Metamorphic History
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Metamorphic History |
| Prograde pressure-temperature-deformation-time (P-T-D-t) paths for rocks of different grade in the Cooma Complex have been suggested by Johnson & Vernon (1995a). However, a detailed prograde history cannot be inferred with confidence, because both these melting reactions and the andalusite-sillimanite transformation may all occur at practically the same temperature and pressure. For example, the first appearance of leucosome and prograde fibrous sillimanite at Cooma occurs in the same rock (sample 40). The absence of muscovite from non-migmatitic rocks of the cordierite–K-feldspar and andalusite–K-feldspar zones (except in local retrograde zones), indicates that the muscovite + quartz dehydration reaction was crossed before melting occurred, and consequently that muscovite dehydration did not contribute to the leucosomes. The metapelitic leucosomes appear to have formed by reaction of quartz + biotite and andalusite. Possible reactions are: Qtz + Bt + And = Crd + Kfs + melt or Qtz + Bt + And + Kfs + water = Crd + melt, depending on the availability of water vapour (Grant, 1985; Ellis & Obata, 1992). | |