Abstract

Paleoproterozoic (1770–1735 Ma) bimodal volcanic rocks in central Colorado have been considered to represent southward growth of Laurentia by arc accretion. Although the bimodality of these rocks suggests an extensional continental setting rather than continental or oceanic arcs, there has been little evidence for pre–1800 Ma crust south of the Wyoming craton other than the 1840 ± 1 Ma Elves Chasm pluton in the Upper Granite Gorge of the Grand Canyon, Arizona. We report SHRIMP U-Pb ages of inherited zircons from metarhyolites and plutons in central Colorado that are latest Archean–earliest Proterozoic (2520–2000 Ma) and Trans-Hudson–Penokean (1878–1814 Ma). Associated quartzites contain detrital zircons with mean ages of 1735 Ma, indicating only local derivation. A meta-arkose, however, contains detrital zircons of Trans-Hudson–Penokean and Archean ages. We believe it likely that the 1900–1800 Ma Trans-Hudson–Penokean orogens, including Archean enclaves, extended farther south and west than is currently thought, and were the source of the bimodal volcanic rocks and associated plutons during the period 1770–1700 Ma.

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