Magnetizations of some Late Cretaceous glassy tuffs, volcanic breccias, and altered basalts of the Elkhorn Mountains volcanic field, Western Montana

Abstract

New magnetization data for Late Cretaceous glassy welded tuffs, volcanic breccias, and altered basalts from the Elkhorn Mountains volcanic field, together with geologic, mineralogic, and K-Ar data, indicate that (1) the glassy tuffs have unusually strong, uniform remanent magnetizations which are reversely polarized, much of the remanence perhaps residing in submicroscopic single-domain iron oxide particles within the glass itself, (2) breccias emplaced at minimum temperatures of about 400°C have moderately dispersed remanent magnetization directions which are normally polarized, and (3) basalts that occur within a broad area of hydrothermal alteration have remanent magnetization directions which are normally, horizontally, and reversely polarized. Present and previous studies indicate the occurrence of from two to eight complete field reversal cycles during emplacement of the volcanic field about 78 m.y. ago. Among the rocks studied, only basalt from the Zosell mining district is important as a major aeromagnetic anomaly source.