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

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Verbeeten, A (1996) Petrology, geochemistry and tectonic implications of magmatism along the northern hunter ridge and Kadavu Island group, Fiji. PhD thesis, University of Tasmania.

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Abstract

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.

Item Type: Thesis (PhD)
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Date Deposited: 17 Feb 2013 22:17
Last Modified: 15 Sep 2017 01:06
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