Independence Volcano: A Major Eocene Eruptive Center, Northern Absaroka Volcanic Province

Abstract

The Independence volcano was a major eruptive center in the northern part of the Absaroka volcanic province during middle to late Eocene time. A huge strato-volcano developed that led to the accumulation of thick volcani-clastic deposits of intermediate composition in the surrounding low-lying areas. A large volume of calc-alkalic and potassic-alkalic volcanics was extruded, reworked, and distributed over the Independence area during a period of volcanic activity that probably extended over a relatively long time. The extrusive rocks are divided into five groups: Group I volcanics, which consist of dacitic, rhyodacitic, and trachyandesitic breccias and minor lava flows; Group II volcanics, consisting of a series of quartz latitic lava flows; Group III volcanics, which consist of trachyandesitic pyroclastic breccias, tuffs, agglomerates, volcanic mud-flows, conglomerates and sandstones; Group IV volcanics, which is a sequence of well-bedded, reworked epiclastic volcanic conglomerates and sandstones; and Group V volcanics, which consist of alkalic basalts and minor felsic tuffs. The volcanic rocks show a normal line of descent from trachybasalts to trachyandesites and rhyodacites. Coarse-grained intrusive equivalents of the extrusive rocks which occur in the core of the Independence volcano are syenogabbro, monzonite, and granodiorite; the general felsic to mafic extrusive sequence suggests tapping of a differentiating magma chamber at successively deeper levels. Several small plugs aligned with a northwest trend were emplaced along a major fault zone during the main period of volcanism.