HR: 16:50h
AN: V54A-04 INVITED     [Abstracts]
TI: Rejuvenated Galapagos Volcanism
AU: Harpp, K
EM: kharpp@mail.colgate.edu
AF: Colgate University, Geology, Hamilton, NY 13346 United States
AU: * Geist, D
EM: dgeist@uidaho.edu
AF: University of Idaho, Geological Sciences, Moscow, ID 83844 United States
AB: Floreana Island, which lies in the southern part of the Galapagos, has revealed the first evidence for rejuvenated volcanism in the archipelago. In the Galapagos, erosion is extraordinarily slow, so older phases of volcanism are not exposed. Thus, it has not been established whether the volcanoes maintain their compositions for their entire active lives, or if there is an evolutionary trend as the volcanoes are carried away from the focus of the hotspot. Exposed subaerial lavas on Floreana range from about 1 Ma to 20 Ka, but the volcano was at the focus of the Galapagos hotspot at about 3 Ma. Floreana lavas are notably alkaline, and they form an enriched isotopic endmember with the most radiogenic Pb and Sr isotopic ratios in the archipelago. The lavas also have a chemical imprint of metasomatism, as well as major and trace element evidence for shallow melting. Some mafic and ultramafic xenoliths have isotopic compositions unlike Floreana lavas but instead are similar to the volcanoes at the hotspot focus in the western Galapagos (i.e., Sierra Negra). Moreover, a submarine rift zone that extends from the island is constructed of lavas that also have the isotopic and trace element imprint of the western Galapagos source, distinct from that of Floreana. Therefore, in the southern Galapagos, volcanoes start with a dominantly plume signature, tholeiitic magmas are produced, and melting is dominantly within the garnet facies. After about 2 million years of inactivity, alkaline magmas with more enriched Sr and Pb (but not Nd) isotopic ratios are produced, and most melting is shallow. Because the metasomatic signature is much older than the lithosphere on which Floreana lies, we attribute the origin of these rejuvenated lavas to enriched plums within the southern part of the Galapagos plume. At the cool margin of the plume, this is the only component that melts, consequently melting is only at shallow depths. If rejuvenation occurs in the central part of the archipelago, it has completely different petrologic consequences, being dominated by entrainment of depleted upper mantle.
DE: 3615 Intra-plate processes (1033, 8415)
DE: 3619 Magma genesis and partial melting (1037)
DE: 3640 Igneous petrology
DE: 3690 Field relationships (1090, 8486)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005