Petrology and geochemistry of the central North Fiji Basin spreading centre (Southwest Pacific) between 16°S and 22°S

Abstract

The North Fiji Basin (NFB) is a 12 m.y. old back-arc basin that has a complex multi-stage history. The presently active spreading system can be divided into four segments between 16°S and 22°S, which from north to south trend N160, N15 and N-S (the fourth segment is the N-S trending segment located near 174E). The main N-S segment is morphologically similar to other medium-rate oceanic ridges, whereas the other segments have rougher morphologies which have been severely disturbed by a triple junction at 16°45′S and several instability features such as overlapping spreading centres (OSCs) and propagating rifts. The spreading rate seems to diminish from 7.8 cm/yr near 20°S to 4.6 cm/yr near 18°S. Mineralogical, pertological and geochemical data were obtained on 24 new stations located along all the four segments. The petrogenesis of the basalts collected is essentially controlled by low-pressure crystal fractionation of plagioclase±olivine±clinopyroxene (plagioclase>olivine>clinopyroxene) with 52% of the NFB basalts reaching the four-phase cotectic. Locally, some magma mixing occurs, but this is limited to magma batches of closely related composition, as might be expected to occur inside a magma reservoir. The N-S segment, which, since 3 m.y., is the only steady-state segment, is also petrologically and geochemically very comparable to other medium-rate oceanic spreading centres, producing moderately evolved LILE and LREE-depleted N-MORB. In contrast, the three other segments produce basalts of much more variable petrology and geochemistry characterized by LILE and slightly LREE-enriched magmas cf back-arc basin basalt (BABB) affinity (but not as enriched as, for example, the Mariana BABB); MORB is, however, also found on the N160, N15 and 174E segments. Diagrams using Ba, Rb, K/P and (K/Ti) N (Normalized to the chondrites) plotted against latitude clearly show along-strike variations. Beneath the recently formed segments, the mantle source is heterogeneous, and locally has some BABB affinities, whereas beneath the more steady-state N-S segment the magma source is more homogeneous, being generally depleted in LILE and REE as is the case for classical N-MORB mantle sources. Simple evolution from an early stage of BABB production to MORB described in the Lau basin and proposed for the NFB does not seem to occur. Present-day activity still produces large amounts of BABB along the less-stable and more recently created segments, and MORB was produced in the earlier stages of the development of the NFB.