INCOMPATIBLE-ELEMENT RICH ANDESITIC AMPHIBOLITES FROM THE ARCHEAN OF MONTANA AND WYOMING - EVIDENCE FOR MANTLE METASOMATISM

The incompatible-element-rich andesitic rocks of the Beartooth and Bighorn Mountain areas of Montana and Wyoming represent melts derived from a mantle source during the Archaean. Major- and trace-element abundances in conjunction with isotopic data suggest that the source region of the melts was enriched in incompatible elements during, or immediately before, the melting event. Similar models have been proposed for more modern rock series where the enriched phase may have been derived from another part of the mantle or from a subducting slab. Regardless of the source of this enriched phase, it seems clear that the mantle in this area was developing regional-scale chemical heterogeneities by approx 3000 m.y. ago.- A.P.