Contributions to Mineralogy and Petrology

, Volume 162, Issue 3, pp 547–563 | Cite as

Dynamics of melting beneath a small-scale basaltic system: a U-Th–Ra study from Rangitoto volcano, Auckland volcanic field, New Zealand

  • Lucy E. McGeeEmail author
  • Christoph Beier
  • Ian E. M. Smith
  • Simon P. Turner
Original Paper


The Auckland volcanic field is a Quaternary monogenetic basaltic field of 50 volcanoes. Rangitoto is the most recent of these at ~500 year BP and may mark a change in the behaviour of the field as it is the largest by an order of magnitude and is unusual in that it erupted magmas of alkalic then subalkalic basaltic composition in discrete events separated by ≤50 years. Major and trace element geochemistry together with Sr–Nd and U-Th–Ra isotopes provides the basis for modelling the melting conditions that brought about the eruption of two chemically different lavas with very little spatial or temporal change. Sr–Nd isotopes suggest that the source for both eruptions is similar with a slight degree of heterogeneity. The basalts show high 230Th-excess compared with comparable continental volcanic fields. We show that the alkalic basalts give evidence for lower degrees of partial melting, higher amounts of residual garnet, a longer melting column and lower melting and upwelling rates compared with the subalkalic basalts. The low upwelling rates (0.1–1.5 cm/year) modelled for both magmas do not suggest a plume or major upwelling in the mantle region beneath Auckland; therefore, we suggest localised convection due to relict movement from the active subduction system situated 400 km to the southeast. A higher porosity for the initial alkalic basalt is based on 226Ra-excesses, suggesting movement of melt by two different porosities: the initial melt travelling in fast high porosity channels from greater depths preserving a high 230Th-excess and the subsequent subalkalic magma travelling from a shallower depth through lower porosity diffuse channels preserving a high 226Ra-excess; this creates a negative array in (226Ra/230Th) versus (230Th/238U) space previously only seen in mid ocean ridge Basalt data. This mechanism suggests the Auckland volcanic field may operate by the presence of discrete melt batches that are able to move at different depths and speeds giving the field its erratic spatial and temporal pattern of eruptions, a type of behaviour that may have implications for the evolution of other continental volcanic fields worldwide.


Monogenetic Auckland volcanic field U-series 226Ra-excess Two-porosity model 



This study has been funded by the Earthquake Commission through the DEVORA (DEtermining Volcanic Risk in Auckland) project. Andrew Needham provided samples and access to the Rangitoto dataset. Christoph Beier thanks Coopers PA for inspiration and was funded by a Feodor Lynen fellowship of the Alexander von Humboldt-Foundation. Lucy McGee appreciates the Australian wildlife and medical services for their participation during the labwork of this project. Simon Turner is supported by an Australian Research Council Professorial Fellowship. This work used instrumentation funded by ARC LIEF and DEST Systemic Infrastructure Grants, Macquarie University and Industry. We thank K.Sims and an unfortunately anonymous constructive reviewer whose comments and suggestions substantially improved the manuscript. This is contribution 706 from the Australian Research Council National Key Centre for the Geochemical Evolution and Metallogeny of Continents.


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Copyright information

© Springer-Verlag 2011

Authors and Affiliations

  • Lucy E. McGee
    • 1
    Email author
  • Christoph Beier
    • 2
    • 3
  • Ian E. M. Smith
    • 1
  • Simon P. Turner
    • 2
  1. 1.School of EnvironmentUniversity of AucklandAucklandNew Zealand
  2. 2.GEMOC, Department of Earth and Planetary SciencesMacquarie UniversitySydneyAustralia
  3. 3.GeoZentrum NordbayernUniversität Erlangen-NürnbergErlangenGermany

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