CHARACTERIZING A PROTEROZOIC LEUCOGRANITE WITHIN THE ARCHEAN LITTLE ELK TERRANE, BLACK HILLS S.D

The Little Elk Terrane (LET) is a window into the Archean basement in the NE margin of the Black Hills. Currently the Archean rocks are split into two units; the Little Elk Granite (LEG) and the Biotite-Feldspar Gneiss (BFG). Field mapping in the LET identified several small discontinuous bodies of post-tectonic leucogranite in both Archean units. This magmatism may be key to understanding the relationship between the LEG and BFG and the timing of shearing recognized across the LET. This research focused on two questions related to this leucogranite; 1) is there a structural element controlling granite locations, and 2) are all exposures of leucogranite from a single source or multiple sources. To answer these questions my research compared the host rock lithology and structural characteristics, and characterized the mineralogical variation for the granite at each granite location.

The leucogranite cross-cuts both the BFG and LEG and commonly form sill-like bodies parallel to the NW-striking mylonitic fabric. In hinge areas where the fabric is folded to NE striking and overprinted by a younger NW-striking fabric, it is found intruding parallel to either or both. Therefore, the fabrics control the granite body’s shape, pre-existing structures do not control where in the LET the melt migrated and recrystallized.

Thin section analysis of ten granite samples from across the LET compared mineralogic criteria to determine if a single source for the melt is possible. These included; 1) modal abundances for major minerals 2) plagioclase An content, and 3) accessory minerals. All samples contained similar amounts of quartz, and feldspars, but biotite showed minor variation in mode. Preliminary Anorthite content estimated using the Michel-Levy method are similar in all samples and at present no accessory minerals have been identified. Because biotite was scarce and disseminated in samples collected interior to melt bodies, and present as schlieren in samples collected proximal to the host rock, we interpret this variation a result of contamination by the host rock, not a source differences. Although not conclusive, we found no evidence to suggest more than one source for the melt. Therefore, we interpret a single post-tectonic intrusion for all the discontinuous bodies across both lithological units during a single melting event.