PETROGRAPHY AND GEOCHEMISTRY OF THE COPPER LAKES INTRUSIVE SERIES, ABSAROKA VOLCANIC PROVINCE, WYOMING

In order to study late-stage crystallization processes at shoshonitic magmatic centers, this study examines shallow intrusive rocks of the Copper lakes intrusive series, Absaroka Volcanic Province, Wyoming. The intrusive series represents the late-stage differentiated core of the Sunlight volcano, the type locality for shoshonitic magmas worldwide. Data examined include whole-rock major and trace element compositions, ⁴⁰Ar/³⁹Ar age determinations, and SEM and XRD data. Compositionally, rocks of the Copper Lakes intrusive series are shoshonitic to trachytic (52.3 to 64.1 wt% SiO₂) and represent the most evolved and compositionally diverse sequence of magmatic rocks at the volcano.

Important results of this study include the following. (1) Four compositional units are recognized within the intrusive series, including shoshonite, latite, quartz-poor trachyte, and quartz-rich trachyte. These units grade laterally from least evolved shoshonite at the margins to most evolved quartz-rich trachyte in the core. The latite unit has been contact metamorphosed by the quartz-poor trachyte and thus is older. (2) Whole-rock chemical compositions and modal abundances of phenocrysts suggest evolution of these shoshonitic series magmas largely through fractional crystallization involving separation of augite, apatite, and oxides at ~52 wt% SiO₂, K-feldspar and andesine at ~56 wt% SiO₂, and albite at ~60 wt% SiO₂. (3) The presence of marialitic scapolite along intrusion-country rock contact zones attests to compositional modification of some rocks by metasomatic alteration from NaCl-rich deuteric fluids related to intrusion of the Copper Lakes pluton. (4) ⁴⁰Ar/³⁹Ar age determinations and continuous trends on variation diagrams suggest that potassic trachybasaltic lavas of the Upper Trout Peak Trachyandesite (48.10 ± 0.07) may represent parental magmas for Copper Lakes intrusive series rocks (48.07 ± 0.06 Ma). (5) The large compositional range of intrusive series rocks, relative to earlier eruptive units, reflects the terminal stages of magmatism at the Sunlight volcano due to slowing or cessation of parental basalt replenishment to the system.