CALCULATED P – T PATHS FOR MESOPROTEROZOIC METAMORPHISM OF METAPELITIC ROCKS IN THE BELT SUPERGROUP, SNOW PEAK AREA, NORTHERN IDAHO

Intersecting isograds, pseudomorphs, and abrupt radial changes in garnet composition in metapelites of the Belt Supergroup in the Snow Peak area of northern Idaho all indicate garnet growth through two distinct episodes of metamorphism, M1 and M2. Lu-Hf dating of garnet from these samples documents initial, M1, garnet growth at approximately 1.3 Ga followed by later garnet growth, M2, at approximately 1.1 Ga (Nesheim, et al., 2012). We have examined textures and garnet zoning in samples from the garnet zone, staurolite zone, staurolite-kyanite zone and kyanite zone. Some garnet and garnet-staurolite zone samples show evidence of only one metamorphic episode, M1 or M2. Some staurolite zone and most staurolite-kyanite and kyanite zone samples contain garnet that shows evidence of growth during two metamorphic episodes, which are separated by an abrupt increase of X_Ca in garnet.

P – T paths for samples that show evidence of two metamorphic episodes calculated using THERIAK-DOMINO (deCapitani and Petrakakis, 2010) indicate that M1 garnet growth took place at relatively low pressure ranging from 520°C at 3.4 kbars to 610°C at 6.5 kbars. M2 conditions for multiple samples are constrained at higher pressure (ranging from 7.0 to 9.0 kbars) and slightly higher temperature by garnet composition rimward of the euhedral boundary where X_Caincreases abruptly. There is no evidence of cooling or retrogression between M1 and M2 garnet growth (even though they were separated by 200 Ma.) Calculated isopleths for volume percent garnet in the staurolite and staurolite-kyanite field are near horizontal and volume % garnet is predicted to increase from 1 to 5% along typical M1 to M2 paths. The THERIAK-DOMINO modeling produces nested P – T paths for increasing zones showing garnet growth during an abrupt increase in pressure followed by heating and decompression along a clockwise P – T path.

de Capitani, C., Petrakakis, K., 2010. The computation of equilibrium assemblage diagrams with Theriak/Domino software. American Mineralogist, 95, 1006–1016.

Nesheim, T.O., Vervoort, J.D., McClelland, W.C., Gilotti, J.A. and Lang, H.M., 2012, Mesoproterozoic syntectonic garnet within Belt Supergroup metamorphic tectonites: Evidence of Grenville-age metamorphism and deformation along northwest Laurentia, Lithos, 134-135, 91-107.