NEW EVIDENCE FOR LATE DOWNGRADE METAMORPHISM IN THE PELITES FOUND IN THE EASTERN BLACK HILLS, SOUTH DAKOTA

Field research near the eastern margin of the Black Hills, near Rockerville, SD, has revealed a Proterozoic shearing event that post-dates intrusion of the Harney Peak Granite (HPG). Dahl et al. (1999) proposed a series of folding events related to the suturing of the Wyoming and Superior provinces during the Proterozoic that culminated with doming of the region during intrusion of the HPG. Peak metamorphism, ranging from biotite- to sillimanite-grade, is interpreted to be late syntectonic, but prior to intrusion of the HPG.

Field and petrographic evidence support the previous interpretations with the exception of a late, downgrade metamorphism constrained within narrow, meter-scale, high-strain zones. The rocks crosscut by the shear zones are black, due to biotite forming the foliation, and contain the mineral assemblage bt-grt-musc-qtz-plag. Within the high-strain zones, the foliation is defined by chlorite giving the rocks a light green color. In these rocks, garnet is not present; however, garnet-shaped enclaves with the mineral assemblage bt-chl-qtz are abundant. Commonly garnet-bearing rocks and those with garnet-shaped enclaves are found only a few meters apart.

Petrographic examination of enclave-bearing rocks reveals an important texture. The foliation within the high-strain zone is wrapped around the enclaves similar to fabrics surrounding syntectonic porphyroblasts; however, the minerals within the enclaves are strain-free. For example, quartz grains display sharp extinction and the biotite and chlorite are randomly oriented with respect to the foliations within the rock. Because the minerals are not deformed within the enclaves, and the high-strain deformation would have been too great for the enclave to remain circular, it is likely that the downgrade reaction is younger than the development of the high-strain zones. Additionally, the shape of the enclaves is interpreted to suggest the presence of a strong porphyroblast, such as garnet, that was resistant to deformation during the high-strain event. These observations are interpreted as recording a downgrade reaction postdating all deformation in the area, including that related to intrusion of the HPG.

Dahl, P. S., et al., 1999, New constraints on the timing of Early Proterozoic tectonism in the Black Hills (South Dakota), with implications for docking of the Wyoming province with Laurentia: GSA Bulletin, v. 111, p. 1335-1349.