ISOTOPIC STUDIES IN THE ROSKRUGE MOUNTAINS, PIMA COUNTY, ARIZONA
K-Ar dating of igneous rock units in the Roskruge Mountains, Pima County, Arizona, indicates that periodic activity occurred in mid-Cretaceous time, during the Late Cretaceous–early Tertiary (Laramide) interval, and in mid-Tertiary time. The general sequence of rock types and periods of activity are very similar to those in the adjacent Tucson Mountains. The oldest date (108 m.y.) is from an andesite in the Cretaceous basement of the range. The Laramide ages (74–66 m.y.) are on a series of ash flows and an associated pluton which give initial Sr 87 /Sr 86 ratios ranging from .706 to .709. The mid-Tertiary activity includes the emplacement of a small pluton at 34 m.y., the extrusion of basaltic andesites at 23 m.y. (initial Sr 87 /Sr 86 ratio of .709), and the formation of a small well-preserved ash-flow sheet at 13 m.y. (initial Sr 87 /Sr 86 ratio of .707). The latest known igneous activity in the range is represented by basalt dikes and a flow(?) which are 10 m.y. old and have initial Sr 87 /Sr 86 ratios of .704.