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.
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:
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 to become a gneiss!
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.
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.
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!
Thank you for telling me about my own backyard so to speak. I drive Highway #7 several times a year and have been facinated by the rock cuts. You have explained some of them. Too bad you seem to have missed the ones further east including the outcrops of yellow cake and lots more “pink granite” etc. It doesn’t sound as if you got north of Bancroft to Quadville where one can “go on a dig’ for minerals. Wish I knew the right name(s) for the decaying Feldspar &/or quartzite stained yellow with ?? along the St. Lawrence River.
Carol – Glad you liked the posts! No, we didn’t go much farther east or north on this particular trip, probably because the focus of the class was on a certain metamorphic progression. Sounds like I’ve got lots more to see the next time I visit, though!