K-Ar Apparent Ages, Peninsular Ranges Batholith, Southern California and Baja California

Abstract

More than 200 K-Ar apparent ages have been determined from minerals from the Peninsular Ranges batholith of southern California and northern Baja California. In general, the apparent ages show a progressive decrease from about 120 m.y. in the southwestern (coastal) pan of the batholith to less than 70 m.y. in the northeastern (desert) part. The gradients for biotite and hornblende ages can be represented by contours of equal ages. Both concordant and discordant hornblende-biotite pairs and minerals, from a variety of plutonic and metamorphic rock types, share in the apparent-age gradient. Ages for hornblende average 5 m.y. older than the ages for coexistent biotite. Isotopic U-Pb and Pb-α measurements on zircon indicate ages greater than those calculated from K-Ar ratios of hornblende or biotite. It is believed that in the Peninsular Ranges province, the U-Pb ages for zircon approximate the ages of emplacement, whereas concordant K-Ar ages may or may not approximate the ages of emplacement, depending on the depth of emplacement and the rate of uplift and denudation.