Encyclopedia of geology, five volume set, volume 1 5 (encyclopedia of geology series) ( PDFDrive ) 1961

118 OCEANIA (INCLUDING FIJI, PNG AND SOLOMONS)

century and still comprise the largest single reserve
of nickel worldwide. Chromite has been mined directly from the ultramafic rock. Manganese deposits are
associated with the Poya terrane basalts. Copper, lead,
and zinc deposits have been mined from volcanosedimentary horizons in the metamorphic rocks of
the Diahot valley in northern New Caledonia. Goldbearing quartz veins have been mined in the Diahot
valley and gold and molybdenum deposits are
associated with the Miocene granodiorite intrusions.

Fiji

basaltic and andesitic volcanic rocks and associated
sediments (radiometric ages, 5.6–4 Ma). These cover
much of the northern half of the island. The centre of
the island was elevated at about 4 Ma. In the southeast, sediments were deposited in relatively deep water
during the latest Miocene and through the Early Pliocene, the Messinian sea-level low being represented
by a subaerially eroded coral reef. Shallow-water
sediments were laid down in the latest Pliocene. In
the south-west, sandstone and siltstone, overlain by
limestone, were deposited in the Early Pliocene.
Vanua Levu

The Fiji archipelago (Figure 7) comprises 800 islands,
most of which lie between 16 and 19 south,
177 east, and 178 west. The Lau Group of islands
in the east extends to 21 south (Figure 1). The largest
islands are Viti Levu and Vanua Levu. The island of
Rotuma, 650 km north of Viti Levu, is also a part of
the Republic of Fiji. The islands are mainly volcanic
and of island-arc origin, of basaltic, basaltic andesite,

andesitic, and dacitic composition, and tholeiitic,
calc-alkaline, or shoshonitic affinities. The younger
volcanic rocks are mostly more alkaline and mainly
belong to the alkali–basalt suite, and are intraplate
rather than subduction related.

Vanua Levu comprises Late Miocene to Pliocene volcanic rocks (7–2.8 Ma). Volcanic activity appears to
have progressed with time from east–north-east to
west–south-west. The volcanic rocks are generally
tholeiitic to calc-alkaline and include basalt, andesite,
and some dacite, including a strong development of
dacite and rhyolite in the north-east. Boninitic basalts
were erupted on Cikobia to the north-east. Vanua
Levu was elevated at about 3.5 Ma and Seatura Volcano began to form in western Vanua Levu not later
than 3.3 Ma. Clastic sediments were deposited along
the south-east coast.

Viti Levu

Yasawa Group

The oldest rocks in the Fiji archipelago are exposed
on Viti Levu. These are the volcanic and plutonic
rocks of the Late Eocene to Early Oligocene Yavuna
arc, and of the Late Oligocene to Middle Miocene
(and possibly Late Miocene) Wainimala arc. The two
volcanic suites are separated by an unconformity. The
Yavuna arc rocks are exposed in western Viti Levu
and include basaltic pillow lavas and volcaniclastic
rocks, dacite, tonalite, and limestones of both Eocene

and Oligocene ages. The basalts are geochemically
primitive to arc like.
The Wainimala arc rocks are exposed throughout
southern and central Viti Levu and on the islands of
Malolo and Waya, immediately to the west. The
rocks include mainly dacites in the centre of the island
and basalts overlain by pelagic limestone and dacitic
and andesitic lavas and volcaniclastics in the southwest. On Malolo and Waya, there are basaltic pillow
lavas and breccia, and swarms of basalt dykes, indicating the local arc axis. Gabbro and tonalite of
the Colo Plutonic Suite are thought to be part of the
Wainimala arc activity and have mid- to Late Miocene radiometric ages (12–7 Ma). They were exposed
to erosion in the Late Miocene. The Colo, Yavuna
and Wainimala rocks show the effects of some deformation and weak metamorphism. They are unconformably overlain by latest Miocene to Early Pliocene

The Yasawa and other islands north-west of Viti
Levu, other than those of the Wainimala arc, were
formed by volcanic arc activity (Yasawa arc) in the
Late Miocene, from around 8 Ma. Early submarine
dacites and basalts were succeeded by subaerial
basalts in the central Yasawa islands, and by varied
andesites. Strata generally dip towards Viti Levu, but
at the north-eastern and southern ends of the chain
have been strongly deformed with the development of
overturned folds and thrust faults.
Islands Immediately South of Viti Levu

Vatulele, Beqa, and Yanuca (Serua) are Early Pliocene
volcanoes south of Viti Levu. On Vatulele, shoshonitic
rocks were erupted through limestone.
Islands of the Koro Sea

The islands of the Koro Sea (Lomaiviti and the Moala
Group) mostly formed by volcanic activity in the
Early Pliocene. Exceptions are the island of Koro,
which formed in the Late Pliocene, and a Late Pleistocene cone on Nairai. The rock types range from
tholeiitic through calc-alkaline to alkali basaltic and
shoshonitic, and are mostly basaltic, with some hornblende andesite, monzonite, olivine monzonite, and
tonalite. Olivine nodules occur in basalts on southern
Moala and the western Koro islands.