Magmagenesis at the Eocene Electric Peak–Sepulcher Mountain complex, Absaroka Volcanic Province, USA

Igneous rocks exposed on Electric Peak and Sepulcher Mountain represent the preserved intrusive and extrusive components of an early Eocene, calc-alkaline eruptive center in the Absaroka Volcanic Province (AVP) of Montana and Wyoming, USA. The Electric Peak stock has an outcrop area of ∼1 km2 and consists of six discrete phases ranging in composition from quartz diorite to granite, representing multiple small intrusions of compositionally distinct magmas. Lava flows and dikes on Sepulcher Mountain are basaltic–andesitic to dacitic in composition with silicic rocks generally peripheral to mafic rocks. Important geochemical characteristics of the rocks from both localities include high Ni and Cr concentrations in andesitic and dacitic rocks, lower rare earth element (REE) concentrations in evolved rocks relative to more mafic rocks, and variable Sr and Nd isotopic ratios. Although petrographic evidence permissive of magma mixing is limited, compositional data suggest that andesitic rocks are hybrids formed by mixing variably fractionated and contaminated mantle-derived mafic magmas with diverse composition silicic crustal melts derived from partial melting of amphibolitic lower- to mid-crustal rocks. Isotopic and trace element systematics of mafic rocks, such as high ratios of large ion lithophile to high field strength elements, also suggest that parental magmas were derived from melting of subduction-modified lithospheric mantle. The significance of these results is that the calc-alkaline nature of the center was inherited from crustal processes, whereas the arc-like trace element signature reflects generation of the magmas in a tectonic environment related to geometrically complex subduction of the Farallon plate under the North American continent.