Relating deformation fabrics to multiple metamorphic events preserved in metapelites of the Belt-Purcell Supergroup in the snow peak area of northern Idaho

Kacey Jo Largent, West Virginia University


Previous work has shown that two significant metamorphic/deformation events (M1/D1 and M2/D2) affected the metapelites of the upper Wallace Formation in the Snow Peak area of northern Idaho. This study's goal was to distinguish the effects of the two metamorphic and deformation events by studying garnet growth and structural fabrics preserved in the metapelites in the Snow Peak area. All the samples within the M1/D1 affected area have the earlier foliation (S1) as the dominant matrix foliation. S1 in these samples was overprinted by S2 which is defined by a crenulation cleavage and/or sheared strain shadows around porphyroblasts that did not transpose S1. Garnet zoning in these samples shows that an abrupt increase in Ca in garnet separates M1 and M2 growth and the majority of garnet growth occurred during the M1/D1 event. The high-Ca boundary is marked by a ring of coarse-grained quartz inclusions. The proportion of M2 garnet growth increases with metamorphic grade towards the southwest and garnet ages reflect this. Garnets with more M2 growth have younger ages and garnets with minimal M2 growth are the oldest. The oldest dated garnet in the area is located in the garnet zone in the far eastern portion of the study area and is dated at ∼1314 Ma. This is the best estimate of the age of the M1/D1 event. The youngest sample is located in the northwestern portion of the map, furthest from the M1 relict isograd and is dated at ∼1085 Ma. This sample shows no signs of polymetamorphism and has a different matrix foliation orientation than the other samples. It was determined that this sample's dominant foliation is S2 and that it was only affected by the M2/D2 event. Garnet growth for this sample occurred entirely during M2.