Abstract

Across a 5000-km2 area of central Colorado, previously unstudied Paleoproterozoic metabasalts (amphibolites) and metarhyolites (felsic gneisses) comprise a bimodal metavolcanic association within a dominantly metasedimentary terrain. Extending southward from about 39° N latitude in the Southern Front Range to about 38° 15′ N latitude in the Wet Mountains, and from the mountain front westward to the Wet Mountain Valley and Pleasant Valley fault system, this area includes the exceptional exposures within the lower Arkansas River Canyon from Howard downstream to Canon City. Regional metamorphism from garnet to sillimanite grade, pervasive deformation, and intrusion by three generations of Proterozoic plutons have largely obscured original stratigraphic relationships and primary structures within these metamorphic basement rocks, although a few pyroclastic features persist locally within the felsic members. These metavolcanic rocks are compositionally similar to the much better preserved bimodal section in the Salida area, dated at 1728 ± 6 Ma by Bickford (1986), which emerges from beneath Paleozoic cover rocks about 15 km beyond the western edge of the area of this report.

Geochemical studies of 45 samples (30 amphibolites and 15 felsic gneisses) delineate two groups of metavolcanic rocks ranging in silica content from 45–55% in one group and from 65 to almost 80% in the other. Along a 100-km transect from north to south, metavolcanic rocks of the lower-silica group (amphibolites) show an increase in total alkalies (from 2% t o 5–6%) and large ion lithophile trace elements as well as an increase in degree of enrichment in light rare earth elements (from LaN/LuN < 2 to LaN/LuN ∼5). Rocks with higher silica content (felsic metavolcanic rocks) occur mostly in the Arkansas Canyon area and contain 6–8% total alkalies; they show strong fractionation between light and heavy rare earth elements with moderate to pronounced negative europium anomalies.

Tectonic discriminant diagrams using relatively immobile high-field-strength elements indicate volcanic arc settings for both mafic and felsic populations. Metavolcanic rocks from the northern Wet Mountains and Arkansas Canyon suggest a mature arc environment, possibly on an expanding continental margin. The isolated metabasalts to the north in the southern Front Range, where no felsic metavolcanic rocks have been identified, are more primitive island-arc tholeiites; they may represent pyroclastic rocks with a source beyond the study area.

These new data from a wide area of central Colorado reinforce results from the well-studied Paleoproterozoic bimodal are assemblages to the west near Salida and Gunnison. They also allow the extension across a wider geographic area of previous tectonic models for the Paleoproterozoic evolution of the Colorado province (as defined by Bickford et al., 1986). These models (Condie, 1986; Reed et al., 1987; Karlstrom et al., 1987) portray the rapid addition of juvenile crust to the southern margin of the Wyoming province by accretion of individual volcanic arcs or larger, previously amalgamated are terranes, resulting in the southward expansion of the craton by about 1300 km from 1800–1650 Ma.

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