Precursors to a continental-arc ignimbrite flare-up: Early central volcanoes of the San Juan Mountains, Colorado, USA

Our newly acquired and recently published map, geochronologic, and compositional data for early intermediate-composition central volcanoes in the northeastern San Juan Mountains provide insights about the broad magmatic precursors to the large continental-arc ignimbrite flare-up in the mid-Cenozoic Southern Rocky Mountain volcanic field (SRMVF). Initial volcanism migrated from central Colorado to northern New Mexico ca. 40–29 Ma, as part of a more regional trend of southward-progressing mid-Cenozoic magmatism in the U.S. segment of the North American Cordillera. Within the San Juan locus, which represents the largest preserved erosional remnant of the SRMVF and site of most intense eruptive activity, new ⁴⁰Ar/³⁹Ar and U-Pb zircon ages show that eruptions at many individual edifices began nearly concurrently, at ca. 35 Ma, with peak activity at 34–32 Ma. Broadly similar precursor effusive volcanism characterizes other major loci of continental-arc ignimbrite magmatism along the western American cordilleras, but none of these sites records early volcanism as voluminous, spatially widespread, well exposed, or compositionally diverse as the San Juan locus in Colorado.