Cordierite and orthopyroxene (or orthoamphibole) are widespread in migmatitic terranes, and partial melting of pelitic rocks may be important in their production. In particular, the reaction quartz 4-albite.4, biotite 4-garnet.4, water vapor = cordierite 9 4.orthopyroxene or orthoamphibole+melt was among reactions discussed by Grant (1973) but poorly constrained in pressure-temperature space.
This reaction involves too many phases to be readily studied experimentally. Therefore simpler melting and dehydration reactions involving quartzalbite-biotite-cordierite-orthopyroxene were investigated.
In conjunction with the work of Hoffer (1976, 1978) these experiments place useful constraints on the above reaction and on the reaction quartz -4-albite -4-aluminosilicate -4-biotite -4-vapor = cordierite -4-garnet,4, melt. In pelitic rocks near the second sillimanite isograd, cordierite and garnet may coexist with melt as low as 660~ and cordierite and orthopyroxene may coexist with melt at temperatures less than 675 ~ C. In the absence of significant Mn or Ca, in pelitic rocks within the realm of melting, biotite+garnet assemblages are probably limited to pressures greater than 2kb and aluminosilicate ~-biotite assemblages to pressures greater than 3kb.