ELUCIDATION OF AN UNDEFINED RELATIONSHIP: A STUDY OF THE LITTLE ELK GRANITE AND BOXELDER CREEK QUARTZITE AS TO DECIPHER PRECAMBRIAN BASIN DEVELOPMENT IN THE BLACK HILLS, SD

The Precambrian tectonic history of the Black Hills remains enigmatic, with unresolved questions surrounding the relationship between the Little Elk Granite (LEG-2560 Ma augen gneiss) and the Boxelder Creek Quartzite (BCQ- >2480 Ma), which are among the oldest rocks exposed in the Black Hills. The intrusion of the Blue Draw gabbro (2480 Ma) sets a lower age limit on the BCQ. The BCQ is an orthoquartzite with stretched pebble metaconglomerate layers thought to be sourced from crust exposed, including the LEG, in the east during a Paleoproterozoic rifting event. However, the absence of granite clasts in the BCQ does not support the interpretation of a predominantly granitic source terrain, yielding the possibility that the BCQ may be older than the LEG or is derived from other areas. This study aims to determine the structural evolution, geochemical characteristics, and maximum depositional age of the BCQ to determine what features may be shared with the LEG. Outcrops of the BCQ have steeply SW-dipping foliation (135, 77) and contain sheared clasts that record top-to-the-left kinematics. Length measurements of elongated cobbles in outcrops reveal distinctive clustering within the flattening field on Hsu and Flinn-Ramsay plots. In thin section, static recrystallization structures and undulose extinctions are observed among quartz grains. No feldspar grains were observed in thin section, weakening a potential link to the LEG. Compositional analysis conducted with scanning electron microscopy reveals the presence of iron ooids and various REE (La, Ce, and Nd) substitution into monazite. Silver and chromium mineralization are present. X-Ray fluorescence will be utilized to derive a chemical analysis of the BCQ and compare with published geochemical data from the LEG. Forthcoming laser ablation U-Pb dating of detrital zircons will reveal the age of sediment source areas for the BCQ, including whether it contains ca. 2560 Ma zircon grains that could be linked to the LEG. This study contributes valuable insights into the tectonic evolution of the BCQ, shedding light on the processes involved in development of the Paleozoic rifting event that would later close during the Trans-Hudson Orogeny.