HR: 0800h
AN: V51B-0574    [Abstracts]
TI: Geochemical Evolution of the Hikurangi Oceanic Plateau, New Zealand
AU: Hoernle, K
EM: khoernle@ifm-geomar.de
AF: Dynamics of the Ocean Floor, IFM-GEOMAR, Wischhofstr. 1-3, Kiel, 24148 Germany
AU: Hauff, F
EM: fhauff@ifm-geomar.de
AF: Dynamics of the Ocean Floor, IFM-GEOMAR, Wischhofstr. 1-3, Kiel, 24148 Germany
AU: * Werner, R
EM: rwerner@ifm-geomar.de
AF: Tethys Geoconsulting GmbH, Wischhofstr. 1-3, Kiel, 24148 Germany
AU: Mortimer, N
EM: N.Mortimer@gns.cri.nz
AF: Insitute of Geological and Nuclear Sciences, Private Bag 1930, Dunedin, 31-312 New Zealand
AU: van den Bogaard, P
EM: pbogaard@ifm-geomar.de
AF: Dynamics of the Ocean Floor, IFM-GEOMAR, Wischhofstr. 1-3, Kiel, 24148 Germany
AU: Geldmacher, J
EM: jgeldmacher@ifm-geomar.de
AF: Dynamics of the Ocean Floor, IFM-GEOMAR, Wischhofstr. 1-3, Kiel, 24148 Germany
AU: Garbe-Schoenberg, D
EM: dgs@gpi.uni-kiel.de
AF: Institut fuer Geowissenschaften, Kiel University, Ludewig-Meyn-Str. 10, Kiel, 24118 Germany
AB: The Hikurangi oceanic plateau or large igneous province (LIP), located east of the North Island of New Zealand, covers an area of 350,000 km3 and is located at a depth of 2,500-3,500 b.s.l. The Hikurangi plateau was possibly connected to the Manihiki LIP (now located 3000 km to the north) but may have been separated by Cretaceous seafloor spreading at the Osbourn Trough (Billen and Stock, 2000, J. Geophys. Res., 106, 13481-13489). Therefore it may have formed part of the "greater Ontong Java Plateau event" (Coffin and Eldholm, Geology, 21, 515-51), the largest magmatic event preserved on Earth. During the R/V Sonne SO168 ZEALANDIA cruise, 77 dredge hauls containing igneous samples were recovered from the Hikurangi Plateau. Volcanic rocks were obtained from 1) the plateau basement along the 1 km high Rapuhia Scarp, 2) large guyot-type seamounts within the plateau, and 3) ridge-type seamounts associated with rifting of the NE plateau margin (Hoernle et al., 2004, EOS). The recovered plateau rocks range from basalts, dolerites and gabbros with tholeiitic and alkali basaltic to trachybasaltic compositions. The seamount volcanic rocks have more Si-undersaturated compositions than the plateau rocks and range from alkali basalts through mugearites to basanites through tephrites to nephelinites. The plateau basement rocks have flat rare earth element (REE) patterns similar to enriched mid-ocean-ridge basalt (MORB) and basement rocks from other oceanic LIPs, such as Manihiki, Ontong-Java and the Caribbean. The late-stage seamount lavas show enrichment in the light REE and all strongly to moderately incompatible elements, having incompatible element characteristics similar to the HIMU (high time-integrated U/Pb) component in ocean island basalts (OIB). Although the Pb isotopic composition has been extensively effected by seawater alteration, the freshest samples have enriched (EM-type) Sr-Nd-Pb isotopic compositions similar to Ontong-Java and Manihiki basement rocks, suggesting derivation from a common source. In contrast, the late stage seamount lavas have Sr-Nd-Pb isotopic compositions similar to HIMU OIB. In conclusion, increasing Si-undersaturation of the volcanic rocks with decreasing age suggests that the degree of melting decreased and melting depths increased, possibly due to increasing lithospheric thickening, during the waning stages of plateau growth. The change from EM to HIMU-type trace element and isotopic signatures indicates that the low-degree, late-stage seamount lavas were derived from different source material than the large-degree plateau basement volcanic rocks, either reflecting a heterogeneous source or distinct sources for the plateau and seamount volcanism. Ar/Ar age dating is underway.
DE: 5480 Volcanism (8450)
DE: 1035 Geochronology
DE: 1040 Isotopic composition/chemistry
DE: 1010 Chemical evolution
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting