A QUANTITATIVE STUDY TO DIFFERENTIATE SIMILAR GNEISSES IN THE ARCHEAN LITTLE ELK TERRANE, BLACK HILLS, SOUTH DAKOTA

The Little Elk Terrane (LET) in the northeastern boundary of the Black Hills contains one of only two exposures of Archean basement rocks in the hills. Currently the LET is subdivided into three lithologic units that include two Archean-aged crystalline basement units and one Proterozoic-age metasedimentary unit. The Archean rock units include the Biotite Feldspar Gniess (BFG) and the Little Elk Granite (LEG). The LEG is a coarse-grained granite rich in feldspar and the BFG is a biotite gneiss with coarse-grained feldspar augen. Two criteria commonly used to distinguish the two are; coarser-grained feldspar in the LEG and greater abundance of biotite in the BFG. Recent mapping by WSU students in the LET raises concerns about the location of the previously mapped BFG-LEG contact. In some exposures mapped as LEG, the rocks are extensively sheared and appear very similar to the BFG due to grain-size reduction in the feldspar and an increase in biotite that occurred during shearing. This makes distinction of these two units very difficult in many outcrops.

The goal of this project was to define quantitative criteria that will allow the classification of the two units in order to define better the contact location. Two independent methods were employed. The first will compare feldspar size by measuring individual grains with a millimeter scale and calculating the area using an ellipse formula. The second will look at variations in biotite abundance, and separation between biotite foliation.

A size vs. frequency graph shows that BFG samples have grains smaller than 1.5 mm² with greater than 30% biotite. LEG samples have grains larger than 5mm², with a majority greater than 10mm², and contain less than 20% biotite. Some data are transitional and plot between the BFG and LEG end members, with grain size decreasing corresponding with biotite percentage increasing. Additionally, one sample collected from a 30 cm wide shear band in an outcrop of irrefutable LEG is indistinguishable from the BFG using these criteria. We interpret these findings to suggest that the BFG may well be sheared LEG and the “transitional” samples demonstrate varying strain amounts between the typical LEG and BFG.