CRYSTALLIZATION OF METASTABLE MINERALS IN HIGH PRESSURE AND LOW TEMPERATURE METAMORPHISM AND ITS IMPLICATIONS TO P^T PATH STUDY

Chemically heterogeneous minerals are common in low grade area of high pressure regional metamorphic terrains. Some of heterogeneity has been ascribed to fractional crystallization as for bell-shaped compositional profile of Mn of garnet, and reversely zoned (Mn-enrichment at rim ) garnet has been ascribed to its resorption by retrograde metamorphism. Garnet with minimum Mn content in the center of crystal is formed when the temperature of the host rock suddenly increased to bring them in the reaction loop of garnet forming reaction. Vicial sector-zoned garnet occurs in limited areas with particular geology.

Sector-zoned epidote group minerals are common. Sharp compositional gap of epidotes across the boundary of chemical sectors have been often regarded as corresponding to miscibility gap. Even if heterogeneous, as far as minerals exist, they have mass and structure, for which thermodynamic properties are defined. Three areas with different grade of epidote Epumpellyite - actinolite assemblage are distinguished using the average compositions of epidotes in sets of samples from small areas.

The textural study of metagreywackes of the Franciscan in the Pacheco Pass area revealed that rocks with stable albite entered the jadeite - quartz stable P-T domain. A kind of possible sector zoning of jadeite from the jadeitite of the Cazadero area was observed in Na-poor omphacite of the Sanbagawa belt in Shikoku, too. Morphological core to rim compositional change of such pyroxenes, however, does not corresponds to the change of the temperature and pressure of metamorphism, unless it is proven that apparent rim grew later than the apparent core.

The presence of chemically heterogeneous minerals does not necessarily negates the rigorously established tradition of phase petrology, but it warns naive attitude to simply send the analyses to geothermobarometer program without looking into chemical structure.