GROWTH OF SOUTHWESTERN LAURENTIA BY RETREATING, COMPOSITE, OCEANIC ARCS

The mechanism for Paleoproterozoic growth of the continental crust in the southwestern U.S. remains contested after several decades of study. Three main tectonic models have been proposed: 1) growth by accretion of retreating juvenile oceanic arcs (together with their associated backarcs); 2) contractional growth by repeated arc accretion with little or no extensional crust (any older basement blocks would be exotic); and 3) growth by continental crustal extension (with the original continental crust now buried). Convergence-only models are difficult to reconcile with the prevalence of extension in modern oceanic arcs and the abundance of metasedimentary and non-arc-like metaigneous rocks in northern Colorado. Extension-only models are difficult to reconcile with the juvenile arc-like geochemistry of many units and the paucity of pre-1.8 Ga rocks.

We present new Hf-in-zircon and new and compiled Nd whole-rock isotopic data from both mafic and felsic metaigneous rocks from the northernmost accreted arc terrane in the Colorado province. Samples from both the bimodal ~1.78 Ga Green Mountain arc and the bimodal ~1.76 Ga suite that intruded the arc during subsequent (inferred extensional) magmatism were analyzed. The felsic magmas of both suites are interpreted geochemically to be dominantly lower crustal melts, not fractionates of the coeval mafic magmas. Both the Hf and Nd isotopic data indicate a more evolved source for the felsic rocks [εHf(t) = +5.0-6.7; εNd(t) = +1.3-4.2] than the mafic rocks [εHf(t) = +7.4-8.7; εNd(t) = +3.9-4.5] suggesting that the Green Mountain arc formed on older basement (model age ~1.8–1.9 Ga).

An earlier detrital zircon study from the Barber Lake block of the Cheyenne belt suggested that sometime after ~1.8 Ga, rocks of either the Penokean or Trans-Hudson orogens were emplaced along the Cheyenne belt by transcurrent faulting. We suggest that these rocks became the basement on which the subsequent arcs were built. During trench retreat, these Laurentian basement fragments were dragged oceanward at the base of the arcs while backarc basins opened behind them (analogous to the modern composite Japanese arc and Sea of Japan). Periodic contractional episodes inverted, thickened, and stabilized the backarc basins and sutured the composite arcs and inverted backarcs to Laurentia.