Abstract

The Beartooth Mountains of the northern Wyoming province contain a suite of incompatible-element-enriched amphibolites of andesitic composition that contains both calcalkalic and tholeitic varieties. These amphibolites formed about 2800 Ma and show trace-element and lead isotopic signatures that suggest an origin similar to some modern andesites produced in convergent margin environments. In particular, these rocks show depletions of the high-field-strength elements (e.g., Nb, Ti, Zr) and enriched Pb isotopic compositions indicative of recycling of Pb from a crustal reservoir into the mantle. These data and other geologic considerations suggest that the northern Wyoming province may have been the site of a Late Archean convergent continental margin.

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