Primitive shoshonites from Fiji: Mineralogy, melt inclusions and geochemistry

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.