Metamorphism of pelitic rocks in the Snow Peak area, northern Idaho: Sequence of events and regional implications

Effects of two episodes of regional metamorphism are recorded in pelitic rocks in the Snow Peak area of northern Idaho. The first event (M1) produced a strong foliation and large porphyroblastic minerals. Metamorphic grade during M1 increased to the south. The second metamorphic episode (M2) occurred under relatively static conditions. Complete equilibration of metamorphic minerals to M2 conditions occurred, although pseudomorphs of M1 minerals are common. The progressive sequence of M2 mineral assemblages ranges from chlorite-biotite to kyanite grade via a low-variance transition zone in which some samples contain chlorite, biotite, garnet, staurolite, kyanite, muscovite, quartz, ilmenite, and graphite. This second metamorphic episode occurred after thrusting of Prichard over Wallace Formation rocks. M2 metamorphism occurred at moderate pressure but lacks evidence of penetrative deformation. It is suggested that formation of kyanite in the Goat Mountain area to the southwest was contemporaneous with Snow Peak M2, and that M2 was closely associated with deep-seated Cretaceous plutonic activity in the Idaho batholith. During Tertiary plutonic activity and uplift, rocks in the Goat Mountain area remained hot, and kyanite inverted to andalusite. The Snow Peak area cooled more rapidly and was affected only by minor retrograde hydration of staurolite. In the Snow Peak samples, therefore, assemblages produced during the moderate pressure event, M2, are preserved.