AGE AND PETROLOGY OF IGNEOUS CLASTS IN THE CUTLER FORMATION NEAR OURAY, COLORADO: NEW EVIDENCE FOR MESOPROTEROZOIC VOLCANISM IN SOUTHWESTERN COLORADO
Beds of conglomerate in the Cutler Formation near Ouray are meters to tens of meters thick, clast to matrix supported, and composed of a variety of rounded to subangular clasts of Proterozoic rocks (granite, gneiss, quartzite, phyllite), Paleozoic sandstone and limestone, and intermediate to felsic volcanic and hypabyssal clasts. The latter make up 9 to 13 percent of the total clast populations.
U-Pb zircon analyses on selected volcanic and hypabyssal clasts reveal dominant age populations of 1.8-1.7 Ga and ~1.4 Ga, and very minor populations at ~1 Ga. The 1.8-1.7 Ga zircons are inherited from older basement rocks, and the ~1 Ga zircons are mostly discordant and appear to have undergone Pb loss. The volcanic and hypabyssal clasts are thus assigned a crystallization age of ~1.4 Ga.
Petrochemical analyses indicate that the volcanic clasts are subalkaline. They have arc-like geochemical signatures as revealed by enriched LREE relative to chondrites, and LIL enrichment with distinct Ta-Nb troughs as compared to MORB.
Proterozoic basement assemblages in the region contain deformed, upper greenschist to amphibolite facies, 1.8 to 1.7 Ga volcanic arc rocks that range from basaltic to rhyolitic in composition. This older basement is intruded by 1.4 Ga plutons. Previous studies suggested that the extensive 1.4 Ga plutonic complexes in the southwestern Colorado are arc related, but no other evidence in support of this idea was published and there are no preserved volcanic and hypabyssal correlatives.
This study documents the detritus from a previously undefined ~1.4 Ga volcanic arc complex that was exposed northeast of Ouray, as indicated by published paleocurrent data. The 1.4 Ga plutons in the region likely share a genetic relationship to this previously undocumented arc. These volcanic assemblages were uplifted and completely eroded during the Uncompahgre uplift leaving only fluvial detritus in the record.