USE OF GARNET ZONING AND PSEUDOSECTION MODELING TO CONSTRAIN THE P-T PATH OF METAPELITIC TECTONITES IN THE SNOW PEAK AREA, NORTHERN IDAHO

The Snow Peak area in northern Idaho contains metapelitic rocks that document polymetamorphism (Hietanen, 1968; Lang & Rice, 1985). Recent studies have demonstrated that these rocks experienced two periods of metamorphism and garnet growth at approximately 1.3 and 1.1 Ga (Nesheim, et al., 2012).

We have examined detailed compositional maps of garnet in samples with different relationships to the two metamorphic episodes and different grades of metamorphism. Garnets from some samples, particularly those with pseudomorphs from the earlier M1 episode in the M2 staurolite-kyanite zone, experienced an abrupt increase in Grs and significant garnet growth during M2.

Pseudosections calculated for several of these samples show that the high Sps, low Grs M1 interiors of garnets in these samples apparently grew in the range of 550-630°C with pressure weakly constrained at 4-7 kilobars in staurolite-bearing assemblages. The low Sps, high Grs M2 rims grew at substantially higher pressure of 7 to 8 kilobars in the range of 650-660° in staurolite + kyanite-bearing assemblages. The staurolite-kyanite-garnet field on the calculated pseudosections is quite narrow, putting a relatively tight constraint on peak M2 metamorphic temperature and pressure for each sample. The abrupt increase in X_Grs requires an abrupt increase in pressure of at least 500-1000 bars in a narrow temperature range. Garnet growth and zoning textures, such as idioblastic zoning and rings of quartz inclusions, and pseudosections contoured for garnet composition and modal % indicate that garnet grew along a continuous P-T path. The path included an abrupt step to significantly higher pressure without an intervening period of garnet consumption or diffusion between the M1 and M2 episodes, even though their ages are substantially different.

Hietanen, A., 1968. Belt Series in the region around Snow Peak and Mallard Peak, Idaho. USGS Professional Paper 344-E.

Lang, H.M., Rice, J.M., 1985. Metamorphism of pelitic rocks in the Snow Peak area, northern Idaho: sequence of events and regional implications. GSA Bulletin, 96, 731–736.

Nesheim, et al., 2012. Mesoproterozoic syntectonic garnet within Belt Supergroup metamorphic tectonites: Evidence of Grenville-age metamorphism and deformation along northwest Laurentia, Lithos, 134-135, 91-107.