Petrogenesis and tectonic implications of paleoproterozoic mafic rocks in the Black Hills, South Dakota

Abstract

Four suites of mafic to intermediate intrusions and flows that occur in the Precambrian core of the Black Hills were analyzed for major and trace elements to establish tectonic settings of magmatism during the late Archean and early Proterozoic history of the region. Most of the mafic rocks were later metamorphosed to amphibolites during the Black Hills orogeny, the collision of the Wyoming and Superior cratons which began at ∼1760 Ma. Compositions of the mafic suites 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, is differentiated with compositions ranging from gabbro to granodiorite. It is a calcalkaline intrusion with relatively high SiO 2 from 51.8 to 60.5 wt.% and K 2O mostly >1 wt.%. REE patterns are slightly concave upward and show LREE enrichment. The intrusion is also enriched in Ba, Rb, Th, and Zr. These and other geochemical discriminants are typical of modern calcalkaline basalts. The intrusion's calcalkaline character is similar to the neighboring, slightly older 2.56 Ga Little Elk granite-gneiss. These two intrusions support the presence of late Archean/early Proterozoic arc magmatism on the eastern margin of the Wyoming craton. The three other mafic suites comprising metamorphosed flows and sills occur within a sequence of deep marine sediments, including black shales, limestones, iron formations, and cherts. U–Pb zircon ages suggest their emplacement between 2.01 and 1.88 Ga. Mafic rocks in the eastern half of the Precambrian core in the Mt. Rushmore and Pactola Dam quadrangles and near Bear Mountain on the western margin of the Precambrian core have slightly depleted to enriched tholeiitic compositions with SiO 2 ∼50 wt.% and K 2O generally <0.5 wt.%. They have flat to slightly light-enriched REE patterns. The chemical characteristics of these two suites point to E-MORB-type magmatism that characterizes present-day, mantle plume-related magmatism at oceanic spreading centers. A mantle plume source is especially evident in the composition of mafic rocks in the Rochford district in the north-central part of the Precambrian core. These mafic rocks have LREE-enriched patterns. They have the highest TiO 2 concentrations, typically >2.5 wt.%, and the lowest SiO 2 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 may indicate an extended presence of a mantle plume under the eastern Wyoming craton. The plume may have led to the break-up of the previously proposed paleocontinent Kenorland.