Sunday, 5 October 2008
George R. Brown Convention Center, Exhibit Hall E
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.