278 D50. Precambrian Geology (Posters)

Tuesday, 24 October 2017
9:00 AM - 6:30 PM
Halls 4EF

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

Paper number: 278-3
Program: Technical Sessions
Time: 9:00 AM – 6:30 PM

Authors

Abstract

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 1450o 1.7 GPa to 1650o 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.

Geological Society of America Abstracts with Programs. Vol. 49, No. 6

doi: 10.1130/abs/2017AM-303495

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