Petrology, geochemistry and tectonic implications of magmatism along the northern hunter ridge and Kadavu Island group, Fiji

The submarine Hunter Ridge separates the inactive South Fiji Basin from the actively spreading North Fiji Basin, and is a newly recognised intra-oceanic arc in the SW Pacific. Magmatic activity along the Hunter Ridge is linked to the northward subduction of the crust of the Oligocene (26.0-32.5Ma) South Fiji back-arc basin. Subduction began in response to establishment of an E-W orientated spreading ridge at -7Ma across the North Fiji Basin, and the accompanying anticlockwise rotation of the Fiji Platform. Rocks suites dredged along the northern part of the Hunter Ridge include basalts to dacites transitional between high-Ca boninites and typical arc tholeiites, and calc-alkaline basalts to rhyolites. Mineral compositions (e.g. Cr-spinel with Cr#>70), coupled with whole rock high CaO/ Ah03 (0.91-1.13) and low abundances of HFSE (0.37-0.54% Ti02; 0.27- 0.S6ppm Nb; 0.02-0.05ppm Ta), Y (1l-15ppm) and HREE of the basalts relative to N-MORB show that both the arc tholeiite and calc-alkaline basalts are derived from sources more refractory than the N-MORB source, probably peridotite residual after production of North Fiji Basin oceanic crust. The enrichment in LILE and LREE of the Hunter Ridge rocks reflects addition to this refractory source of a slab-derived fluid for the arc tholeiitic suite and a slab melt component for the calc-alkaline suite. The Kadavu Island Group, in southwesternmost part of the Fiji archipelago, is the northeastern exposed end of the Hunter Ridge and can be divided into four geochemically distinct magmatic groups. The Astrolabe Group shoshonites (-3.4Ma) have major and trace element and isotopic compositions very similar to other shoshonites in Fiji. The low HFSE abundances (-0.64%Ti02, O.llppm Ta, 2.0ppm Nb), high CaO/ Al203 values (0.SS-1.1) and high Cr# of Cr-spinel (CrLS5) of the mafic lavas (absarokites) of the shoshonite suite indicate a particularly refractory peridotite source for the Astrolabe lavas. The strong LILE and LREE enrichments of the Astrolabe shoshonites are extreme variants of the same enrichment shown by primitive arc tholeiites on the Hunter Ridge and in the Vanuatu arc, and are attributed to relatively low degrees of partial melting of a mantle source similarly affected by ingress of slabderived hydrous fluids. The mantle metasomatism responsible for producing the source peridotite of the Astrolabe suite magmas may have ,,(curred during Oligocene to Miocene subduction associated with the Viti.l!' em: system. Emplacement of the Fijian shoshonites, including the :\\~lrLlbbe Croup, is considered to be related to lithospheric extension in the early-mid Tertiary Fijian arc, in response to reorganisation of spreading systems in the North Fiji Basin and initiation of spreading in the adjacent Lau Basin. Pleistocene to Recent (2.9-0.4SMa) volcanism on Kadavu is represented by the Western Kadavu and Central/Eastern/Ono Croups, .mel records the effects of initiation of subduction of the South Fiji Basin Cl'ust beneath Fijian arc lithosphere . The dominant rock types are medium to high-K adakitic andesites and dacites. Low abundances of Y (15.6-19.6ppm), high Sr contents (553-1667ppm), high Sr/Y (79.2-SS.9), and strongly fractionated REE patterns (La/YbN =12-25), are consistent with an origin involving partial melting of subducted basaltic oceanic crust consisting mainly of garnet and clinopyroxene (eclogite). Furthermore, Sr, Nd and Pb isotopic analyses plot within the range of Pacific MORB, consistent with derivation of these magmas by partial melting of subducted MORB, with no pelagic sediment involvement. Lavas of the Ngaloa Croup form a volumetrically small part of the Kadavu Island Croup and consist of unusual basalts and basaltic andesites with high Na20 (2.7-5.5%), Ti02 (1.4-1.7%), and Sr (2055-2957ppm) and low FeO' (5.S-6.S%) and relatively high Nb contents (S-16ppm) for suprasubduction basalts. They are temporally and spatially associated with the Western Kadavu adakitic andesites and dacites, but major and trace element considerations rule out any direct genetic link between them through differentiation. However, Ngaloa Croup lavas also have specific geochemical features (Sr/Y=93-205, La/YbN=19-29 and MORB-like isotopic compositions) similar to the Western Kadavu and Central/Eastern/Ono Croup adakites, indicating involvement in their petrogenesis of a component formed by partial melting of the subducted oceanic crust. Their high MgO (5.1-7.9%), and Ni (up to 200ppm) contents and primitive phenocryst compositions (e.g. F089-91) however, preclude their derivation solely from the partial melting of the subducted crust. Trace element considerations suggest that the Ngaloa Croup basalts were prod uced by partial melting of mantle wedge peridotite that had interacted with and been strongly modified by slab melts probably similar to the Western Kadavu adakitic lavas. Lavas compositionally similar to the Ngaloa Croup volcanics in some other arcs also occurring in close association with adakitic andesites, have been termed Nb-enriched arc basalts (NEAB) and are also thought to have formed by melting of a slab-melt metasomatised mantle wedge. Partial melting of subducted oceanic crust in eclogite facies (-50- 80km depth) is thus implicated in the petrogenesis of the Ngaloa Group and the Central/Eastern/Ono and Western Kadavu adakitic lavas. As the subducted slab is too cool to partially melt at the amphibolite-eclogite transition beneath normal forearc regions of oceanic arcs, a mechanism to elevate the isotherms beneath the northern end of the Hunter Ridge is required. The subducting South Fiji Basin oceanic crust is too old (26.0- 32.5Ma) and cold to provide the necessary heat. An alternative heat source is ascending asthenospheric mantle beneath the northwestern end of the Lau backarc basin. This convecting MORB-source mantle is hypothesised to move via the mantle window at the end of the subducting plate to abut subducted South Fiji Basin oceanic crust. Conductive heat transfer from this hot asthenospheric mantle enabled partial melting of the subducted South Fiji Basin slab, and partial melting of the same asthenosphere mantle is considered to be responsible for generation of the unusual OlB-type basalts that occur directly above the slab window between Viti Levu and Vanua Levu, Fiji.