PETROLOGIC AND GEOCHEMICAL CHARACTERIZATION OF ARCHEAN GNEISSES IN THE LITTLE ELK TERRANE, BLACK HILLS, SOUTH DAKOTA

The Little Elk terrane (LET) is an Archean exposure along the NE margin of the Black Hills, SD. Current interpretations divide the terrane into two units, the Little Elk Granite (LEG, 2559 ±6 Ma) and the Biotite-Feldspar Gneiss (BFG, 2563 ±6 Ma); however, two conflicting petrologic interpretations have been proposed. While both interpret the LEG as intrusive into the BFG, one argues for a supracrustal origin for the BFG and the other argues for magmatic. A third alternative, based on structural studies by Winona State researchers, proposes strain-partitioning of a single granite body to explain the variation in outcrop appearance within the LET. The goal of this research is to determine which of the three models best explains the variation seen in the Archean Rocks of the LET.

Field mapping at 1:8000 identified significant spatial variation across the LET and developed a classification for the two end member Archean lithologies, The LEG is a coarse-grained 2-mica porphyritic granite containing pink microcline megacrysts up to 4.5 cm. The BFG is a medium-grained biotite-rich microcline-augen gneiss with minor muscovite. Both units contain a well-developed shear-fabric interpreted to have developed during the final suturing of the Wyoming and Superior provinces. Typically the BFG is distinguished from the LEG by finer-grained felsic minerals, greater biotite and lesser microcline abundances, a more closely spaced and more continuous biotite foliation, and significantly increased shear-strain indicated by fabric and textures.

During mapping a visually distinct transitional sub-lithology was differentiated. This lithology is defined as distinctly intermediate to the BFG and LEG in all the above criteria. Most distinguishing is that when located within the BFG, it displayed slightly less strain than the BFG, yet when located within the LEG it displayed slightly more strain than the LEG, supporting the interpretation that strain partitioning may control lithologic appearance. Furthermore, fully within the LEG, strong strain partitioning into narrow, 10-50 cm wide high-strain zones result in rocks indistinguishable from the BFG. Pending geochemical analyses from all 3 lithologies will help determine whether strain partitioning of a single rock unit can lead to the variable appearance seen in the Little Elk Terrane.