierra Nevada, because it's so sparkly. Now, welded ash-flow tuffs are very lovely rocks, especially with all those sparkly sanidine crystals. But princesses need company. So, I give you...


(Source: Vermont Geological Survey.)

The Gassetts Schist, Tiara of the Taconian Orogeny. (Or maybe Acadian. Argon gives exhumation ages, I believe. Though there's something creepy, like Brothers Grimm or something, about exhuming princesses.) Which would make the Green Mountains the Princess of New England, unless Maine, New Hampshire, and Massachusetts want to fight about it.

Thin section:


(Source: Union College.)

Muscovite/paragonite, biotite, garnet, staurolite, kyanite, quartz, chlorite... and wait, it's got too many minerals for an AFM diagram. That's ok. If you're a princess, you get additional components, so you can have extra phases, too. Because princesses totally can do thermodynamics.

Edit: If you prefer metals in your tiara, Silver Fox reminds us that there's big money to be made in mining these days. (Dr. Lemming would probably agree, if he weren't in the field at the moment.) So you can make your own tiara, or you can make a lot of money and buy one.

7 comments:

Joseph said...

Hmmm... Green Mountains as the Princess of New England? Especially with the Gassetts that's pretty hard to top for Princess-itude. No argument here from the bedrock mapper for Office of the Mass. State Geologist...

But what about the Princess of the Acadian or Alleghenian? Now that gets a little dicey...epsecially if we got Pete Robinson involved in the conversation. There are schists and gneisses in the Pelham Dome that are over 50% (by a visual guestimate at the OC) fibrolitic silliminite... all whispy and white... that seems pretty princessy to me. I would nominate the Pelham Dome for Princess of the Acadian in Mass...

Most of the other rocks I've been working with in Massachusetts are too hosed-over to be very regal... Maybe a princess the morning after the big party...

I'd have to nominate the Marlborough formation to be the Black Knight of the Acadian and Allegehenian ... a dark and mysterious and many layered character of amphibolite and metavolcanic sequences with epidote and garnet, a mysterious past, unknown age, scarred and battered by events that are as of yet not known to anyone but him...

Kim said...

I've got a bunch of Pelham Dome rocks in my drawers, and yeah, garnet-biotite-sillimanite-cordierite is very sparkly. (Especially with those pinkish-purplish garnets.)

I'm trying to imagine Pete Robinson and others on an NEIGC field trip arguing about whose rock deserved to be the tiara. I'm having trouble, though.

Silver Fox said...

Well, I was thinking it would be nice to respond with some kind of rock/mineral photo - I'll have to dig around in my truck sometime and see what I can find. That's a nice schist. (I don't think that last statement makes any sense!) I haven't seen that schist before - I mean I really like it! (I was going to say it's nice, again - yikes!)

Elli said...

Interesting that you plucked a rock out that's obvious not in EQ and preserves a few reactions--makes it more interesting in my mind :)

Hey, Joe, what are you up to these days? I had a few project ideas that I should discuss with you before this summer at some point... (sorry to get off topic)

Kim said...

Elli - Is it definitely not in equilibrium, or are the extra phases stabilized by Na in the white mica and Mn in the garnet (and maybe Zn in the staurolite)? I've never worked on the rock, just made students look at it, so I'm curious what the texture crowd is saying these days.

And mostly, I picked it because it was the sparkliest schist I could think of, and because I figured that if a rock was at the Smithsonian's Natural History Museum, there should be pictures of it on the web.

And to Silver Fox - there's a huge road cut of this rock in Vermont, so everyone who has worked on metamorphic rocks in New England has visited it. (It's more fun with a big group of petrologists, because they argue about whether it could be in equilibrium or not. We all used it in classes, even if it's not in equilibrium, because you don't find nicer staurolites in thin section anywhere.)

Elli said...

The textures all by themselves state its not in equilibrium, but it would be interesting to plot up a pseudosection and try to pin down better what's frozen in the texture. I looked and can't such a study of the area... anyone know what the bulk composition of the rock is?

Chuck said...

Can I have a lead tiara, if it contains 50g/t silver as a byproduct?

On the other hand, I saw a biotite/kyanite schist- 5 cm crystals with no other visible phases in hand section- from the Arunta that was awesome, in terms of phyllosilicate shininess.

Speaking of biotite, don't forget that Ti in metapelites can substantially increase the stability field of this mica.