Origin of rhyolite by crustal melting and the nature of parentalmagmas in the Oligocene Conejos Formation, San Juan Mountains,Colorado, USA

Abstract

Four closely spaced volcanoes (Summer Coon; Twin Mountains; Del Norte;Carnero Creek) form the east-central cluster of Conejos volcaniccenters. These Conejos rocks range from high-K basaltic andesite torhyolite, with andesite volumetrically the most abundant. Summer Coonand Twin Mountains are composite volcanoes. The Del Norte and CarneroCreek Volcanoes are deeply eroded dacite shields. Rhyolite (10% of ourConejos analyses but a much smaller percentage by volume) is onlyknown from Summer Coon and Twin Mountains Volcanoes, although high-SiO(sub 2) dacite occurs in the Del Norte Volcano. The younger HinsdaleFormation contains a related series ranging from transitional basaltto high-K andesite; we use Hinsdale Formation analyses to representConejos parental magmas. Conejos and Hinsdale magmas evolved throughAFC processes: Basalt, after interacting with lower crust, assimilatedlow K/Rb crust, similar in some ways to Taylor and McLennan (Taylor,S. R., and McLennan, S. M., 1985, The continental crust: itscomposition and evolution. Oxford, Blackwell Scientific) model uppercrust; main series basaltic andesite fractionated to high-K andesite;rhyolite was produced by melting of high K/Ba upper crustal rockssimilar to granite gneiss known from inclusions and basement outcrops.Some rhyolite may have been back-mixed into fractionating andesite anddacite. Field evidence for assimilation includes sanidinite-facies,partially melted, gneiss blocks up to 1 m in diameter. Temperatureestimates (1100-900 degrees C) from two-pyroxene equilibria areconsistent with this interpretation, as are the sparsely porphyriticnature of the most-evolved rhyolites and the absence of phenocrysticalkali feldspar. Our study supports the conclusions of previousworkers on AFC processes in similar, but generally more mafic, Conejosmagmas of the southeastern San Juan Mountains. Our results, however,emphasize the importance of crustal melting in the generation ofConejos Rhyolite. We further speculate that Conejos magmatism, and theSan Juan Volcanic Field (SJVF) in general, may represent an earlyphase of Rio Grande Rift magmatism, the orogenic geochemical signatureof the series having been generated through multi-level and extensiveassimilation of varied Precambrian orogenic and anorogenic rocks.

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