CONSTRAINTS ON THE PRESSURE-TEMPERATURE PATH OF ROCKS IN THE SNOW PEAK AREA, NORTHERN IDAHO DETERMINED BY COMPARING OBSERVED AND COMPUTER GENERATED MINERAL ASSEMBLAGES AND COMPOSITIONS

The Belt-Purcell Supergroup of northern Idaho contains the Wallace Formation which is divided into a lower quartzite and upper schist. The upper schist unit is the focus of this research because it contains pressure and temperature sensitive assemblages that record two metamorphic events (M1 and M2). In the Snow Peak area M2 metamorphic grade increases in a southwesterly direction towards the Cretaceous Idaho Batholith; however, recent determination of Proterozoic garnet ages for M1 and M2 show that metamorphism was not related to the batholith. The software Theriak-Domino was utilized to calculate pseudosections from the bulk compositions of two selected samples, contour major elements in garnet, the volume percent of garnet, the mole fraction of Ca and Na in plagioclase, the volume percent of plagioclase and other phases in the MnNCKFMASH system. The two samples selected for analysis come from the highest grade zones in the Snow Peak area. Sample 09HL07 from the garnet-staurolite zone contains abundant euhedral, sieve-textures staurolite grains. The porphyroblastic garnets contain numerous quartz inclusions which form euhedral rings. This sample also contains pseudomorphs of coarse-grained muscovite after staurolite which are related to M1. The foliated matrix deforms around the porphyroblasts and is composed of quartz, muscovite, and biotite. Sample 09HL04 from the garnet-staurolite-kyanite zone also contains porphyroblastic garnets. The garnets in this sample have larger quartz inclusions with multiple quartz inclusion rings. The M1 pseudomorphs in this sample represent complete replacement of staurolite by kyanite and coarse-grained muscovite with new staurolite growth around the edges of the pseudomorphs. M2 staurolite is also found in this thin section. The matrix is foliated and composed of quartz, muscovite and biotite and is deformed around the porphyroblasts. Garnet maps reveal a high Ca rim around the garnets in these samples from the Snow Peak area. It has been determined that these rims relate to mineral growth during the M2 event. Observed variations in Ca and other components in garnet are compared with calculated contours on the pseudosections to constrain the P-T path for the rocks of Snow Peak, northern Idaho.