Geochemistry of amphibolites from the central Beartooth Mountains, Montana-Wyoming

Trends of variation of major- and minor-element contents in amphibolites from the central Beartooth Mountains strongly suggest that these rocks of andesitic composition are derived from a tholeiitic, mafic igneous parent and not from a sedimentary parent. Discriminant functions based on minor-element content also indicate igneous parentage, whereas functions based on major-element content indicate a sedimentary parent, suggesting that the general assumption that the regional metamorphism is isochemical may not be entirely valid. Furthermore, chemical data on the amphibolites do not fit the fractionation curves of average Beartooth metadolerites, the most likely precursor. If, however, it is assumed that regional metamorphism and formation of amphibolites were accompanied by potassium metasomatism, then the higher concentrations of SiO₂, Al₂O₃, K₂O, and potassium-related minor elements in the amphibolites relative to metadolerite can be explained.