Magmatic andalusite from the migmatite zone of the Aoyama area, Ryoke metamorphic belt, SW Japan, and its importance in constructing the P-T path.

Abstract

Magmatic andalusite is found from the metatexite and from the pelitic rock with boudin necks filled with leucosome in the garnet-cordierite (Grt-Crd) zone of the Aoyama area, Ryoke metamorphic belt, SW Japan. In general, andalusite crystals in the pelitic schist without leucosome from the sillimanite-K-feldspar (Sil-Kfs) zone and low-temperature part of the Grt-Crd zone are partly transformed into sillimanite, suggesting that the peak P-T conditions of the Aoyama area have reached to the sillimanite grade. On the other hand, andalusite crystals found from the leucosome of the low-temperature part of the Grt-Crd zone are subhedral, and have not transformed into sillimanite at all. Absence of an evidence for andalusite-sillimanite transition implies that these crystals did not experience the prograde andalusite-sillimanite transition and thus are retrograde products. They characteristically occur in the leucosome containing euhedral plagioclase or subhedral cordierite crystals and show subhedral shape different from the euhedral shape usually observed in the prograde andalusite. The P-T conditions for the low-temperature part of the Grt-Crd zone is sufficiently high for the partial melting of pelitic rocks, and the andalusite-bearing leucosome has appropriate composition as a frozen melt. These observations suggest that the retrograde andalusite is of magmatic origin. Combining effects of the presence of boron in the melt, addition of Al2O3 to the subaluminous Qtz-Ab-Or system to saturate in aluminosilicates, and preferential incorporation of Fe2O3 into andalusite may have favored the crystallization of magmatic andalusite in the Aoyama area. Finding of the magmatic andalusite from the low-temperature part of the Grt-Crd zone suggests that the P-T path of this zone went through the overlapping region of water-saturated solidus of peraluminous granite and andalusite stability field. The P-T path of the low-temperature part of the Grt-Crd zone may, therefore, not be a hairpin shaped one but a clockwise one when the aluminosilicate phase diagram of Holdaway (1971) is used. Non-hairpin shaped nature of the P-T path may be partly responsible for the preservation of the textural evidences of partial melting in the Aoyama area.