GEOCHEMICAL ANALYSIS OF VARIABLY STRAINED ARCHEAN GNEISSES IN THE LITTLE ELK TERRANE, BLACK HILLS, SOUTH DAKOTA

In the northeastern Black Hills, South Dakota, Archean basement gneisses are exposed in the Little Elk Terrane (LET). Prior to this research, the LET was interpreted as having two distinct units, the Little Elk Granite (LEG) and the Biotite feldspar gneiss (BFG). Recent discoveries by the Winona State University (WSU) research group has called this hypothesis into question, proposing a variably-strained single-unit model. This research will describe and interpret any variation in the geochemistry in the LET, compare it with the strain-related textural analysis from Colbert and Allard (this meeting), and propose an alternative classification and genesis for this terrane.

The current hypothesis for two units in the LET is based on the interpretations of McCombs (2004) and Gosselin (1988, 1989). They interpret the presence of two fabrics in the BFG and only one in the LET, variation in both abundance of micas and feldspar grain size, and minor difference in ages to support this interpretation. However, research by WSU discovered that the two fabrics are present across the entire LET, and propose the mica and grain size variation is a function of strain intensity with higher strained rock having more micas and smaller feldspar grains. Therefore, these observations, coupled with the overlap in ages for the two units, permit the LET to be a single unit.

Field mapping assigned a “strain group” based on mica percentage and feldspar strain ratio to every gneissic rock exposed in the LET. The eastern third is dominated by rocks of the lowest strain group with fewer micas and lower strain ratios, whereas rocks in the western third are mostly from the highest strain group. The central third is transitional between the high and low strain areas with intermediate values. Interestingly, rocks from all strain groups are identified in all three domains. In fact, strain partitioning has created narrow zones of high-strain rocks in the eastern third that are indistinguishable from high-strain rocks in the western third, and vice-versa. To investigate a single unit hypothesis, samples representing each strain group were collected from within each third of the LET for geochemical analysis. This geochemistry will be compared to the textural study to create a new classification for the LET, and will determine if a single unit interpretation is supported.