Abstract
An 80-mi2 area adjacent to Salmon Lake, mapped at a scale of 1:36,680, includes a section of the Nome Group schist that is at least 2,000 m thick. It is relatively intact, with relict primary lithologic layering, and not mélange-like or highly disrupted by faults. The Nome Group metasediments include pelitic schist, graphitic quartzite, pure and impure marble, calcareous schist, chlorite-albite schist, chloritic quartz-albite, and mafic schists. Metaigneous rocks include Fe-Ti-rich metabasite and granitic to tonalitic orthogneiss. The protoliths are Precambrian(?) to Paleozoic in age, and radiometric ages indicate that the high-pressure metamorphism of the Nome Group is Late Jurassic. Blueschist-facies assemblages occur in Nome Group metabasites and in most metasedimentary schists. The blueschists are synkinematic; they formed during an episode of widely distributed penetrative deformation and mesoscopic intrafolial isoclinal folding of consistent orientation. The Nome Group of the Salmon Lake area underwent a prograde monocyclic polyfacial high-P/T metamorphism. Initial metamorphism was in the lower-T regime of the lawsonite-glaucophane field (300 °C, 7 kb?) and prograded with increasing T and P into the epidote-almandine-glaucophane field through an apparent P/T maximum, as indicated by the assemblage glaucophane-omphacite-garnet (420 °C, 9–10 kb). The later stages of the metamorphism were characterized by increased T/P, likely during initial stages of decompression and the return of the “normal” geothermal gradient, to the transition between the blueschist facies and the high-P albite-epiclote-barroisite field (450 to 525 °C, 7–8 kb?). This high-P/T cycle was followed by an incipient static retrogressive metamorphism at low greenschist-facies conditions during the later uplift of the Nome Group in the Late Cretaceous and early Tertiary.
