An application of R-mode factor analysis to the study of metamorphic petrogenesis

R-mode factor analysis of modal and chemical data clarifies the trends of mineralogical evolution and associated chemical variations in amphibolites from the Northwest Adirondack Mountains, New York and the Willyama Complex, Broken Hill District. In both areas the principal mineralogical reaction attending metamorphism, as described by the first factor, appears to have been Hornblende + Quartz → Calcic pyroxene + Orthopyroxene + Plagioclase. For the Adirondack amphibolites, the derived reaction coefficients viewed against the modal contents of the pyroxenes suggest another hornblende breakdown reaction producing calcic pyroxene, which is corroborated by the first appearance of the phase a little NE of Emeryville. The other factors extracted reveal the underlying parameters responsible for the development of biotite, sphene, opaques and quartz in the Adirondack metabasites and albite, anorthite, ilmenite and quartz in the Willyama Complex amphibolites.The analysis also demonstrates that the principal mineralogical reactions in both the areas have been combinations of thermal and ionic types and further that to decipher the net mineralogical changes in high grade metamorphic basic and pelitic rocks it is necessary to extend the integration of the ‘part reactions’ occurring in different domains to the level of a hand specimen.