ISLAND-ARC SERPENTINITE IN WEST-CENTRAL IDAHO, ANALOG FOR MODERN-DAY MARIANAS SERPENTINITE?

Situated west of Idaho’s Salmon River suture near Riggins are a series of ultramafic lenses, some of which are serpentinite. The Riggins ultramafic pods appear to be concordant with the foliation and are present at and near the base of the Jurassic(?) Squaw Creek Schist. This schist is carbonaceous and calcareous, having formed from immature arc-derived sediments. The pods are metasomatically altered, and are dominantly serpentinite, though they contain talc-carbonate, and amphibolite (high magnesium amphibole) locally. The serpentinite pod north of Riggins at Time Zone Bridge contains “black wall” sequences at the contacts with country rock, where serpentinite reacted with hydrous silica-rich rock to form assemblages of chlorite, talc, amphiboles and other hydroxylated silicates. These assemblages surround angular fragments of country rock within the serpentinite pod, suggesting the ultramafic pod was likely emplaced as a cold “mush” of serpentinite. Some ultramafic rocks are known to follow major thrust faults in collisional (compressional) terranes. An alternative (for at least the emplacement mechanism) for the Riggins exposures is movement of serpentinite upward along normal faults in a forearc depositional setting. This hypothesis would be consistent with the presence of serpentinite within a sedimentary package as is seen in the Squaw Creek schist at Riggins, as well as the lack of high-pressure blueschist facies metamorphism. A present-day example is the Marianas forearc, where serpentinite has apparently moved up along forearc normal faults and been emplaced on the sea floor as mud volcanoes. There, the initial product of subduction zone recycling is serpentinite. This proposed setting eliminates the need of a major thrust boundary to emplace ultramafic rocks at Riggins. Later compressional deformation of the Squaw Creek Schist has likely complicated the earlier extensional history through transposition, faulting, and folding.