GARNET ZONING AS A RECORD OF PROTEROZOIC POLYMETAMORPHISM IN BELT-PURCELL SUPERGROUP METAPELITES, WESTERN LAURENTIA

Reexamination of metapelites in the Snow Peak area of northern Idaho is warranted in light of Mesoproterozoic Lu-Hf garnet ages from many samples from the Belt-Purcell basin of Western Laurentia. Snow Peak Lu-Hf garnet ages and U-Th-Pb monazite ages cluster at ~1.3 Ga and ~1.1 Ga. Metapelites from the Snow Peak area display a wide range in metamorphic grade and show evidence of two distinct metamorphic episodes in that they contain coarse grained muscovite ± kyanite pseudomorphs after St ± And. Rocks in the northern part of the area are fine-grained and show little evidence of polymetamorphism; however, rocks in the southwestern part of the area reached higher metamorphic grade and show evidence of a more complex metamorphic and deformational history. Inclusion trails in garnet preserve evidence of at least two episodes of penetrative deformation and multiple episodes of garnet growth.

X-ray maps of garnet from Snow Peak metapelites preserve a detailed record of the metamorphic history that also suggests polymetamorphism. All of the mapped garnets have cores with idioblastic zones that vary slightly in Grs content, most commonly low, high, low, high, but in some cases oscillatory. Some garnets, mainly from rocks in the southwestern part of the area that contain St or St+Ky, display an abrupt increase in Grs content near the garnet rim, coincident with a textural change to inclusion-rich, almost dendritic garnet growth. Grossular content changes from approximately 3 mole% Grs in the cores to approximately 14 mole% Grs in the initial garnet rim and drops off to the outer rim. Spessartine content of the garnet cores shows a typical bell-shaped profile with Sps content up to 18 mole% in some cores, dropping off to 3 mole% Sps in the rims. High Grs rims, if present, show uniformly low Sps content. There is no evidence of garnet consumption between episodes. Garnets in the one sample from Clarkia (~50 km west) from which it has been possible to date 1.3 Ga cores and 1.1 Ga rims separately also have low Grs cores and higher Grs rims, suggesting that the high Grs rims on Snow Peak samples also grew in the later 1.1 Ga event.

We are currently examining isochemical phase diagrams to evaluate P-T paths that are consistent with continued garnet growth and an abrupt increase in Grs content during a second metamorphic episode.