TheFiji Islands are remnants of a once continuous
Solomon-Vanuatu-Fiji-Tonga volcanic arc (the Vitiaz arc), located at the
boundary between the Indian-Australian and Pacific plates. They record a history
of:
(i)
arc volcanism the late Eocene
to late Miocene,
(ii)
an
extensive magmatic response to arc
rifting (5.5 - 3.0 Ma) during which volcanism was dominated by
shoshonitic to tholeiitic basalts, and as rifting continued,
(iii)
a
transition from arc to intraplate volcanism.

The Hunter Ridge-Hunter Fracture Zone separates the
inactive South Fiji Basin
from the actively spreading North Fiji
Basin, and has been considered
to be a transform zone, almost parallel to the motion vector of the Australian
plate. However rocks dredged during the 1993 Alize' cruise, from along
the northern part of the Hunter Ridge are typical arc tholeiites, and therefore
demand that subduction has occurred beneath the Hunter Ridge sometime during
the last ~5 Ma.
The
Kadavu Island group, south westernmost Fiji, lies at the north-eastern end of the Hunter
Ridge-Hunter Fracture Zone, and possibly results from oblique subduction of
South Fiji Basin
oceanic crust, which may have accompanied the anticlockwise rotation of
Fiji during opening of the
North
Fiji Basin. These islands consist dominantly of 3.4 - 0.5 Ma old
medium-high K andesites and dacites, with many traits typical of subduction-related volcanics,
ie predominance of intermediate members (55 - 65%
SiO2) no Fe enrichment, TiO2<1%, and negative nb-ta>
anomalies. These features contrast strongly with volcanics from elsewhere in
Fiji around this time, where volcanic
centres produced Om-like intraplate olivine basalts. The Kadavu Islands become gradually younger to the
southwest, and show a significant decrease in K2O with time. The north
easternmost islands (Astrolabe islands) containing the oldest rocks (~3 Ma)
have shoshonitic affinities and are considered to be of similar age and
composition to the youngest shoshonites found elsewhere in Fiji.