The inception and progression of melting in a monogenetic eruption: Motukorea Volcano, the Auckland Volcanic Field, New Zealand Lucy E. McGee a, ⁎, Marc-Alban Millet b, 1 , Ian E.M. Smith a , Károly Németh c , Jan M. Lindsay a a School of Environment, The University of Auckland, Private Bag 92019, Auckland, New Zealand b School of Geography, Environment and Earth Sciences, Victoria University of Wellington, PO Box 600, Wellington, New Zealand c Volcanic Risk Solutions, Massey University, PO Box 11222, Palmerston North, New Zealand abstract article info Article history: Received 23 March 2012 Accepted 26 September 2012 Available online 2 October 2012 Keywords: Auckland Volcanic Field Monogenetic Alkalic basalt Nephelinite Pb isotopes Compositional variation through basaltic monogenetic eruptive sequences provides a unique view into the processes and source heterogeneity of small-scale magmatic systems. A well-exposed, continuous sequence on Motukorea volcano in the Auckland Volcanic Field, New Zealand, consists of an early tuff ring, scoriaceous deposits and late lava flows which allow the evolution of the eruption to be studied at very high resolution. The deposits show a spectrum of basaltic compositions from Mg# 60 nephelinite (early tuff ring) to Mg# 70 alkalic basalt (lava). Within the deposits of each main eruptive phase (i.e. tuff, scoria and lava) very little var- iation is observed in major element chemistry, suggesting that fractional crystallisation has a limited effect. Systematic changes in trace element chemistry, however, are significant through the sequence. The major and trace element features observed through the sequence are inferred to be primarily due to the mixing of two magma batches, with a two-fold increase in the degree of melting between these. Variation in Pb-isotopic compositions up-sequence indicates subtle changes in mantle source with samples representing the start of the eruption displaying higher 207 Pb/ 204 Pb than the latter parts of the eruption. This chemical change coincides with a switch in the mode of eruption, with the arrival at the surface of magmas produced by larger degrees of partial melting resulting in the beginning of a more effusive eruption phase. The silica-undersaturated, high total alkali, low Al 2 O 3 and higher 207 Pb/ 204 Pb nature of the samples from the tuff units suggests that these samples were produced by melting of relatively young eclogite domains. The lower 207 Pb/ 204 Pb, higher silica, lower total alkali nature of the samples from the scoria and lava reflects the exhaustion of these domains and the resultant melting of the surrounding garnet-peridotite matrix. This detailed study shows that the petrogenesis of small volcanic centres may be far more complex than their physical volcanology suggests. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Monogenetic eruptions of basalt where there has been only limited modification in magma chambers or by crustal assimilation provide valuable information regarding the deeper processes of magma genera- tion and extraction. The term ‘monogenetic’ describes small volume, typically basaltic volcanoes that have been built in a continuous erup- tion sequence within a relatively short time span (on the scale of days up to several decades) (Connor and Conway, 2000; Kienle et al., 1980; Valentine and Gregg, 2008). An assumption is that monogenetic volca- noes are built by the eruption of a single compositionally discrete batch of magma. Recent detailed work on individual centres in volcanic fields, however, shows that they are more often the product of relatively complex magmatic processes, commonly involving more than one magma batch within a single eruptive episode (e.g. Bradshaw and Smith, 1994; Brenna et al., 2010, 2011; Needham et al., 2011; Valentine and Gregg, 2008; Valentine and Hirano, 2010; Valentine and Keating, 2007). Detailed studies of volcanic sequences have investigated systematic relationships of chemical composition to stratigraphic posi- tion (and therefore to time in an eruption sequence) and found that they are far from simple. For example, Strong and Wolff (2003) docu- mented the compositional changes through several monogenetic se- quences in the Southern Cascades, USA, and described differences both within scoria deposits, and between the scoria and lavas of the same centre. This was attributed to the involvement of several distinct sources over the course of one eruption. This study (in addition to others, e.g. Blondes et al., 2008; Brenna et al., 2010; Cebriá et al., 2011; Garcia et al., 2000; Reiners, 2002; Smith et al., 2008) demonstrates the significance of monogenetic volcanism in sampling heterogeneous source regions, and also the complexity of simple sequences. The observation that chemical compositions, melting processes and source characteristics can be highly variable from one eruptive phase to Lithos 155 (2012) 360–374 ⁎ Corresponding author. Tel.: +64 9 373 7599x88824; fax: +64 9 373 7435. E-mail addresses: lucy.e.mcgee@gmail.com, l.mcgee@auckland.ac.nz (L.E. McGee), millet@uchicago.edu (M.-A. Millet), ie.smith@auckland.ac.nz (I.E.M. Smith), K.Nemeth@massey.ac.nz (K. Németh), j.lindsay@auckland.ac.nz (J.M. Lindsay). 1 Now at: Origins Laboratory, Department of Geophysical Sciences, University of Chicago and Enrico Fermi Institute, 5734 South Ellis Avenue, Chicago, IL 60637, USA. 0024-4937/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.lithos.2012.09.012 Contents lists available at SciVerse ScienceDirect Lithos journal homepage: www.elsevier.com/locate/lithos