18 June 2012

Blown away by Bancroft: Part III

Posted by Jessica Ball

On the second afternoon of our trip, we finally began moving out of the greenschist facies into the amphibolite facies – higher pressures, higher temperatures and a different set of minerals.

Our next stop was about 2 km north of ON-7 on ON-62, the road leading back to Bancroft. Here we had a chance to check out some excellent examples of glacial striations.

Glacial striations on top of a roadcut on ON-62

Striations again. All the glacial striations in this area are oriented roughly 020 degrees (N20E)

The outcrop (a metapelite) also had chloritoid crystals, which indicate that we’re in a higher metamorphic facies than most of the previous ones. This was our last stop in the greenschist facies – the chloritoid plus a lack of chlorite probably meant that this stop was in the biotite zone of the Barrovian metamorphic grades.

After our chloritoid encounter, we took a brief compositional detour (but still on ON-62) to look at some gabbros:

Dikes(?) in gabbro

Contact between zones of large and small phenocrysts

But we didn’t linger at the igneous rocks long. Our next stop, just north of St. Ola Road, had enough amphibole to place us firmly in the amphibolite facies. There were also some quartz veins present, showing that the silicic minerals were starting to segregate themselves away from the more mafic minerals. This outcrop was trying┬áto become a gneiss!

Amphibole phenocrysts

More amphibole

Our final stop of the day was a little farther north on 62 (around house number 26122), where we saw some beautifully layered, tilted metapelites. The texture was gneissic but not quite there yet – another rock trying to become a gneiss.

Top of the outcrop

We saw more amphibole and some garnets, but the most visually striking part of the outcrop were the boudins, which contained lots of biotite.


The part that really got me excited, though, was at what would have been the ‘bottom’ of the metapelites had they been untilted. Because at this outcrop, you can see basalt! Not just any basalt, but basalt with chilled and altered outer margins. A lava flow! I don’t remember if these showed any evidence of being pillow basalts, but they were certainly emplaced in an environment where they cooled quickly.

The whole crowd looking at the lava flow

Closer view of the basalt

Inclined layers in the outcrop

Basalts and metapelites: not a bad way to end the day. For the last installment, get ready to see my favorite stop – the one with leucite crystals as big as your face!