Melting of the Cooma Metapsammites
  The Cooma high-grade feldspathic metasediments (metapsammites and metapsammopelites) consist of quartz, K-feldspar, plagioclase and biotite, with local cordierite, andalusite and fibrous sillimanite. Metapsammite melting first occurs at slightly higher grade than the initial metapelite melting being only incipient in Spring Creek (Fig. 2). It is most abundant in the highest-grade rocks, which are high-strain, stromatic and diatectic migmatites. The melting reaction appears to have involved breakdown of biotite, quartz and feldspars in the presence of water, probably requiring a higher temperature than the metapelite melting reaction (e.g., Ashworth, 1985, p. 12). Detailed studies are in progress.

Disruption and boudinage of the metapelite leucosome imply that it solidified before injection of the metapsammite leucosome, and this is a possible source of water for the late hydration of cordierite that is widespread in the high-grade rocks of the Cooma Complex (Vernon, 1978). Therefore, later melting of the metapsammites (1) required an external source of water and (2) involved re-heating, at least of the high-grade parts of the Cooma Complex. The heat source for the metapsammite melting is unknown, but Johnson et al. (1994) suggested that intrusion of the main Cooma Granodiorite body may have re-heated the higher grade rocks, producing syn-D5 sillimanite while retrograde minerals were growing in the lower-grade rocks.
 
Previous Section Home Next Section