HR: 0800h
AN: V51A-01    [Abstracts]
TI: How ‘Monogenetic' is the Auckland Volcanic Field?
AU: Spargo, S R
EM: s.spargo@auckland.ac.nz
AF: Geology-SGGES, University of Auckland, PB92019, Auckland Mail Centre, Auckland, 1142, New Zealand
AU: * Smith, I E
EM: ie.smith@auckland.ac.nz
AF: Geology-SGGES, University of Auckland, PB92019, Auckland Mail Centre, Auckland, 1142, New Zealand
AU: Wilson, C J
EM: cjn.wilson@auckland.ac.nz
AF: Geology-SGGES, University of Auckland, PB92019, Auckland Mail Centre, Auckland, 1142, New Zealand
AB: The Auckland Field is the youngest basaltic intraplate volcanic field in New Zealand; it is located about 350-400 km behind the present day active convergent plate boundary. The field contains about 50 recognised late Pleistocene to Holocene eruptive centres generated by the rise and eruption of very small volume (mainly less than 0.35 km3) batches of magma. The field covers approximately 100 km2 of the Auckland urban area and has been termed monogenetic, implying that individual centres erupt single magma batches during brief eruptive periods. Detailed studies of individual centres reveal significant compositional diversity. The following trends are recognised: 1). Single trends from early evolved to later less evolved compositions representing deep near source fractionation of a single magma batch generated in the garnet peridotite stability field (e.g. Crater Hill about 29 ka, 0.1 km3), this is demonstrably monogenetic behaviour. 2). Multiple compositional trends in magmas from a single eruption event signifying the sequential rise and fractionation of magma batches generated from different sources (3-8 percent melt of a garnet peridotite source at depths of about 80-50 km and 5-12 percent melt of spinel peridotite at depths about 50- 22km), for example Pupuke (about 250 ka, 0.1 km3) this is polygenetic behaviour. 3). Multiple compositional trends in temporarily discrete eruption events from the same centre (Rangitoto, 8 to 700 a, 2.3 km3) this is also polygenetic behaviour. The chemical diversity observed within these three volcanic centres, representing the life span of the Auckland Volcanic Field, questions how well we actually understand this very common type of global volcanism. The range of compositions observed in individual centres of the Auckland Volcanic Field reflects the interplay of melting and fractionation processes at different depths in the mantle and calls into question the use of the term monogenetic to describe them.
DE: 1033 Intra-plate processes (3615, 8415)
DE: 3615 Intra-plate processes (1033, 8415)
SC: Volcanology, Geochemistry, and Petrology [V]
MN: 2007 Joint Assembly