Geol 285 - Petrology, Dr. Helen M. Lang, West Virginia University, Spring 2010 Generation of Subduction-related Andesites Calc-alkaline Volcanism occurs inboard of Subduction Zones at Convergent Plate Boundaries When mantle melts beneath Mid-Ocean Ridges or at Hot-spots, uniform tholeiitic basalt is formed • Why does melting of mantle at subduction zones produce such different magmas? • The magmas of subduction-related volcanoes are calc-alkaline magmas, mostly andesites. Things about subduction-related magmatism that must be explained by any model • The magmas (and rocks) are calc-alkaline, not tholeiitic – i.e., they show no Fe-enrichment • Magmas (and rocks) are dominantly andesites with higher SiO2 than basalts • More varied magma types are produced than in Hawaii or at Mid-ocean ridges • Bimodal volcanism (basalt + rhyolite) is common • Eruptions are commonly explosive Generation of Calc-alkaline volcanic rocks above Subduction Zones Refer to the Handout – Cross-section of an ocean-continent subduction zoneWhat happens at each of the locations numbered 1-9? 1 – 2 – 3 – 4 – 5 – 6 – 7 – 8 – 9 – Processes that change magma composition: Crystal Fractionation Contamination Magma mixing Assimilation and othersThings about subduction related magmatism that must be explained by any model • The magmas (and rocks) are calc-alkaline, not tholeiitic – i.e., they show no Fe-enrichment • Magmas (and rocks) are dominantly andesites with higher SiO2 than basalts • More varied magma types are produced than in Hawaii or at Mid-ocean ridges • Bimodal volcanism (basalt + rhyolite) is common • Eruptions are commonly
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