Paleoproterozoic rocks of central Colorado: Accreted arcs or extended older crust?

Paleoproterozoic (1770-1735 Ma) bimodal volcanic rocks: in central Colorado have been considered to represent southward growth of Laurentia by arc acc retion. Although the bimodality of these rocks suggests an extensional cont inental setting rather than continental or oceanic arcs, there has been lit tle 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 Gran d Canyon, Arizona. We report SHRIMP U-Pb ages of inherited zircons from met arhyolites and plutons in central Colorado that are latest Archean-earliest Proterozoic (2520-2000 Ma) and Trans-Hudson-Penokean (1878-1814 Ma). Assoc iated quartzites contain detrital zircons with mean ages of 1735 Ma, indica ting only local derivation. A meta-arkose, however, contains detrital zirco ns of Trans-Hudson-Penokean and Archean ages. We believe it likely that the 1900-1800 Ma Trans-Hudson-Penokean orogens, including Archean enclaves, ex tended farther south and west than is currently thought, and were the sourc e of the bimodal volcanic rocks and associated plutons during the period 17 70-1700 Ma.