PETROGENESIS AND TECTONIC IMPLIATIONS OF MAFIC ROCKS IN THE PRECAMBRIAN CORE OF THE BLACK HILLS, SOUTH DAKOTA

The Precambrian core of the Black Hills of South Dakota records evidence of the metamorphic, structural and magmatic events that occurred in the area during the Proterozoic collision of the Wyoming and Superior provinces. While these syn-collisional events are now well understood, the geologic setting and tectonic events that occurred prior to the collision are still enigmatic. In this study we used petrology of metabasalts and amphibolites to define the mantle component of the pre-collisional tectonic history of the Black Hills.

There are two main suites of mafic rocks in the Black Hills which occur on opposite sides of the Silver City Fault. This and other faults run parallel to the NNW-trending collisional fabric along the eastern side of the Black Hills. East of the fault, the mafic rocks are mostly gabbros, including the Blue Draw Metagabbro (BDM) near Nemo. This is the largest single mafic body in the Black Hills, and has been dated at 2.48 Ga (Dahl et al., 2003; GSA Abstract). The BDM has distinct calc-alkaline affinities, not unlike the somewhat older Little Elk Granite that occurs in the vicinity. The chemical characteristics of these rocks suggest a volcanic arc setting related to a subduction zone. Many of the metagabbros show mineralogic layering and cumulate textures which may suggest differentiation in layered mafic intrusions.

To the west of the Silver City faults, the oldest basement rocks are Archean metapelites and the 2.55 Ga Bear Mountain granite (Redden et al. 1990). These units are overlain by sequences of pelites and localized iron formations, cherts, and calcareous sedimentary rocks, all of which were intruded by basaltic dikes and sills. Calcite is present in some of these mafic rocks, suggesting possible sea water alteration. All of the mafic rocks here have distinctly different ages (~1.96 Ga; ibid) and compositions compared to rocks to the east of the fault. They have tholeiitic signatures close to those of mid-ocean ridge basalts and are generally finer grained, suggesting generation along an oceanic spreading center.

The contrasting characteristics of the mafic rock suites suggest that distinctly different pre-collisional tectonic regimes are juxtaposed by faults within the Precambrian core of the Black Hills.