TECTONIC IMPLICATIONS OF PALEOPROTEROZOIC MAFIC ROCKS IN THE BLACK HILLS, SOUTH DAKOTA

Metamorphosed mafic to intermediate intrusions and flows that occur in the Precambrian core of the Black Hills served to establish the tectonic settings of magmatism during the early Proterozoic history of the region. The rocks define at least two distinct tectonic settings of magmatism between 2480 and 1880 Ma. The 2.48 Ga Blue Draw intrusion, located on the eastern edge of the Precambrian core, has compositions ranging from gabbro to granodiorite. It is a calc-alkaline pluton with SiO₂ from 51.8 to 60.5 wt.% and K₂O >1 wt.%. REE patterns are slightly concave upward with LREE enrichment. The intrusion is also enriched in Ba, Rb, Th, and Zr. The geochemistry is typical of modern calc-alkaline rocks and is similar to the neighboring, 2.56 Ga Little Elk granite-gneiss. These two intrusions indicate the presence of late Archean/early Proterozoic arc magmatism on the eastern margin of the Wyoming craton.

The other mafic rocks occur within sequences of deep marine sediments. Published U–Pb zircon ages are between 2.01 and 1.88 Ga. In the Mt. Rushmore and Pactola Dam quadrangles and near Bear Mountain, these rocks have slightly depleted to enriched tholeiitic compositions with SiO₂ ∼50 wt.%, K₂O generally <0.5 wt.%, and flat to slightly light-enriched REE patterns. The chemical characteristics of these two suites point to E-MORB-type magmatism that characterizes present-day plume-related magmatism at oceanic spreading centers. A mantle plume source is especially evident in the composition of mafic rocks in the Rochford district. They have LREE-enriched patterns, have the highest TiO₂ concentrations, typically >2.5 wt.%, and the lowest SiO₂ concentrations, typically <50 wt.%. They also have the highest Nb/Yb and Ta/Yb ratios and are relatively depleted in Y. With the Bear Mountain and Rushmore–Pactola suites, the Rochford suite provides evidence for rifting of the crust over a mantle plume that has produced a submarine spreading center. This plume-related magmatism in the Black Hills was apparently coeval with 2.01 Ga basic magmatism in southeastern Wyoming and together these locations indicate an extended presence of a mantle plume under the eastern Wyoming craton at this time. The plume may have led to the break-up of the previously proposed paleocontinent Kenorland.