ORIGIN OF RHYOLITE OF THE OLIGOCENE CONEJOS FORMATION, EAST-CENTRAL SAN JUAN MOUNTAINS, COLORADO

Four closely-spaced volcanoes (SC-Summer Coon; TM-Twin Mountains; DN-Del Norte; CC-Carnero Creek) form the east-central cluster of Conejos volcanic centers. Conejos rocks range from high K basaltic andesite to rhyolite, with an average of high K dacite (63.6 wt.% SiO2). The younger Hinsdale Formation contains a related series ranging from basalt to high K andesite. SC and TM are composite volcanoes; DN and CC deeply eroded dacite shields. Rhyolite (10 percent of our Conejos analyses) is only known from SC and TM, although rhyodacite occurs in DN.

Conejos and Hinsdale magmas evolved through AFC processes in three stages: 1. Basalt, after interacting with lower crust, assimilated low K/Rb, high K/Ba crust, similar in some ways to Taylor model upper crust; 2. Main series basaltic andesite fractionated to High K andesite; 3. Rhyolite was produced by large-scale melting of high K/Ba upper crustal rocks similar to granite gneiss known from inclusions and outcrops. Also, some dacite and rhyodacite of SC and DN volcanoes show incompatible trace element ratios suggestive of assimilation of a local high K/Rb syenitic crustal component, also present as inclusions. Field evidence for such assimilation includes sanidinite-facies, partially-melted gneiss blocks up to one meter in diameter. Temperature estimates (1100-900 degrees C) from two pyroxene equilibria for the magma series are consistent with this interpretation, as well as the absence of phenocrystic alkali feldspar.

Our study supports the conclusions of previous workers on AFC processes in similar but, generally more mafic, Conejos rocks of the southeastern San Juan Mountains. Our results, however, emphasize the importance of crustal melting in the generation of Conejos rhyolite, which forms a much larger proportion of our data set.