Phase relations in andalusite‐sillimanite type Fe‐rich metapelites: Tono contact metamorphic aureole, northeast Japan

Abstract

ABSTRACTMineral assemblages in metapelites of the contact aureole of the Tono granodiorite mass, northeast Japan, change systematically during progressive metamorphism along an isobaric path at 2‐3 kbar. The bulk rock compositions of metapelites are aluminous with A′ values on an AFM projection larger than that of the chlorite join. The metapelites commonly contain paragonite in the low‐grade zone. With increasing temperatures, andalusite is formed by the breakdown of paragonite. The importance of pyrophyllite as a source of Al2SiO5 polymorphs is limited in typical pelitic rocks.The most common type of metapelite in the study area has FeO/(FeO + MgO) = 0.5–0.6, and develops assemblages involving chlorite, andalusite, biotite, cordierite, K‐feldspar, sillimanite and almandine, with paragenetic changes similar to other andalusite‐sillimanite type aureoles. Rocks with FeO/(FeO + MgO) > 0.8 progressively develop chloritoid‐bearing assemblages from Bt‐Chl‐Cld, And‐Bt‐Cld, to And‐Bt at temperatures between the breakdown of paragonite and the appearance of cordierite in the more common pelitic rocks in the aureole. The paragenetic relations are explained by a KFMASH univariant reaction of Chl + Cld = And + Bt located to the low‐temperature side of the formation of cordierite by the terminal equilibrium of chlorite. A P‐T model depicting the relative stability of chloritoid and staurolite at low‐ and medium‐pressure conditions, respectively, is proposed, based on the derived location of the Chl + Cld = And + Bt reaction combined with the theoretical phase relations among biotite, chlorite, chloritoid, garnet and staurolite.