Crustal growth by magmatic overplating in the Galápos

The isotopic compositions of xenoliths hosted in lavas from Floreana Island indicate that they formed from magmas unlike those at present-day Floreana. Instead, the xenoliths are geochemically more similar to magmas now erupting from Sierra Negra and Cerro Azul volcanoes, at the leading edge of the Galápagos hotspot. This is the first evidence for compositional evolution at a Galápagos volcano and indicates increasing contributions from an isotopically enriched source with time as the volcano is carried away from the focus of the hot-spot. Clinopyroxenes in many of the xenoliths exhibit positive anomalies of Sr and Eu, which are attributed to the breakdown of plagioclase. The growth of clinopyroxene at the expense of plagioclase results from compression as the crust cools. Compression is caused by growth mostly from above, as shallow intrusions and lavas load the middle and upper oceanic crust. © 2007 The Geological Society of America