Date of Graduation
Eberly College of Arts and Sciences
Geology and Geography
Helen M Lang
Kathleen C Benison
The Snow Peak area in northern Idaho experienced two distinct metamorphic episodes, M1 and M2. Garnets, especially those that experienced both episodes, are strongly zoned. The assemblages observed in metapelites from garnet to staurolite/kyanite zones were compared to computer generated assemblages calculated using Theriak-Domino (deCapitani and Petrakakis, 2010). The comparison and analysis of the assemblages, garnet zoning, and modal contours provides constraints on the path the rocks must have followed between metamorphic events called M1 and M2. The most useful constraints on the proposed P-T path are provided by comparing the content of Ca in garnet and the observed assemblage to a computer generated assemblage. The Ca content is pressure sensitive and increases in Ca can be used as indicators of increases in pressure (Caddick et al., 2013). The samples in this study that experienced M1 and M2 all have an abrupt, euhedral zone of high calcium near their inner rims, which was interpreted to be the beginning of M2. Therefore, Ca is a reliable constraint for this study. Once the observed assemblage was reconciled with the computer generated assemblage and the garnet analysis was used to further constrain the range for peak metamorphic conditions, the abrupt change in P and T the rock must have experienced from M1 to M2 was apparent. The M1 (core) to M2 (rim) records a significant pressure increase and modest temperature increase for each sample that experienced two metamorphic episodes. The change in metamorphic condition reflects an increase in pressure of at least 0.5 kbars and there is also a small accompanying increase in temperature. Some samples showed larger increases in pressure and temperature, but all samples followed the same general trends. The garnet zone M1 peak conditions range from 500°C to 545°C at ~2.5-5 kbars. The M2 garnet zone peak conditions are 525°C to 555°C at ~3-5.5 kbars. Staurolite zone M1 peak conditions ranged from 570°C to 595°C at ~5-7 kbars. M2 staurolite zone conditions were 590°C to 595°C at ~7.25-7.75 kbars. Staurolite/kyanite zone M1 conditions ranged from 580°C to 645°C at ~4-9 kbars. M2 staurolite/kyanite zone conditions were 605°C to 650°C at ~7.75-9.5 kbars. The P-T paths for the Snow Peak samples of different metamorphic zones are nested (Spear et al., 1984). From the analysis of the pseudosections and the abrupt increase in pressure from M1 to M2, the only tectonic setting which would match this pattern of pressure/temperature change is due to a continental collision involving over-thrusting. During the Mesoproterozoic, specifically at 1.3 Ga, parts of the Belt Supergroup underwent regional contraction in what is present-day northern Idaho due to the East Kootenay Orogeny (Nesheim et al., 2012). The thickened crust and subsequent burial resulted in M1, which produced at least staurolite grade metamorphism (Nesheim et al., 2012). Snow Peak sample 08TN82/09HL09, the M1 garnet from the garnet zone, was dated and returned an age of 1315 +/- 2.3 Ma, which corresponds with the regional contraction and burial of the Belt Supergroup (Nesheim et al., 2012). M2 resulted from the collision of Laurentia with an unknown plate during the Grenville orogeny at approximately 1.1 Ga (Nesheim et al., 2012). Sample 08TN80a, from the Snow Peak M2 garnet zone, was dated to 1085 +/- 2.4 Ma which corresponds to the collision that caused the increase in pressure which produced M2 assemblages (Nesheim et al., 2012). Both orogenies are associated with the formation of Rodinia. Snow Peak rocks have recorded the multiple metamorphisms which occurred as the supercontinent grew.
Johnson, Sarah Elizabeth, "Constraints on the pressure-temperature path for Belt-Purcell Supergroup metapelites from Snow Peak, northern Idaho" (2015). Graduate Theses, Dissertations, and Problem Reports. 5895.