Leucogranites in the Black Hills of South Dakota: The consequence of shear heating during continental collision

Leucogranites are common features of convergent orogens. However, the heat necessary for producing leucogranite magma from pelitic sources in convergent orogens is a major unresolved issue. We suggest that the generation of the Harney Peak Granite, Black Hills, South Dakota, during the Proterozoic Trans-Hudson orogeny is best explained by shear heating during thrusting of a Proterozoic sedimentary sequence over the Archean Wyoming basement. Our thermotectonic model successfully reproduces pressure-temperature-time constraints on early regional metamorphism of schists and subsequent generation of the late orogenic Harney Peak Granite. The model is consistent with isotopic data for the granite, which require both Proterozoic and Archean source rocks, and its emplacement as small batches of melt. The similarity of geologic relationships in the Black Hills to those of other collisional orogens suggests that shear heating may be a viable mechanism for leucogranite generation in thickened crusts.