2 November 2011

The Ottertail Pluton

Posted by Callan Bentley

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):