Sequential, syndeformational porphyroblast growth during Hercynian low‐pressure/high‐temperature metamorphism in the Canigou massif, Pyrenees

Abstract

The sequence of growth of garnet, staurolite and aluminosilicate in Fe-rich metapelitic rocks from the Canigou massif, Pyrenees, is established using evidence of inclusion, reaction and pseudomorphing textures between the different minerals, compositional zoning patterns in garnet and staurolite (that can be related to the KFMASH reaction grid), and the geometric relations between inclusion trails in the porphyroblasts and the matrix microstructures. The evidence indicates that garnet and staurolite commenced growth before aluminosilicate in all cases, even where all three are in textural equilibrium. Interpretation of the reaction textures between the porphyroblasts and of the compositional zoning in garnet and staurolite in terms of the KFMASH reaction grid indicates the importance of continuous reactions in the development of these phases. Some garnet and staurolite porphyroblasts underwent renewed growth during breakdown, producing rims enriched in Mn and Zn respectively. The presence of aluminosilicate in these assemblages (i.e. the absence of a clear andalusite-absent zone in the field) is attributed to a strong pressure-dependence for the aluminosilicate-producing reactions. Porphyroblastmatrix microstructural relations indicate that Hercynian metamorphism in the massif was synchronous with the development of the regional subhorizontal foliation (S3).

EMP-monazite age controls on P-T paths of garnet metapelites in the Variscan inverted metamorphic sequence of La Sioule, French Massif Central

“…It could be explained by a continuous staurolite-consuming reaction: staurolite + muscovite = andalusite + biotite + H 2 O. According to the P-T grid proposed by Gibson [1992], this staurolite-consuming and andalusite-producing reaction should have proceeded in the andalusite stability field along a P-T trend involving decompression.…”

Polymetamorphic origin of muscovite+cordierite+staurolite+ biotite assemblages: implications for the metapelitic petrogenetic grid and for P–T paths

“…Similar examples have been described from the Pyrenees by de Bresser et al . (1986 ) and Gibson (1992).…”

Regional vs contact metamorphism of garnet metapelites in the vicinity of Late Variscan granites (Central Armorican Domain, Brittany, France)

“…Arrows are prograde paths from micaschists and hornfelses. Broken arrows are retrograde paths from micaschists, inferred from zonations in outer garnet rims and the interpretation of mineral assemblages in the petrogenetic grids by Spear and Cheney (1989) and Gibson (1992) temperatures calculated from Mg-rich inner garnet rims and corresponding S 2 -biotites range around 600-640°C (Fig. 8a).…”

“…Following the interpretation of Gibson (1992, p. 640), the divariant field of the assemblage andalusite+staurolite is limited towards the lower-pressure one-phase andalusite stability field by a univariant curve which is isobaric in the P-T space. According to the P-T grid proposed by Gibson (1992), the staurolite-consuming and andalusiteproducing reaction should have proceeded in the andalusite stability field along a P-T trend involving decompression. Due to the decompression involved, it appears likely that this reaction was pertinent for retrograde regional metamorphism, rather than for a pure contact metamorphic stage (Fig.…”