Thermal Relaxation of a Magmatic Arc Complex, Mount Barcroft Area, Eastern California: 40Ar/39Ar Geochronologic Study of Hornblende and Biotite from Igneous and Metamorphic Rocks

Abstract

The central and southern White Mountains constitute a well-exposed section of the middle to late Mesozoic convergent margin of western North America. New 40Ar/39Ar analyses were obtained for 8 Ca-amphibole and 13 biotite specimens from igneous and metamorphic wall rocks (8 rock samples from the Barcroft pluton, a single McAfee Creek-type aplite, 7 mafic dikes/metadikes, and two Andrews Mountain quartzites). Most of the spectra are very complicated. Except for one relatively fresh and four apparently fresh diabase dikes, all 9 granitoids, the 2 metadiabasic dikes, and both contact metamorphic quartzites partially lost (or gained) argon during heating and/or deuteric-hydrothermal alteration over ~100 m.y. of igneous arc evolution. New granitoid ages are similar to—or younger than—published U/Pb ages for the Middle Jurassic Barcroft and the Late Cretaceous McAfee Creek plutons, and are similar to published K-Ar and 40Ar/39Ar ages of the Cottonwood/Beer Creek and other plutons in the central and southern White Mountains. Contact metamorphic ages of wall rocks approximate nearby granitoid emplacement ages. Mafic dikes invaded the area episodically; most yield disturbed 40Ar/39Ar spectra exhibiting a range of cooling ages. Some, metamorphosed by the Barcroft pluton, must be older than 165 Ma; others give apparent ages of 144 (Independence dike swarm), 115-118, and 98 Ma. Clearly, multiple stages of mafic dike injection accompanied mid- and late-Mesozoic accretion of this sector of the North American continental margin.