Lu-Hf depleted mantle model (TDM) ages, obtained by analysis of zircons previously dated by U-Pb methods, demonstrate that ca. 1.75 Ga bimodal Paleoproterozoic rocks in the Gunnison-Salida region of central Colorado, and by extension in much of the southwestern U.S., were formed by partial melting of preexisting crustal rocks, the ages of which mostly ages greater than 2.0 Ga indicate that range from 1.83 to 1.87 Ga. Some calculated Lu-Hf TDM even older crust was probably involved, consistent with the limited presence of ca. 2.5 Ga xenocrystic zircons in some rocks. These results suggest that rocks related to the Trans-Hudson and Penokean orogens are cryptically present much farther to the south than previously believed. Coupled with the bimodality of the volcanic suite in central Colorado, and indeed in much of the southwestern U.S., these results indicate that, in contrast to current juvenile arc-accretion models, melting of older crust related to extensional tectonics played an important role in the genesis of many magmatic rocks between 1.6 and 1.8 Ga.

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