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Primitive shoshonites from Fiji: Mineralogy, melt inclusions and geochemistry

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Leslie, RA (2004) Primitive shoshonites from Fiji: Mineralogy, melt inclusions and geochemistry. PhD thesis, University of Tasmania.

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Abstract

Fundamental issues regarding the origin and evolution of primitive shoshonitic
magmas are addressed using mineralogical, melt inclusion and geochemical data from
Fijian shoshonites. Melt inclusions in olivine phenocrysts from primitive Fijian
shoshonites are used to critically assess the issues relating to melt sampling by high Fo
olivine phenocrysts and address to what degree trapped melt compositions reflect
the larger volume magmatic system as a whole.
Shoshonitic magmas erupted in Fiji during the Pliocene (5-3Ma) from 11 main
volcanic centres along three broad ENE and NNW trending lineaments. The most
mafic shoshonitic lavas (absarokites) range from 8.4-15.2wt% MgO and are highly
porphyritic, containing phenocrysts of olivine (to F093.2) and clinopyroxene (to Mg#
93.3).
The vast majority of melt inclusions in high-Fo-olivine phenocrysts from Fijian
shoshonites have anomalous major element composition, mainly characterised by
high CaO contents and high-CaO/Al203. Anomalous melt inclusion compositions are
interpreted to reflect localised, grain-scale dissolution-reaction-mixing processes
within the magmatic plumbing system where hot, primitive magma comes in contact
with wall-rocks and/or pre-existing semi-solidified mush zones. Injection of hot
primitive melt causes partial dissolution of the mush-zone phases, which are not in
equilibrium with the primitive melt and mixing of the reaction products with the
primitive magma. Rapid cooling at the margins of the magma body induces fast
crystallisation and efficient trapping of numerous and large melt inclusions, with
anomalous major element composition. Populations of melt inclusions in high-Fo
olivine phenocrysts from Fijian shoshonites, and arguably many other subduction related
suites, are naturally biased toward anomalous compositions.

Item Type: Thesis (PhD)
Copyright Information:

Copyright 2004 the Author - The University is continuing to endeavour to trace the copyright
owner(s) and in the meantime this item has been reproduced here in good faith. We
would be pleased to hear from the copyright owner(s).

Date Deposited: 10 Jan 2013 00:11
Last Modified: 11 Mar 2016 05:54
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