Nine new chemical analyses of supposed para-amphibolites from the southern Bear-tooth Mountains are used with previous data to show that these rocks were derived from basic igneous rocks rather than clastic-carbonate sediment mixtures. Trends of chemical variation, rubidium-strontium and lanthanum-cerium ratios suggest a metaigneous origin. Banding in the amphibolites is due to metamorphic differentation. Two analyses of ultrabasic rocks are also presented.

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