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

Major and trace element data are presented for a suite of lavasand gabbroic xenoliths from the northern Mariana islands inthe west Pacific. Fractional crystallization of a gabbroic mineral assemblage,similar to that observed in the xenoliths, appears to be thepredominant control on major and trace element variation withinthe 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 thefractionating assemblage. Consideration of REE data, in particular the pronounced negativecorrelation between Eu/Eu^* and silica, allows the identificationof a ‘parental’ magma composition, representingthe most primitive lavas erupted. These are basaltic andesitein composition with approximately 53 wt.% silica. More evolvedlavas can be produced by the fractionation of a gabbroic assemblage,as noted above, while simultaneous cumulus enrichment processesmay produce apparently less evolved, more basic compositions.Mineral medal data for the lavas provide corroborative evidencefor the operation of this process, which may be common to otherintra-oceanic arc settings. Fractional crystallization appears to be selective, with titanomagnetitebeing removed from the magmatic system more efficiently thanplagioclase, suggesting a control by differential crystal settling. Comparison of the Mariana ‘parent’ with picriticprimary magmas from the Solomons and Vanuatu arcs shows thatthe former can be derived from the latter by simple fractionalcrystallization of olivine and clinopyroxene, which also readilyaccounts for the low Ni and Cr concentrations observed in thesuite. In addition the Mariana ‘parent’ appearsto have been influenced by phase relations involving a reducedplagioclase field, possibly under conditions of moderate P_loadand a_H2O. The model has much in common with those currently in favourfor the generation of continental flood basalts, OIBs and someMORBs in that primary magmas are picritic and the crust actsas a ‘density filter’ which prevents the ascentof primary magmas and results in volcanic products dominatedby low pressure fractionates.