NORTH FIJI BASIN BASALTS AND THEIR MAGMA SOURCES .1. INCOMPATIBLE ELEMENT CONSTRAINTS

A systematic compilation of the geochemical data collected during the STARMER programme is presented, in addition to all published data gathered along the North Fiji Basin (NFB) spreading system. From the 194 samples selected, the geochemical variations observed along the different spreading segments can be related to uni- or multi-modal origin of distinctive magma sources. These variations are interpreted as directly linked to the geodynamical features which surround the NFB. In the southern NFB, on the N174-degrees-E segment, N-MORB are present, but the low rate (or dead) subduction along the Hunter ridge still marks its influence as several basalts show a significant subduction-related contamination with negative Nb anomaly and high LOI. Several others are marked by a weak E-MORB source contribution, that may be related to subducted-OIB seamounts from the South Fiji Basin. In the central NFB, along the N S segment which represents the most active and morphologically regular spreading ridge of the NFB, the magma source produces only N-MORB, with depleted LILE, HFSE, and LREE patterns, except in its northernmost part where magmatic signatures similar to that of the N15-degrees segment appear. Along the N15-degrees segment, three distinctive sources coexist and produce a wide range of geochemical signatures on the basalts collected; (1) a N-MORB source signature; (2) a transitional towards E-MORB source signature with a negative Nb anomaly indicating that some subduction related contamination still exists beneath the NFB (the basalts derived from this source might also have been called BABB previously); and (3) a source signature transitional towards E-MORB or OIB marking the start of an influence that increases towards the northern NFB as the Rotuma-Samoan hot spot lineament is approched. Around 17-degrees-S, on the Kaiyo station 4 site explored and sampled by the Nautile deep-sea submersible, as well as in the triple junction area, the same variability derived from three distinctive mantle sources is observed. Along the N 160-degrees segment, the three sources still coexist, but the influence of the E-MORB or OIB source (hot spot-related) increases, whereas the influence of the subduction-related source decreases. Along the Pandora-Rotuma ridge, the OIB-derived lava type is the only one present, the eventual contribution from other sources being completely diluted. Thus, the geochemistry of the NFB basalts is directly influenced by (1) the regional geodynamic environment, such as subduction zones and/or hot spot trails, (2) the geodynamical regime and stability of this type of back-arc system. The influence of the New Hebrides subduction, located some 500 km west of the active spreading ridge, is still perceptible, although weak, in the whole northern half of the NFB. However, this infuence is not directly linked to the presently active subduction, but originates rather from the partial melting of an upper mantle source that suffered subduction contamination during the clockwise rotation of the New Hebrides arc leading to the opening of the NFB during the past 12 Myr. In the northern NFB, many basalts result from the mixing of an N-MORB and a OIB source, similar to transitional alkalic lavas from oceanic intra-plate magmatism. This sources mixing increases northwards from 18-degrees-20'S to 12-degrees-S.