TECTONIC EVOLUTION OF THE BLACK HILLS, SOUTH DAKOTA FROM NEW U-PB AND HF ANALYSES OF IGNEOUS AND DETRITAL ZIRCON

The Black Hills uplift located on the eastern margin of the Wyoming craton preserves a remarkable archive of tectonic events spanning the Neoarchean assembly of the eastern Wyoming Province through terminal collision of the Superior and Wyoming cratons in the late Paleoproterozoic. Five sedimentary basins, formed between 2.6 and 1.78 Ga, record the tectonic setting of the eastern Wyoming craton and provide a geochronologic framework for comparison to events within adjoining cratons.

We present results of in-situ U-Pb and Hf analyses of detrital zircon from a geographically and stratigraphically representative sampling of metasedimentary units as well as analyses of igneous zircon from Black Hills granitoids. Older Paleoproterozoic sedimentary units contain exclusively Archean detritus with major peaks of 2.7, 2.85, and 2.95 Ga age and subordinate populations extending back to 3.43 Ga. These are interpreted to be intracratonic (rift) and passive margin (drift) sediments. Voluminous turbidite greywackes previously assigned to several different ~1.88 Ga formations show detrital age peaks of 1820, 1830, 1860, and 1980 Ma as well as five discrete age peaks between 2100 and 2400 Ma and minor Archean detritus. These units are interpreted to be a single turbidite basin that was deposited after 1830 Ma and that marks the initiation of arc assembly along the eastern Wyoming craton. Widespread debris flow conglomerates in the Bluebird Formation contain abundant zircons <1800 Ma and are interpreted to be a younger turbidite package that formed during Green Mountain arc accretion along the southern Wyoming craton.

New Hf analyses of plutonic rocks show that the ~2.6 Ga granites in the Little Elk and Missouri Buttes exposures are underlain by Mesoarchean crust of 3.1-3.3 Ga age, confirming the detrital zircon evidence that the Black Hills represent Archean Wyoming Province crust.

In a new model, we construct a series of time slice maps for the tectonic evolution of the Black Hills including the birth of Mesoarchean substrate, Neoarchean assembly of the eastern Wyoming Province at 2.6 Ga, episodic Paleoproterozoic rifting 2.5-2.0 Ga, juvenile Trans Hudson arc collision 1.88-1.83 Ga and its trench turbidites, 1.78 Ga turbidites from Green Mountain arc collision, and final orogenic assembly from 1.78 to 1.69 Ga.