2008 Joint Meeting of The Geological Society of America, Soil Science Society of America, American Society of Agronomy, Crop Science Society of America, Gulf Coast Association of Geological Societies with the Gulf Coast Section of SEPM

Paper No. 15
Presentation Time: 8:00 AM-4:45 PM

Was the Black Hills Domain (South Dakota) Always Part of the Archean Wyoming Craton?: New Evidence from ~3.4-2.7 Ga Detrital-Zircon Populations Found in Both ~1.9 and ~2.5 Ga Strata

DAHL, Peter S., Department of Geology, Kent State Univ, Kent, OH 44242, FREI, Robert, Institute of Geography and Geology, University of Copenhagen, Øster Voldgade 10, Copenhagen, DK-1350, Denmark and WHITEHOUSE, Martin J., Swedish Museum of Natural History, Box 50007, Frescativägen 40, Stockholm, SE-104 05, Sweden, pdahl@geology.kent.edu

A longstanding question concerning the Archean Wyoming craton is whether the so-called Black Hills domain has always comprised its eastern margin, as suggested by published Pb- and Sm-Nd-isotopic tracer studies or, instead, was juxtaposed later along a prominent Paleoproterozoic shear zone. This question is being addressed further by detrital-zircon microchronometry of Precambrian metasedimentary rocks in the Black Hills. Two such strata are the ~2.56-2.48 Ga Boxelder Creek Formation, a conglomeratic quartzite (Nemo), and the Montana Mine Formation, a younger BIF unit (Rochford); these rift-related units were deposited directly below and well above the ~2.48-2.02 Ga Estes unconformity, respectively. Intercalated with the BIF is a felsic tuff in which magmatic zircon spot-dated by ion microprobe yields a 207Pb/206Pb upper-intercept age of 1887 ± 7 Ma (2σ, MSWD = 0.90, mean Th/U = 0.75, n = 16). This result is interpreted as the age of local BIF deposition, which correlates with regional Trans-Hudsonian magmatism and global superplume activity, both of which are dated at ~1.9 Ga. In contrast, thin, dark laminae of mafic phyllite occurring within the ~1.89 Ga tuff contain only detrital components of magmatic zircon, whose >90%-concordant spot-analyses yield 207Pb/206Pb ages ranging from ~2.67 Ga (predominant) to ~3.45 Ga (subordinate). Similarly, nearly-concordant detrital zircons of magmatic origin in three samples of the older quartzite range in 207Pb/206Pb spot-age from ~2.56 to ~3.45 Ga (n = 55), with several distinct populations being evident between ~2.56-2.65 Ga (n = 10), ~2.65-2.78 Ga (n = 31), and ~2.80-3.45 Ga (n = 14). Presence of ~3.00-3.45 Ga detrital components in these sedimentary horizons strongly implies that both originated from Wyoming craton sources, which apparently underwent sub-aerial erosion at ~2.56-2.48 and ~1.89 Ga. Thus, we conclude that the Black Hills domain was part of the Wyoming craton at least as long ago as 2.6 Ga.