Partial melting and P-T evolution of migmatitic metapelites from the southwestern Gagnon Terrane, northeastern Grenville Province

Jordan, Sherri Lynn (2003) Partial melting and P-T evolution of migmatitic metapelites from the southwestern Gagnon Terrane, northeastern Grenville Province. Masters thesis, Memorial University of Newfoundland.

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Abstract

The Gagnon terrane (Parautochthonous belt, northeastern Grenville Province) is situated between the Grenville Front and the tectonically overlying High Pressure belt to the southeast. It was metamorphosed during the late stages of the Grenvillian orogeny (~1000 Ma) at P-T conditions increasing from greenschist facies near the Grenville Front to amphibolite and HP granulite facies farther south in the higher structural levels. Migmatitic metapelites from three thrust slices, which form part of the structurally higher levels, display evidence of extensive partial melting in the kyanite stability field and can be classified as HP granulites. These rocks are composed of variably intermixed leucosome and restite and have a peak assemblage of garnet + plagioclase + quartz + K- feldspar + kyanite ± biotite which is locally overgrown by retrograde biotite ± kyanite ± plagioclase ± quartz ± muscovite. The prograde sub-assemblage K-feldspar + kyanite is consistent with dehydration melting of micas. Textures as well as compositional patterns in garnet, in cases where growth zoning is preserved, are consistent with a reaction sequence involving: (a) dehydration melting of muscovite by the discontinuous NaKASH reaction: muscovite + quartz ± plagioclase = K-feldspar + kyanite + liquid [R1] followed by (b) dehydration melting of biotite by the continuous NaKFMASH reaction: biotite + quartz + kyanite ± plagioclase = K-feldspar + garnet + liquid [R2]. In one particular case, however, inclusion and zoning patterns in garnet are suggestive of dehydration melting of phengite instead of muscovite, followed by a discontinuous NaKFMASH reaction before entering the field of reaction [R2]. Muscovite has only been observed as a replacement of kyanite or K-feldspar, suggesting that final crystallization of the melt took place in the muscovite stability field. Biotite is commonly developed at the expense of garnet, consistent with operation of reaction [R2] in the reverse sense, but it is not clear if all biotite is retrograde. Compositional data suggest that the metapelites crossed reaction [Rl] at P-T conditions between 1140-1445 MPa and 750-780°C while the melt crystallized at conditions between 930-1100 MPa and 722-748°C. These data are consistent with a clockwise P-T path involving little decompression between the prograde and retrograde parts of the path. Migmatic metapelites also occur within a shear zone that marks the southern boundary of the upper structural levels in the southwestern Gagnon terrane, however, these rocks contain retrograde sillimanite (and K-feldspar) rather than kyanite. The presence of retrograde sillimanite suggests that final melt crystallization and possibly dehydration melting of micas by reactions [R1] and [R2] occurred at lower pressures at this locality. -- In the lower structural levels located in the northeastern Gagnon terrane, the transition between muscovite-bearing metapelites to kyanite + K-feldspar-bearing migmatitic metapelites is marked by a zone in which leucosome locally occurs in muscovite-bearing rocks. Textures and garnet zoning in these mid-crustal levels are suggestive of the fluid-present melting reaction: muscovite + quartz + albite + H₂O = kyanite + liquid [3] that occurs at lower temperatures than [R1], followed by garnet growth by a divariant vapour-absent reaction of the type: biotite + kyanite + plagioclase + quartz = garnet + muscovite. Farther south, on the other side of the muscovite-out isograd, this reaction sequence was followed by reactions [R1] and [R2] with the former reaction being crossed at approximately 1525 MPa and 795°C. Thus, the main difference between kyanite + K-feldspar migmatitic metapelites from the higher and the mid-crustal levels is that in the former there is no evidence of fluid-present melting reactions predating dehydration melting of micas, whereas in the latter there is. -- The metamorphic evolution of kyanite-bearing metapelites of the southern Gagnon terrane is consistent with previously proposed tectonic models involving NW-directed tectonic transport of the High Pressure belt over the Gagnon terrane, with incorporation of deeply buried rocks from the upper structural levels of the Gagnon terrane into the ductile shear zone near the interface with the HP belt. Thus the upper structural levels of the Gagnon terrane share metamorphic characteristics with both the tectonically overlying HP belt and the tectonically underlying remainder of the Gagnon terrane.

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