HR: 0800h
AN: V31B-1440    [Abstracts]
TI: The Elusive Intrusive: A Petrologic, Structural, and Geochemical Analysis of a Basaltic Body in an Abandoned Rift, Vatnsdalsfjall, Northern Iceland
AU: * McClanahan, P M
EM: pmcclana@wso.williams.edu
AF: Williams College, Dept . of Geosciences 947 Main St., Williamstown, MA 01267 United States
AU: Wobus, R A
EM: rwobus@williams.edu
AF: Williams College, Dept . of Geosciences 947 Main St., Williamstown, MA 01267 United States
AB: The Hjallin Lens is a thick lenticular basaltic unit in Vatnsdalsfjall near the southern end of the Skagi Peninsula, northern Iceland. The lens was named and first studied by Annells (1968), who interpreted it as a shallow laccolith. However, recent field observations followed by extensive petrographic, geochemical, and geochronolgical analyses support an alternate interpretation: that the lens is actually an extrusive feature formed by the ponding of a thick basalt flow in a topographic low developed above a monoclinal flexure (Ackerly, 2004). The lens extends approximately 3000 meters NNW-SSE and 800 meters ENE-WNW. It reaches its greatest thickness (over 150 meters) near its southern end; its upper contact, however, is not exposed along the ridge crest. The lithology of the lens is virtually uniform throughout: a very hard, fine-grained aphyric basalt that has cooled in well developed columns. It is underlain by a rhyolitic ash flow tuff (previously interpreted as a chilled margin) along a sharply defined lower contact. At the southern end of the lens the ash flow tuff overlies a poorly lithified sediment with well preserved fragments of carbonized and silicified wood. The presence of these relatively undisturbed surficial units immediately beneath the lens is one line of evidence supporting the extrusive origin of its basalt. Geochemically the basalt of the lens is a tholeiite enriched in incompatible elements and with Zr/Nb values less than 10, indicating a plume overprint of N-MORB composition. The striking textural, mineralogical, and geochemical homogeneity of the lens basalt suggests that the entire body must have cooled very rapidly, without the internal differentiation that would likely have accompanied a slower-cooling intrusive basalt of this thickness. New argon-argon dating provides an age of 6.98+/-0.18 Ma for the lens basalt, indicating that it formed during the waning stages of volcanic activity in the Snaefellsnes Rift, which was an active spreading center from about 15-7 Ma.
DE: 8434 Magma migration
DE: 3035 Midocean ridge processes
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting