M.E. Bickford et al., Dept. of Earth Sciences, 204 Heroy Geology Laboratory, Syracuse University, Syracuse, New York 13244-1070, USA. Pages 555-558.
In the Gunnison-Salida region of central Colorado, there are exposures of 1750 million-year-old (Paleoproterozoic) volcanic rocks, including both basalt and rhyolite, and associated granite bodies. These rocks have been thought to represent ancient island arcs that were accreted to the growing North American continent at this time. The bimodal (basalt-rhyolite) composition, however, suggests instead that they were derived from preexisting older continental crustal rocks. In two earlier studies authors Bickford and Hill used the uranium-lead method to date zircons (ZrSiO4) from these rocks, finding that, in addition to the 1750 million-year-old zircons that date the crystallization age, there are numerous "inherited" older zircons whose ages are mostly about 1850-1870 million years. These presumably were derived from preexisting crust that was melted to produce the rhyolite and granite, as hot basaltic magma was introduced from the mantle during crustal rifting. In the current study, the same zircons were analyzed for their hafnium (Hf) isotopic composition. The results of analyses reported by Bickford et al. yielded Hf model ages of 1850 to 2000 million years, significantly older than the crystallization age of the rocks, and confirming their derivation from melting of older crustal sources. This result suggests that in the Gunnison-Salida area, and likely across much of the southwestern United States where similar bimodal volcanic assemblages are known, volcanic and related magmatic activity was related to a period of extension, rifting, and re-melting of previously formed crust. This scenario has important implications for the Proterozoic growth of North America
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