INVESTIGATING SHEAR-HEATING MODEL FOR LEUCOGRANITE GENERATION IN ARCHEAN BASEMENT, BLACK HILLS, SOUTH DAKOTA

Leucogranites are common at continental collisional boundaries, (i.e.: the Black Hills, Central Maine Belt, High Himalayas) suggesting a strong connection between deformation, metamorphism, and granite generation. In the Black Hills a shear heating model is proposed for the Harney Peak Granite (Nabelek and Liu, 1999; Nabelek et al., 2001) during final suturing of the Wyoming and Superior provinces (D3) at ca. 1715 Ma. Within the Little Elk Terrane in the NE Black Hills, Archean basement gneisses are strongly sheared during D3, and intruded by numerous leucogranites. The goal for this research is to determine whether a shear heating model, similar to that proposed for the Harney Peak Granite, can be applied to the leucogranites in the Little Elk Terrane for the same tectonic event.

Field mapping at 1:8000 within the Little Elk Terrane, identified numerous small cross-cutting leucogranitic bodies including narrow dike-like bodies and more irregular-shaped, pluton-like bodies. The dike-like bodies range from millimeter to centimeter in thickness, extend longer than several meters, and were mapped across the entire Archean terrane. The slightly larger pluton-like bodies range from one to ten meters in diameter and are commonly located in areas where the shear fabric has been folded by late shear-related map-scale folds. These bodies show both evidence that they cut across the fabric within the fold hinge as well as evidence that they may have been reoriented slightly with the shear fabric, suggesting a late-syntectonic relationship with the folding of the shear fabric.

The typical leucogranites within the Little Elk Terrane are grey weathered, pale-beige fresh, fine-grained, granular, and composed almost entirely of quartz, plagioclase, and K-feldspar, with minor biotite. Other than the two-mica content and the course-grained porphyritic texture of the Little Elk Granite, the leucogranites are mineralogically similar to the their host rock, making the Little Elk granite permissible as a source rock for the leucogranitic melt. Thin section analysis and geochemical data from 24 samples of leucogranite and the suspected Archean source rock samples are pending. These analyses will help determine if the Archean host could be the source for the leucogranite melt, and whether the shear-heating model is probable.