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Title:
The `Daly Gap' and implications for magma differentiation in composite shield volcanoes: A case study from Akaroa Volcano, New Zealand
Authors:
Hartung, E.; Kennedy, B.; Deering, C. D.; Trent, A.; Gane, J.; Turnbull, R. E.; Brown, S.
Affiliation:
AA(Geological Sciences, University of Canterbury, Christchurch, New Zealand ), AB(Geological Sciences, University of Canterbury, Christchurch, New Zealand ), AC(Earth and Space Sciences, University of Washington, Seattle, WA, USA ), AD(Geological Sciences, University of Canterbury, Christchurch, New Zealand ), AE(Geological Sciences, University of Canterbury, Christchurch, New Zealand ), AF(Geological Sciences, University of Canterbury, Christchurch, New Zealand ), AG(Geological Sciences, University of Canterbury, Christchurch, New Zealand )
Publication:
American Geophysical Union, Fall Meeting 2010, abstract #V52B-02
Publication Date:
12/2010
Origin:
AGU
Keywords:
[3610] MINERALOGY AND PETROLOGY / Geochemical modeling, [3618] MINERALOGY AND PETROLOGY / Magma chamber processes, [3620] MINERALOGY AND PETROLOGY / Mineral and crystal chemistry, [3640] MINERALOGY AND PETROLOGY / Igneous petrology
Bibliographic Code:
2010AGUFM.V52B..02H

Abstract

The origin of compositional gaps in volcanic deposits that are found worldwide, and in a range of different tectonic settings, has challenged petrologists since Daly's first observations at mid-ocean ridges. In the shield-forming Akaroa Volcano (9.6 - 8.6 Ma) of Banks Peninsula, New Zealand, a dramatic compositional gap exists in both eruptive and co-genetic intrusive products between basalt and trachyte, and between gabbro and syenite respectively. Rock compositions display mildly alkaline affinities ranging from picritic basalt, olivine alkali basalt and hawaiite to trachyte. Intermediate mugearite and benmoreite (50 - 60 wt. % SiO2) are not exposed or absent. Equivalent plutonic diorite, monzodiorite and monzonite (45 - 65 wt. % SiO2) are also absent. Previously, the formation of the more evolved trachyte (and syenite) has been ascribed to crustal melting. However, our analysis of new major and trace element data from minerals and bulk-rocks of this hy-normative intraplate alkalic suite provide evidence for an alternative model based on crystal fractionation and punctuated melt extraction. Observed major and trace element trends in bulk-rocks can be reproduced by Rayleigh fractional crystallization from dry melts (< 0.5 wt. % H2O) at oxygen fugacities one unit below the quartz-fayalite-magnetite-buffer (QFM -1). The results of our MELTS models are in agreement with experimental studies, and indicate a fractionation-generated compositional gap, where trachytic liquid (62 - 64 wt. % SiO2) has been extracted after the melt has reached a crystallinity of 65 - 70 %. The fractionated assemblage of clinopyroxene, olivine, plagioclase, magnetite and apatite is left in a mafic cumulate residue (44 - 46 wt. % SiO2). Calculated values of specific trace and minor elements (Sr, Cr, P) from a theoretical cumulate are consistent with measured concentrations from cumulate xenoliths. Compositional trends from individual mineral analysis are also supportive of fractional crystallization, but illustrate a disrupted liquid-line-of-decent for each mineral phase. Olivine compositions progressively decrease in Mg concentration (Fo83-42) in basaltic melts and show high Fe concentration in trachytic melts (Fo5-10). Clinopyroxene analyses also display higher Fe/Mg ratios in more evolved rocks. Ternary feldspar compositions shift from plagioclase (An84-56) in basalt to alkali feldspar (Or8-65Ab53-33An39-2) in trachyte, but also lack the intermediate compositions. On the other hand, analysis of mafic cumulate xenoliths reflect more evolved mineral compositions towards the rim than volcanic equivalents and complete observed fractionation trends. In summary, our results indicate that these compositional gaps formed from punctuated melt extraction within an optimal crystal fraction window (60 - 70 % crystallinity).
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