Geochronology and geology of late Oligocene through Miocene volcanism and mineralization in the western San Juan Mountains, Colorado

This paper presents 25 new 40Ar/39Ar dates from the main calc-alkaline ash-flow sheets and related younger plutons of the western San Juan volcanic field, the ash-flow sheets of the Lake City caldera cycle, and veins and other altered rocks in the Lake City region. The goal of the study was to produce similar quality 40Ar/39Ar ages to those currently published for the eastern and central San Juan Mountains. These new data provide a much more precise chronological framework for interpreting durations of events and their relationship to mineralization than do previously published conventional K-Ar dates for the western San Juan Mountains.