Rare-earth element distributions in volcanic rocks from Archean greenstone belts

Rare-earth element distributions in Archean volcanic rocks from the South Pass (Wyoming), Yellowknife (NW Canada) and Abitibi (Quebec) greenstone belts and from the Upper Fig Tree Group of the Barberton (S. Africa) greenstone belt reveal two distinct types of Archean volcanism. One type, herein referred to as the arc-type, is characterized by flat (or slightly enriched) REE distributions in tho leiites and enrichment in total and light REE and a variable negative Eu anomaly in more siliceous volcanic members. The second type, herein referred to as the Abitibi-type, is characterized by rather flat REE patterns and negative Eu anomalies in all volcanic rock types. REE distributions in the arc-type volcanic successions can be produced by either progressive shallow fractional crystallization of tholeiitic magma or by decreasing amounts of equilibrium melting of a plagioclase-bearing mantle source. REE distributions in the Abitibi volcanic rocks are most readily explained in terms of progressively decreasing amounts of fractional melting of a source area in which REE are contained chiefly in minor minerals (with low melting temperatures) that are depleted in Eu. The melting models seem to necessitate the existence of one or more pre-greenstone magmatic episodes as well as a continuously replenished mantle source. Replenishment of source material could be accomplished in either of the melting models in subduction zones but the analogy to Phanerozoic plate tectonics should be used with caution. Melting models also imply either (or both) a decreasing geothermal gradient with time or systematic changes in mantle source-area composition.