Melting processes in small basaltic systems: the Auckland Volcanic Field, New Zealand

Abstract

Geochemical analyses of melts erupted in small basaltic volcanic fields have the power to reveal particulars of their magmatic system from the inception of melting, to the processes that affect magmas on their ascent. The Auckland Volcanic Field (AVF) of northern New Zealand is a Quaternary intraplate basaltic volcanic field of c. 50 eruptive centres located over an area c. 360km2. The field has been extremely well-sampled over the past few decades thus providing an excellent basis from which to study the inner workings of a small volume monogenetic system. New analyses of rocks have been added to the existing geochemical dataset of the AVF, including Sr-Nd-Pb isotopes and a subset of U-Th-Ra isotopes. In addition to a whole-field study, two case studies of well-exposed volcanic centres in the AVF are presented: Rangitoto and Motukorea. Major and trace element and isotopic whole rock data are used to construct a melting model for the AVF. Furthermore, intricacies of the magmatic system beneath monogenetic volcanoes are revealed; these relate to the nature and behaviour of the source, the reasons for large compositional variability in such volcanic fields, the ways in which monogenetic eruptions may progress volcanologically and chemically, and the mode by which melts move in such systems. The AVF operates by the presence of discrete melt batches which are each governed by their own sets of melting parameters; this is exemplified by the case of two chemically different eruptions at Rangitoto. In all cases, melting began in the garnet-bearing asthenosphere (containing discontinuous regions of recycled material in the form of eclogite veins), and the extent of asthenospheric melting is hypothesised to control the size of volcanic eruption. In each melting event, magmas are variably mixed with a cryptically subduction-metasomatised lithospheric source. The three source components involved are observed in Pb-isotopic space. The geochemistry of eruptive products is also used to deduce the development of channelized flow in the mantle, and the progressive depletion of the source and wallrock. The geochemical complexities modelled suggest that ‘monogenetic’ is a term that does not adequately describe small basaltic systems such as the AVF.