26 June 2012

Blown away by Bancroft: Part IV

Posted by Jessica Ball

I’m feeling particularly uninterested in staring at my model run today, so here’s the last installment of Bancroft. (Only a few months after the fact!) On the last morning of our Bancroft field trip this past April, we continued our journey through the metamorphic faces diagram with a stop at an outcrop north of Bancroft on ON-28, in the amphibolite facies.

Diagram from http://serc.carleton.edu/research_education/equilibria/classicalthermobarometry.html

How did we know we were in the amphibolite facies? Well, we were looking at a lovely garnet-biotite-gneiss (too high for greenschist to begin with)..

Gneiss-ly lit by the morning sun.

Micas and some garnet.

But a careful examination of the top of the outcrop revealed another clue: Kyanite!

1-4 cm kyanite blades on the top of the outcrop.

We did a lot of arguing over what kind of kyanite it was. I don’t know if anyone ever settled on a type, but it was lovely to see in outcrop just the same.

The students were a bit happier about the stop when the sun warmed us up.

But that was all eclipsed by our next stop at the Princess Sodalite Mine. There’s some neat history here; the mine was first opened in the late 19th century, and it became known as the “Princess” Sodalite Mine in the early 1900s, when the Prince and Princess of Wales (King George V and Queen Mary) visited and subsequently ordered tons of the sodalite-bearing rock to decorate Marlborough House in London. The current owner of the mine, Andy, is a great guy who allows UB students to visit every year before tourist season opens up. (He also has a really spiffy Scottish accent, and he knows a lot about the mining history of the region.) Before we went into the quarry to have a look at the sodalite, we checked out a glacially polished outcrop of nepheline syenite behind the rock shop. It was a nice glacial feature, but the most exciting part was the huge leucite phenocrysts! Literally, leucite as big as your face. This mineral forms in silica undersaturated melts where orthoclase is replaced by feldspathoids (also including nepheline).

Part of the polished outcrop (apologies for the odd lighting).

Beautiful euhedral phenocrysts (megacrysts?)

Andy, the mine owner, showed us how to use water to highlight some of the biggest leucite crystals.

The quarry itself is only open to the public on a limited basis. The blasting necessary to get at the best sodalite concentrations means that it can be dangerous to do too much hammering in the quarry, but we got to spend some time breaking up chunks of syenite from already-blasted piles.

Andy showing off some of the best sodalite.

One of the sodalite veins

UB and SUNY Fredonia students sodalite hunting

The Princess Sodalite Mine isn’t just a mineral-hunter’s wonderland, however. Just around the corner from the quarry is a fantastic glacial landscape:

U-shaped valley and some glacial till, anyone?

A beautifully polished syenite outcrop

Andy also keeps a “rock garden” just beyond the outcrop – it’s full of a collection of samples from mines all over Ontario, and apparently a few places even farther afield. If you go to visit the mine, you can pay by the pound to take rocks and minerals from here.

We were drooling over everything here...

Some lovely jasper and mica books

We could have spent hours at the mine, but we had to move on to another mine (the Macdonald Mine, also north of Bancroft, but on ON-62). This was a spectacular granite pegmatite, and was apparently the largest working feldspar mine in the area.

Descending into the mine

K-feldspar galore!

Examining the walls

The floor is covered in feldspar here. It's quite crunchy!

The spoil piles on the hills around the mine are full of crystalline and smokey quartz, and you can find zircons in the smokey quartz if you look very carefully. I managed to snag a piece with multiple zircons, although I didn’t have the foresight to take a photo of it before working on this post.

The last stop of the trip actually took us completely out of the metamorphic phase diagram and into the realm of igneous processes. This outcrop on ON-127 is an excellent example of partial melting in a gneissic rock – or migmatite! (Callan would appreciate this one.) The dark layers here are mostly biotite, while the pink layers are mainly feldspar and quartz (a more granitic composition). This rock was subjected to so much heat and pressure that it ‘sweated out’ (Callan’s words) the granitic melt that you see in the pink areas.

Admiring the results of partial melting

A little closer

This was a seriously messed up metamorphic rock. There were folds everywhere, and even a chunk of skarn that had been cross-cut by a granitic melt.

A fold!

More folds and some flow

There's that skarn

Another nice fold

The group contemplating the migmatite

For a volcanologist, seeing a migmatite among all those metamorphic rocks was kind of comforting. (And considering that we had several hours of driving and a border crossing before we could go home, I definitely appreciated the comfort.)

And thus ends the epic posting! I promised a commenter that I would try and make a Google Map of all the field trip stops, so you will see one more post about Bancroft in the future, but it may take a while.