NEW CLASSIFICATION FOR ARCHEAN GNEISSES IN THE LITTLE ELK TERRANE BASED ON STRAIN AND MICA ABUNDANCES, BLACK HILLS, SOUTH DAKOTA

The Little Elk Terrane (LET), located in the northeastern Black Hills, SD, exposes structurally complex Archean granitic basement. Research by Winona State University (WSU) questions the current interpretation that divides the gneiss into two units. This project describes the mineralogical and textural characteristics of the LET in fine detail, and combined with geochemical analysis by Evenocheck and Allard (This Meeting) will propose an alternative classification and genesis for the LET.

The LET is currently interpreted as two distinct gneissic units by Gosselin (1988, 1989) and McCombs (2004): 1) Biotite-feldspar gneiss (BFG; 2.532+/- 11 Ma) and 2) Little Elk Granite (LEG; 2.549+/- 11 Ma). Arguments for a two-unit model are: minor age differences, variation in mica abundances and feldspar grain size, and the presence of a NE-trending fabric in the BFG, absent in the LEG. Research at WSU identified the NE-trending fabric in both units, and has hypothesized that the mineralogical and textural differences are likely due to variation in strain, where higher strain zones are distinguished by increased mica content and reduced grain size. These observations combined with the overlapping ages allows for a single unit LET with varying degrees of strain.

Detailed mapping of gneisses exposed in the LET assigned a “strain group” to every outcrop, or portion of an outcrop, based on the flattening of feldspar grains (strain ratio), and modal micas. In the eastern third, characteristically LEG, the rock is coarser grained, strain ratios are typically <1.75:1, and modal mica is typically lower than 15%; whereas, in the western third, characteristically BFG, modal mica is >30% and strain ratios are typically >3.75:1. The central third is transitional between these two end members. Furthermore, rocks from all strain groups are recognized in all three domains with low-strain rocks in the western third that are indistinguishable from low-strain rocks from the eastern third and vice-versa. These field observations are consistent with overall increasing strain from east to west across the LET with strain partitioning responsible for the variation in strain within each domain. Samples collected from across the LET will document this variation during ongoing, microstructural and geochemical analysis in order to refine our classification.