HR: 09:30h
AN: V31A-06    [PDF]
TI: A Cenozoic Alkaline Magmatic Province in the SW Pacific Without Rift or Plume Origin
AU: * Finn, C A
EM: cfinn@usgs.gov
AF: U. S. Geological Survey, MS 964, Denver Federal Center, Denver, CO 80225 United States
AU: Mueller, D
EM: dietmar@es.usyd.edu.au
AF: The University of Sydney, Department of Geology and Geophysics, Sydney, NSW 2006 Australia
AU: Panter, K S
EM: kpanter@bgnet.bgsu.edu
AF: Bowling Green State University, Department of Geology, Bowling Green, OH 43403 United States
AB: A large-scale, long-lived (ca. 55 Ma) alkaline magmatic province occupies discontinuous portions of the SW Pacific region including eastern Australia, New Zealand, west Antarctica and the oceanic plates between. Although this province, here named Pacific SW Alkaline Province (PSWAP), is aerially extensive, estimates of the total volume of upper crustal igneous rocks and the magma production rates are rather low. Comparison of the location of magmatic centers and seismic shear wave perturbation models shows that the PSWAP is largely limited to lithosphere less than 80 km thick underlain by distinct low seismic velocity anomalies extending to 200 km depth. Geochemical studies show that the magmatism is largely a result of small degrees of melting of a source enriched in incompatible elements relative to primitive upper mantle. Metasomatic enrichment of the upper mantle, including infiltration of volatile-rich fluids and melts into the overlying lithosphere, may explain the regional geochemical as well as low seismic velocity signatures. Metasomatism most likely occurred during Paleozoic-Mesozoic subduction along the Pacific margin of Gondwana and Jurassic plume-related activity. Models to explain the magmatism in separate parts of PSWAP include adiabatic decompressive melting due to rifting and strike-slip faulting, mantle plumes, or hundreds of hot spots, but all of these associations have flaws. Hot spot formation and associated magmatism have also been linked to faults and propagating fractures in geodynamic models of extending lithosphere above regions where upper mantle has been shielded from the cooling effects of subducting slabs. In addition, boundaries between thick cratons and thin lithosphere (such as in eastern Australia and West Antarctica) may drive small-scale convection, bringing hot sub-cratonic mantle toward thinner lithosphere with eruption occurring during extension- often during major plate reorganizations. But the Cretaceous break-up of Gondwana that rifted Australia and the New Zealand-Campbell Plateau region from Antarctica, regional extension of lithosphere over metasomatized mantle did not cause widespread magmatism as predicted by these models. Although the PSWAP magmatism has been linked broadly to tensional stress-fields, these are not requirements as evidenced by Australian volcanism under mildly compressional stress-fields. Therefore, extension probably did not trigger Cenozoic magmatism in PSWAP. A regional Cenozoic heating event may be more likely. We propose a new model that links sudden detachment and sinking of subducted slabs into the lower mantle beneath the PSWAP in the early Cenozoic, with vertical and lateral flow that triggered melting of metasomatized upper mantle. Small batches of magma leak through pre-existing faults and fractures in most regions, but larger volumes erupt in areas with moderate extension.
DE: 3040 Plate tectonics (8150, 8155, 8157, 8158)
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8499 General or miscellaneous
DE: 9310 Antarctica
DE: 9330 Australia
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting