Assimilation of ocean crust by hawaiitic and mugearitic magmas: an examplefrom Eiao (Marquesas)

Three holes were drilled in the northern (Dominique), central (Sophie) and southern (Naore) parts of the volcanic shield of Eiao island in the Marques as archipelago, French Polynesia. These were continuously cored with an exc ellent recovery and reached depths of 500 m (Sophie) and 800 m (Dominique a nd Naore). K-Ar dating indicates that the volcanic sequence penetrated was emplaced between 5.52 +/- 0.05 Ma and 4.95 +/- 0.04 Ma. Petrographic types include tholeiites, alkali basalts, picrobasalts, hawaiites, mugearites and trachytes. Intermediate lavas (hawaiites and mugearites) are fairly common . They occur within stratigraphic units denoted D1, D2, D3, S1, S2, S3, N1, and N2, respectively (D: Dominique; S: Sophie; N: Naore). The D3, S1 and N 1 hawaiites and mugearites appear to be derived from fractional crystalliza tion of parental alkali basalt magmas. The other groups are characterized b y variable enrichments in Sr (D1, D2 and S3), P2O5 (D2), Th (S2 and N2) and light rare earth elements (D2) or by depletion in heavy rare earth element s (S2 and N2). We suggest that these trace element characteristics reflect open system differentiation processes which occurred during the building of Eiao volcanic shield, Two stages of assimilation coupled with fractional c rystallization (AFC) are proposed. The calculated compositions of the two c ontaminants suggest that they were leucocratic materials located at differe nt levels of the oceanic crust. Finally, a water-induced assimilation proce ss is invoked to explain the origin of the S3 hawaiites: these lavas are en riched in Al2O3, Na2O and Sr, inferred to reflect plagioclase breakdown wit hin the Eiao plumbing system. (C) 1999 Elsevier Science B.V. All rights res erved.