PETROLOGY AND ORIGIN OF THE PROTEROZOIC CROW FORMATION, BLACK HILLS, SOUTH DAKOTA

The Proterozoic Crow Formation occurs in a 20 km-long belt across the southwestern Precambrian core of the Black Hills. Its present thickness ranges from ~40 m in the southeast near the Grand Junction Fault to ~500 m in the hinge of a regional syncline in the northwest, where it continues beneath the Paleozoic unconformity. The Crow separates thick-bedded quartz-mica-feldspar schists of the Bugtown formation from mineralogically similar but thinner-bedded schists of the Mayo Fm. These have been interpreted (Redden and Dewitt, 2008) as proximal and distal turbidites. The present thickness of these units is 2 km and 4+ km respectively. Even with structural thickening, they must represent significant basins of clastic accumulation.

As seen in natural exposures, dominant rock types of the Crow, in order of decreasing abundance, include hornblende-plagioclase rock, actinolite-chlorite schist, hornblende-plagioclase-calcite gneiss, diopside-hornblende–plagioclase gneiss, and minor biotite-microcline schist. A laminated quartzite commonly occurs at or near the top. The hornblende-plagioclase and actinolite-chlorite units are interpreted as metavolcanic rocks, probably flows. The gneisses are interpreted as metamorphosed tuff and pyroclastic rock based on their composition, layering and textures. Enigmatic biotite-microcline rocks may be tuffs or, based on localized occurrence, the result of potassic alteration.

The actinolite-chlorite schists have basaltic komatiite compositions. MgO typically ranges between 15 and 22 wt %, with normative olivine and hypersthene. The hornblende-plagioclase rocks include basaltic komatiites and basalts, with equal abundances of olivine- and hypersthene–normative compositions. HFSE abundance patterns match those of modern E-MORBS.

The technique of Lee et al. (2009) was used to estimate the PT of melting of the actinolite-chlorite schists as ranging between 1450^o 1.7 GPa to 1650^o 3.9 GPa, overlapping their komatiite field and resulting from roughly 30% dry melting of lherzolite mantle. We suggest that the thermal effects of the shallow, hot plume led to uplift and change in sedimentation regime of the enclosing sedimentary units.