Geological and Geochemical Studies of the Roskruge Range, Pima County, Arizona

A combined field and laboratory investigation of the primarily volcanic Roskruge range, Pima County, Arizona, produced the following results: The rocks in the area fall into three major and several minor divisions. The major groups are: (1) The pre-Laramide dark-colored andesites and sediments that floor the range. A K-Ar date on one of the andesites produced a number, 108 m. y. , on a whole rock analysis. (2) The brightly colored and widespread Laramide volcanics and an associated hypabyssal pluton at Cocoraque Butte, which gave K-Ar mineral ages in the 74 m. y. to 66 m. y. spread. (3) The mid-Tertiary potassic basaltic andesites found on an eroded plain that is cut into the Laramide volcanics. These dark-colored extrusives were dated at 23 m. y. by whole rock K-Ar. Amongst the minor units in distribution is the petrographically unusual Recortado ash flow, a small 13 m. ye old unit that preserves a vitrophyre just above its base. Also included in the miscellaneous group of units are the post-orogenic true basalts, which appear as dikes and as a flow(?) at Brawley Wash and which gave whole rock K-Ar dates of about 10 m. y. Initial strontium 87/86 ratios indicate that the Laramide and mid-Tertiary rocks (including the Recortado ash flow) have values ranging from . 7056 to .7092, while the Brawley Wash basalt has a true deep-seated basalt ratio of .7038. The combination of radiochemical assay, petrographic and field study, and isotopic data has established good gross correlation with the already known sequence of formations and plutons in the Tucson Mountains.