Ortho- and para-amphibolites and related silicate and carbonate rocks in the Grenville Province in Ontario have been studied in the field and laboratory in order to determine their genesis. X-ray fluorescence chemical analyses for 31 elements in each of 100 rocks of similar types and of greenschist to granulite facies are used to show that the complex of metasediments was originally an association of tuffs and carbonates. The ortho-amphibolites probably are Na-metasomatized tholeiites. Para-amphibolites were basic tuffs; biotite-plagioclase gneisses were intermediate to acid tuffs. The tuffs were deposited in a region of carbonate sedimentation.Scapolite and clino-pyroxene are the dominant minerals in many of the granulite facies rocks. They are developed from decarbonated calcium-rich rocks to which chlorine was added. The average CO2 loss was 9 to 10 wt.%, and the average Cl addition was 0.8 wt.%. Zn and Pb probably were also added to the high-grade rocks.It is demonstrated that pelite-carbonate mixtures are unlikely progenitors of amphibolites but basic tuffs very likely are. Thin-layered amphibolites could easily be derived from basic tuffs such as occur in unmetamorphosed sedimentary sequences.