Stratigraphy, petrology, and structure of Precambrian metavolcanic rocks in the Iris district, Gunnison and Saguache counties, Colorado
Abdulkader M. Afifi
The Iris district occurs within the Iris and Iris NW 7.5 minute quadrangles, 8-16 km southeast of Gunnison, Colorado. The area is underlain by a mass of Proterozoic (^-1700 m.y.) metavolcanic rocks shown as Dubois Greenstone by Hedlund and Olson (this guidebook). A discussion of the regional geology of Proterozoic rocks exposed along the eastern part of the Gunnison Uplift is presented by Hedlund and Olson. Previous mapping in the Iris district was undertaken by Hedlund and Olson (1974) and Olson (1976). The geology and geochemistry of mineralized areas in the Gunnison gold belt are discussed by Drobeck (1980), and Sheridan and others (this guidebook). The aim of this report is to discuss some aspects of the geology of the small area where detailed mapping was undertaken (Afifi, 1981). Despite two generations of foliation development and metamorphism to the epidote amphibolite facies, relict fabrics and lithologies are locally identified with confidence. For this reason, the prefix "meta" often will be dropped from the following descriptions.
Figure 1 is a simplified geologic map of the Iris district. The stratified rocks are informally divided into three formations: lower metasediments, Iris formation, and upper metasediments. The Iris formation consists of pyroclastic and hyaloclastic rocks and is divided into five mappable members. In chronologic order, these are (1) dacitic member, (2) andesite lapilli tuff member, (3) rhyolitic member, (4) Dirigo Gulch (basaltic) member, and (5) alkali feldspar rhyolite member. Figure 2 is a generalized stratigraphic column for the Iris district showing the proposed stratigraphic nomenclature. The layered rocks are intruded by sheets of metagabbro which are probable subvolcanic equivalents to basaltic rocks of the Dirigo Gulch member. The layered rocks and metagabbro sheets are folded into a tight, steeply plunging fold which is called the Iris syncline. Consequently, all bedding and foliation attitudes are steeply dipping.
- Afifi, Abdulkader M., 1981, Stratigraphy, petrology, and structure of Precambrian metavolcanic rocks in the Iris district, Gunnison and Saguache counties, Colorado, in: Western slope Colorado--western Colorado and eastern Utah, Epis, Rudy C.; Callender, Jonathan F., New Mexico Geological Society, Guidebook, 32nd Field Conference, pp. 287-292.