EVIDENCE FROM LU-HF GARNET GEOCHRONOLOGY OF REGIONAL PROTEROZOIC POLYPHASE METAMORPHISM IN WESTERN LAURENTIA

Lu-Hf geochronology of garnet-bearing, Belt-Purcell Supergroup equivalent, metapelites from northern Idaho yield a range of Mesoproterozoic ages. Samples collected from the Snow Peak area produced Lu-Hf garnet ages of 1085 ± 2.4 Ma, 1198 ± 79 Ma, 1207 ± 8.4 Ma, 1255 ± 28 Ma, and 1314 ± 1.4 Ma. One sample, collected from the Clarkia area, was cut into 2-4 mm-thick slabs and garnets within the slabs were micro-drilled to separate garnet core material from rim material. Core and rim fractions were dated separately, producing a garnet core age of 1347 ± 10 Ma and a garnet rim age of 1102 ± 47 Ma, which span the range of the near-by Snow Peak area garnet ages.

Given the >200 Ma difference between the garnet core and rim ages from the micro-drilled sample, garnet from the Clarkia area likely formed through two separate metamorphic events, M1 and M2. From the Snow Peak area, the oldest and youngest garnet ages, 1314 ± 1.4 Ma and 1085 ± 2.4 Ma, are also interpreted to be the products of two separate metamorphic events, M1 and M2. The intermediate Snow Peak garnet ages within the 1300-1100 Ma range all have higher error values as well as complex inclusion and grossular zoning patterns. This indicates they are mixture ages of M1 and M2 events, and not from separate metamorphic events. Garnet growth has previously been observed to overlap with the development of a regional penetrative deformational fabric, which demonstrates a regional deformational component to the geologic setting of M2.

Metamorphic titanite ages of ~1.3 and ~1.1 Ga have been reported from the Sullivan Deposit in British Columbia, suggesting M1 and M2 occurred throughout the Belt-Purcell basin. While the older of these two ages has been referred to as the East Kootenay Orogeny, the younger (~1.1 Ga) ages overlap with the Grenville Orogeny of eastern and southern Laurentia (North America). Evidence of two regional Mesoproterozoic metamorphic events distributed throughout the Belt-Purcell Supergroup demonstrates that western Laurentia was a complex active tectonic margin during the construction of Rodinia. Future reconstructions of Rodinia need to explain the Mesoproterozoic metamorphism and deformation within the Belt-Purcell Supergroup.