MORE EVIDENCE THAT THE BLACK HILLS DOMAIN WAS ORIGINALLY PART OF THE ARCHEAN WYOMING CRATON: OCCURRENCES OF 3.8-2.6 GA DETRITAL ZIRCONS IN 2.5-1.9 GA METASEDIMENTARY ROCKS

A longstanding question concerning the Archean Wyoming craton is whether the Black Hills domain has always comprised its eastern margin, as suggested by published Pb- and Sm-Nd-isotopic tracer studies or, instead, was juxtaposed at 1.72-1.71 Ga along the Hartville-Rawhide fault, a prominent convergent shear zone of known age in SE Wyoming. This question is being addressed with ongoing U-Pb microchronometry of detrital magmatic zircons in Precambrian metasedimentary rocks from the Black Hills basement. Seven rocks representing diverse lithology, age, and location have been analyzed thus far, including: four 2.56-2.48 Ga conglomeratic quartzites (Boxelder Creek Formation, Nemo); two 2.01-1.89 Ga metapelites (Vanderlehr Formation, Bear Mtn.; unnamed schist, Crook Mtn.); and a 1.89 Ga phyllite (Montana Mine Formation, Rochford). U-Pb concordia plots of all the ²⁰⁷Pb/²⁰⁶Pb spot-dates, and relative probability plots of the >90% U-Pb concordant dates, indicate that the detritral-zircon populations range in age from 3.4-2.6 Ga in each rock analyzed, despite their wide 2.56-1.89 Ga range of depositional ages. Specifically, detrital zircons in: (1) the older rocks exhibit 9 apparent age populations between 3.45-2.59 Ga, with 2.67 and 3.32-3.10 Ga ages predominant in the 3 quartzites and a metaconglomerate, respectively; (2) the intermediate-age metapelites exhibit 8 such populations of 3.43-2.61 Ga age (3.37 Ga predominant), with an outlier at 3.83 Ga; and (3) the younger phyllite exhibits 8 populations of 3.41-2.55 Ga age (2.71-2.68 Ga predominant).

As for potential detrital source(s), crustal ages ranging from ~3.9-2.5 Ga and ~2.9-2.5 Ga characterize the Wyoming and Superior cratons, respectively. Thus, a Wyoming source is more likely for 3.8-2.6 Ga zircons in the Black Hills, indicating also that this domain was contiguous with the craton at 2.56-1.89 Ga. Support for this hypothesis is provided by the cross-bedded quartzites, which independently point to westerly (i.e., Wyoming) sources. We suggest that the Black Hills domain was initially separated from the Wyoming craton during early Trans-Hudsonian back-arc rifting at 1.88 Ga (known ages of tholeiitic dikes) and then rejuxtaposed during terminal Wyoming-Superior collision at 1.715-1.695 Ga (known timing-duration of Harney Peak granitic magmatism).