NEOPROTEROZOIC (?) DIAMICTITES IN CENTRAL IDAHO: EVIDENCE OF FELSIC VULCANISM FROM EMBAYED QUARTZ GRAINS

Diamictite is intersected in two widely-spaced drill holes near Challis, Idaho (locations: 44°29.2'N 114°19.9'W and 44°24.0'N 114°19.2'W). Whereas Neoproterozoic diamictites in central Idaho have typically been attributed to glacial or fluvial processes, previous investigators have proposed a volcanic or intrusive origin for these rocks. In addition, this diamictite unit occurs within the proposed Neoproterozoic Beaverhead impact structure raising the possibility that it is an impact breccia.

This diamictite lies below the Early Paleozoic (?) Bayhorse Creek Dolomite but does not outcrop. Although the lower contact was not intersected, the thickness of the diamictite exceeds 50m. The diamictite and overlying Cambro-Ordovician section have experienced regional lower greenschist facies metamorphism. Clasts make up 25 to 40% of the diamictite and are dominated by metasedimentary rocks including quartzite, phyllite and marble. Monocrystalline quartz grains comprise up to 20% of the diamictite. The quartz grains show flat-field extinction and exhibit both square and hexagonal euhedral cross-sections. The diamictite matrix is microcrystalline but varies in color and degree of recrystallization.

Fabric analysis indicates that the diamictite was emplaced hot. Adjacent to its contact with the Bayhorse Creek Dolomite the diamictite clasts have an average aspect ratio of ~1:2 but show no preferred orientation. Below 25m the matrix flows around the larger clasts and quartz grains. The clasts show evidence of flattening with aspect ratios of 1:4.7. The long axes of the clasts are aligned and define a foliation. At the lowest sampled interval aspect ratios increase to 1:5.8.

Although the monocrystaline quartz grains show no evidence of deformation, they are heavily embayed and contain a microcrystalline infilling. 45 electron microprobe spot analyses from several embayments contain an average of 62.1% SiO₂, 19.4% Al₂O₂ and 12.4% K₂O. Microprobe analyses and SEM imagery shows the infilling is an intergrowth of micron-scale quartz and potassium feldspar grains indicative of a felsic volcaniclastic origin for the diamictite. Neoproterozoic volcanism and detrital zircons along the western margin of Laurentia have been attributed to the rifting during the breakup of Rodinia.