Geochemistry and tectonic setting of Paleoproterozoic metavolcanic rocks of the southern Front Range, lower Arkansas River Canyon and northern Wet Mountains, central Colorado
Reinhard A. Wobus1, Martha J. Folley2, Katherine M. Wearn3, and Jeffrey B. Noblett4
1Department of Geosciences, Williams College, Williamstown, MA 01267, U.S.A.
2USA CRRE; RS/GIS Center, 72 Lyme Road, Hanover, NH 03755, U.S.A.
3Department of Geological Sciences, University of Oregon, Eugene, OR 97403-4702, U.S.A.
4Department of Geology, Colorado College, Colorado Springs, CO 80903, U.S.A.
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 the 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% to 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 discriminate 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 theoliites; they may represent pyroclastic rocks with a source beyond the study area.
Rocky Mountain Geology, v. 36, no. 2, p. 99-118, 13 figs., 2 tables, December, 2001