DETRITAL ZIRCON DATING OF PRECAMBRIAN METASEDIMENTARY ROCKS OF THE CENTRAL AND EASTERN BLACK HILLS, SOUTH DAKOTA

Precambrian rocks of the Black Hills uplift record multiple deformational and metamorphic events related to three orogenic events between 1̃780 and 1690 Ma. Previous workers ascribed these events to several orogenies. Uranium-lead detrital zircon ages were determined to help constrain the maximum depositional ages of metasedimentary units. Data from this study, augmented by detailed mapping, was used to constrain the timing of deposition and subsequent tectonism that formed Laurentia. Twenty-nine samples were collected from fault-bounded blocks within the central and eastern uplift and prepared for detrital zircon analysis. Ten samples were selected for this study based on previously defined lithostratigraphic units and location relative to fault-bounded blocks. Many of the metasedimentary units within the eastern and central uplift have been grouped into lithostratigraphic units. Correlation of many of these units has remained tenuous between fault-blocks based on provenance, chemistry, and crustal affinity. These blocks have previously been interpreted as part of the Wyoming craton margin deformed during the assembly of proto-North America; or alternately as crustal blocks having different tectonic histories that were emplaced along the eastern margin of the Wyoming Craton. All samples have a distinct Trans-Hudson age populations with peaks of approximately 1870 Ma. However, there are distinct differences between older and younger populations which indicate that the fault-bounded blocks received input from multiple sources of various ages, suggesting that previously correlated lithostratigraphic units of the fault-bounded blocks have differing provenance sources and, hence, may have different tectonic histories. This agrees with the detailed mapping and structural analysis. Older Paleoproterozoic and Archean populations have distinct peaks that vary from sample to sample but cluster in peaks of ~2020Ma, ~2348 Ma, ~2540 Ma, and ~2860 Ma. The oldest grain from this study was dated at 3675 Ma and is interpreted to be inherited from the Wyoming Craton. Rocks previously defined as the oldest lithostratigraphic unit in the eastern Black Hills (“Norris Peak rocks” of Norton et al. 2008) contain some of the youngest detrital zircons from this study (~1640 Ma).