Abstract

Mineral assemblages in metapelitic rocks of the Ramah Group from the externides of the Torngat Orogen near Saglek Fiord, northern Labrador have been analyzed in detail. A progression from the upper greenschist to the middle amphibolite facies is preserved along the metamorphic field-gradient, and six metamorphic zones have been defined based on the following mineral assemblages: Zone 1: Chl-Bt; Zone 2: Cld-Chl-And; Zone 3: Cld-Chl-Ky; Zone 4: St-Chl; Zone 5: Ky-Bt; and Zone 6: Sil-St-Bt, Grt-Bt, and Grt-St-Bt. Reaction isograds separating the zones are as follows: And = Ky between zones 2 and 3, Cld+Ky = St+Chl between zones 3 and 4, St+Chl = Ky+Bt between zones 4 and 5, and Ky = Sil between zones 5 and 6. Qualitative and quantitative P-T estimates suggest variation along the metamorphic field-gradient from < or =3 kbar and 450 degrees C to < or =7 kbar and 650 degrees C. The pelites have low concentrations of minor (i.e., non-KFMASH) components and intermediate X Mg , resulting in the stability of chlorite up to middle-amphibolite-facies conditions and restriction of garnet to the highest-grade assemblages. Variations in both bulk-rock Al content and tie-line topologies controlled Al-saturation in the mineral assemblages. At low grades, only the Al-rich bulk compositions were Al-saturated, with reactions involving the Tschermak exchange vector affecting the position of the Al-saturation surface. At higher grades, after the Cld-Chl and St-Chl tie-lines became unstable, Al-saturation was possible in Al-poorer rocks containing biotite. There are systematic variations in X Mg distribution between coexisting Ms, Bt, Chl and possibly Cld that do not seem attributable to the presence of ferric iron in these minerals. This is consistent with the presence of non-ideal mixing in the Ms-Cd and possibly Chl solid solutions.

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