Potassium-argon age and petrography of the Sierra Blanca Peak igneous intrusives Hudspeth County, Texas
Matthews, William K.
Adams, John A. S.
Master of Arts
Rhyolite and diorite intrusives are exposed six miles west of Sierra Blanca (Hudspeth County) in West Texas. StructuraUy, they lie between the relatively undeformed Diablo Platform and the folded and faulted Chihuahua Tectonic Belt (Laramide). In thin sections the rhyolite was homogeneous and the diorite proved more variable in mineralogy. Detailed field mapping (1:24) confirmed the general outlines of the five rhyolite laccoliths described by earlier workers. New details of rhyolite dikes, autobrecciation, and flow banding were recognized. A rhyolite age of 36.1 m.y. was reported by earlier workers. The new mapping documented over fifty previously unreported diorite dikes and sills with a concentrated swarm near Sierra Blanca Peak. A potassium-4/argon-4 age of 88 +4 m.y. from a single hornblende phenocryst from a 3 foot dike in the diorite swarm is the oldest Mesozoic age reported to date in this region. Mineralogical differences between the rhyolite and diorite and the 5 m.y. interval between emplacements increase the probability that they are from unrelated magma sources and different tectonic regimes.