After the awesome outcrops and pavements of strained metaconglomerates from the Quetico / Wabigoon subprovince boundaries of the Superior Craton, my pre-GSA field trip visited the most charmingly-named magma chamber I’ve ever seen, the cuddly-sounding Ottertail Pluton. This is an Algoman-type pluton which is discordant to tonalite-composition gneisses in the area. As with the Giants Range Batholith that we saw near Virginia, Minnesota, the Ottertail Pluton shows lots of cool features that are blog-worthy.
For instance, check out these dark xenoliths along the margin of the pluton, where it inserted itself into pre-existing “host rock”:
At the bottom of the last one, you can see some large phenocrysts of K-feldspar in the phaneritic Ottertail granodiorite. This porphyritic texture was quite pronounced in several places:
Here’s something that really caught my eye: mafic schist boudins strung out in the granite:
Not quite sure what’s up with this area:
Two boudins with a concentration of mafic minerals in the neck; also some small-scale faulting along the same zone of weakness:
Shear zone fabrics were also in evidence in a few places. Here’s a nice example of a small asymmetric shear zone, with a crisp left boundary but the fabric more gradationally returning to “undeformed host rock” on the right:
Lastly, a couple of porphyroclasts, with a dextral sense of shear (right side of photo down relative to left side):
Just wondering where you got that fantastic pen?
It comes (gratis!) from Steve Gough of Little River Research and Design. Google his blog post “Stop Writing Pencil For Scale” on Riparian Rap.