Abstract

A common model for the Paleoproterozoic crustal evolution of southern Laurentia involves southerly accretion of successively younger island arcs from 1780 to 1650 Ma. This model may be oversimplified, however, for although andesite and basaltic andesite, major components of modern island arcs, occur, they are relatively rare, and large volumes of rocks are dominantly bimodal metavolcanic assemblages and related granitoids. Ophiolites occur, but they are also rare, and only one accretionary mélange has been described. Inherited zircons, and common Pb and Nd isotopic data indicate involvement of older crust in ca. 1750 Ma rocks of the Mojave Province of southeastern California and western Arizona and the bimodal assemblages of central Colorado. These data suggest that the bimodal volcanic rocks were derived from pre-existing continental crust, likely of ca. 1850 Ma Trans-Hudson–Penokean age, during extension and partial melting associated with the development of transpressional crustal-scale shear zones. The occurrence of the 1840 ± 1 Ma Elves Chasm pluton in the Grand Canyon demonstrates that rocks of Trans-Hudson–Penokean age are present in southwestern Laurentia.

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