HR: 17:30h
AN: V34B-07    [Abstracts]
TI: Do Processes of Rhyolite Genesis Change as Icelandic Rifts Drift off of the Plume?
AU: * Jordan, B T
EM: bjordan@wooster.edu
AF: Department of Geology, The College of Wooster, Wooster, OH 44691 United States
AB: The abandoned Snaefellsnes rift zone in western Iceland was the on-land manifestation of the Mid-Atlantic Ridge between 15 and 7 Ma. The rift zone was abandoned at 7 Ma, after it had drifted westward off of the Iceland hotspot, generally interpreted as a mantle plume. The position of the abandoned rift was initially recognized as the axis of a regional syncline analogous to the syncline developed in response to active rifting. Previous paleomagnetic and geochronologic studies have confirmed the position of the abandoned rift axis. Recent seismic tomography shows that the abandoned rift is also characterized by relatively thin crust ($<$20 km, versus up to 46 km above the plume). In the context of supervising Keck Geology Consortium undergraduate research projects in northwestern Iceland in 2003 and 2004, I have studied several silicic centers erupted at different times along the northern Snaefellsnes rift. A compilation of preliminary geochemical data from the Skagi area near the rift reveals several interesting trends that bear on the origin of silicic magmas as activity in the rift was waning. The compositional spectrum of silicic rocks in this area is from dacite (67 wt.% SiO$_{2}$) to rhyolite (75 wt.% SiO$_{2}$). Positive correlation between Na$_{2}$O and SiO$_{2}$ is consistent with either fractionation or decreasing degrees of crustal melting to get from dacite to rhyolite. However, Zr correlates negatively with SiO$_{2}$, consistent with zircon fractionation, but inconsistent with variation in the degree of melting unless zircon is present in the source, unlikely for the meta-basaltic crust of Iceland. Therefore, I suggest these rocks reflect extreme ($>$90%) fractionation of a basaltic parent. A similar argument was advanced by Furman et al. (1992, J. Pet., 1405-1445) for rhyolites at Austerhorn in eastern Iceland. Compelling arguments have been previously advanced that most rhyolites erupted in the modern rifts of Iceland are the products of crustal melting. The same has been argued for some older centers erupted from the Snaefellsnes rift (Kroksfjordur, 9-10 Ma?). I propose the hypothesis that as a rift drifts off of the plume, and becomes more like a normal mid-ocean ridge (thinner crust), rhyolite genesis by crustal melting becomes uncommon, and that what rhyolites are generated are the result of extreme fractionation of a basaltic parent. Ongoing studies will test this hypothesis by more detailed trace element and O-isotope studies and the systematic study of a series of silicic centers erupted at the northern Snaefellsnes rift over its history of activity.
DE: 8121 Dynamics, convection currents and mantle plumes
DE: 8150 Plate boundary--general (3040)
DE: 3640 Igneous petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting