Continuing westward, today’s deskcrop comes from the Black Hills of South Dakota, just north of the town of Keystone. You can make out some lovely porphyroblasts of staurolite in this deskcrop, some of which have the distinctive “fairy cross” twinning. Although garnet is not apparent as a phenocryst in this sample it does occur in nearby samples. The metapelites of this region of the Black Hills have mapped isograds suggesting that the heating event responsible for the metamorphism was coincident with the intrusion of the Harney Peak Granite and its related pegmatites.
Explore this and all of my 2010 Deskcrops and Outcrops in Google Earth!