The Origin of Geochemical Variations in Mariana Lavas: A General Model for Petrogenesis in Intra-Oceanic Island Arcs?

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Abstract
Major and trace element data are presented for a suite of lavas and gabbroic xenoliths from the northern Mariana islands in the west Pacific. Fractional crystallization of a gabbroic mineral assemblage, similar to that observed in the xenoliths, appears to be the predominant control on major and trace element variation within the lavas. Mixing calculations indicate that this ‘extract’ has an average composition of PLAG:CPX:MAG:OLIV =60:25:10:5. Amphibole is not thought to be an important component of the fractionating assemblage. Consideration of REE data, in particular the pronounced negative correlation between Eu/Eu* and silica, allows the identification of a ‘parental’ magma composition, representing the most primitive lavas erupted. These are basaltic andesite in composition with approximately 53 wt.% silica. More evolved lavas can be produced by the fractionation of a gabbroic assemblage, as noted above, while simultaneous cumulus enrichment processes may produce apparently less evolved, more basic compositions. Mineral medal data for the lavas provide corroborative evidence for the operation of this process, which may be common to other intra-oceanic arc settings. Fractional crystallization appears to be selective, with titanomagnetite being removed from the magmatic system more efficiently than plagioclase, suggesting a control by differential crystal settling. Comparison of the Mariana ‘parent’ with picritic primary magmas from the Solomons and Vanuatu arcs shows that the former can be derived from the latter by simple fractional crystallization of olivine and clinopyroxene, which also readily accounts for the low Ni and Cr concentrations observed in the suite. In addition the Mariana ‘parent’ appears to have been influenced by phase relations involving a reduced plagioclase field, possibly under conditions of moderate Pload and aH2O. The model has much in common with those currently in favour for the generation of continental flood basalts, OIBs and some MORBs in that primary magmas are picritic and the crust acts as a ‘density filter’ which prevents the ascent of primary magmas and results in volcanic products dominated by low pressure fractionates.

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