Abstract

Paleoproterozoic high-Mg andesites of the Birch Lake assemblage in the Trans-Hudson orogen display trace element enrichments characteristic of Cenozoic boninites, occur in an analogous stratigraphic setting, and share a metallogenic association with the younger rocks. Their geochemical compositions indicate substantial prior depletion and a selective reenrichment of the mantle sources consistent with metasomatism by siliceous melts. These requirements are best accounted for in an island-arc setting and indicate that early extensional stages of subduction-zone development have not changed substantially over the past 2000 m.y.

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