PETROGENESIS AND FIELD RELATIONS OF A SERPENTINITE BODY NEAR RIGGINS, IDAHO

The Riggins region of Idaho consists of Cretaceous to Permian arc related metamorphic and volcanic rocks. Within the metamorphics exists a series of ultramafic pods that trend north-south along Idaho’s Salmon River suture zone. Though the timing of emplacement of these serpentinites is unknown, it is thought to be the result of continental plate overthrust at a former passive margin. This model brings up the question of, what part of the upper mantle was this serpentinite derived from. Surrounding geology and compositional data suggests that these ultramafic bodies may have come from the lower portion of an ophiolite suite. Geochemical data suggests that a serpentinite pod north of “Time Zone Bridge” Riggins, Idaho, was derived from a harzburgite. Compositionally this serpentinite outcrop is predominantly the assemblage antigorite, magnesite, and magnetite. Reaction zones of talc, tremolite, clinochlore, phlogopite, are also present in some exposures. These reaction zones are likely metasomatic in nature, and tend to occur along relic country rock within the serpentinite. Along with country rock, a rodingite dike and possible relic serpentinite mélange are also incorporated in the ultramafic body. Ophiolite deposits outside of this exposure are absent locally, however, later ophiolite deposits occur within the Seven Devils group southwest of the ultramafic slivers.