GEOCHEMISTRY AND TECTONICS OF THE YATES UNIT OF THE POORMAN FORMATION (DUSEL BEDROCK) IN THE CONTEXT OF OTHER NORTHERN BLACK HILLS AMPHIBOLITES

The Early Proterozoic (~2012 Ma) Yates unit of the Poorman Formation is an amphibolite in the northern Black Hills, exposed near Lead, SD. The planned Deep Underground Science and Engineering Laboratory (DUSEL) will be constructed primarily in this unit. The geochemistry of a new suite of surface samples of the Yates unit is considered, from an igneous perspective, in the context of previous data from Homestake Mine drill core (from the 6800’ level), new surface samples from other northern Black Hills amphibolites, and data from an overlapping study to the south by Van Boening & Nabelek (2008: Precamb. Res. 167).

The data was filtered to focus on samples whose normalized major element compositions resembled mafic lavas. Meeting imposed criteria were 11/18 Yates surface samples, 52/104 core samples, and 48/55 regional samples. Omitted samples have compositions reflecting metamorphic modification or cumulate, volcaniclastic, or sedimentary protoliths.

The surface Yates unit suite has bulk compositions of primitive to moderately evolved (12.3-6.0 wt% MgO) tholeiitic basalts. The upper ~275 m of the core is geochemically similar with a broader compositional spectrum. The lower ~350 m of the core is compositionally distinct with higher TiO₂ and P₂O₅. The surface Yates samples are slightly LREE-depleted and plot in the N-MORB fields of many tectonic discrimination diagrams (Nb-Zr-Y, Th/Yb-Nb/Yb, etc.). Based on fewer trace elements, the upper portion of the Yates core is broadly similar, but the lower Yates core is distinctly enriched in incompatible elements and plots in E-MORB to OIB fields on discrimination diagrams.

Regionally, the upper Yates unit is more like N-MORB than any other Black Hills amphibolites. Possibly correlative samples to the south transition to E-MORB compositions. Amphibolites in the Rocheford, SD area are up to 130 m.y. younger and have an OIB affinity. The lower Yates unit may reflect an early phase of continental rifting, with or without mantle plume influence. The strong N-MORB signature of the upper Yates suggests a mature rift. Apparent spatial variations between correlative units could reflect rift architecture or a variation in plume influence, though temporal variation cannot be ruled out. The OIB signature of the younger Rochford rocks may reflect renewed rifting or plume interaction.