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120 OCEANIA (INCLUDING FIJI, PNG AND SOLOMONS) Kadavu Islands The Kadavu islands, south of Viti Levu, are a volcanic island arc related to subduction at the Kadavu Trench Ages range from 3.2 Ma to Holocene Compositions range from basaltic to dacitic and are generally potassic Lau Islands The islands on the Lau Ridge (Figure 1) formed in several periods of volcanic activity, probably beginning in the Eocene The oldest exposed rocks, on Yacata island, are Middle Miocene (14 Ma) Further south, Late Oligocene rocks have been recovered by dredging The exposed volcanic rocks are of three ages: Middle and Late Miocene, Middle Pliocene, and Late Pliocene to Pleistocene Eruptive rocks in the two earlier episodes were tholeiitic to calcalkaline, and in the third episode were alkali basalt Limestone was deposited in the Middle to Late Miocene, Late Miocene to Early Pliocene, and Late Pliocene to Quaternary Quaternary Volcanic Activity Quaternary volcanic activity of mainly alkali basalt composition formed the island of Taveuni immediately south-east of Vanua Levu (where the most recent dated eruption was at 340 Ỉ 70 years), and the island of Rotuma, 650 km north of Viti Levu Many small centres of Quaternary alkali-basalt volcanism are scattered around the archipelago in the north-east, the north-west, in Lomaiviti, adjoining Kadavu and in the Lau Group, but none is known on the Viti Levu platform High-K andesite to dacite erupted at the western end of Kadavu island Economic Geology Gold has been mined at Vatukoula on Viti Levu since 1933 and has been mined at other locations on Viti Levu and at Mt Kasi on Vanua Levu at different times Manganese ore was mined from 1950 to 1971, most production being from Viti Levu in the 1950s Magnetite was mined at Tuveriki in southwest Viti Levu from 1957 to 1963 Deposits of gold, polymetallic sulphides, and disseminated deposits are being explored, notably the Waisoi and other porphyry–copper deposits at Namosi on Viti Levu Bauxite deposits occur in western Vanua Levu, mainly around Wainunu, and some islands of the Lau Group have deposits of phosphatic clay Plate Tectonic Setting The early volcanic arcs that now form the basement of the Fiji platform developed above subducted Pacific plate in the Eocene, Oligocene, and Miocene Initially the Yavuna arc and Lau–Tonga Ridge probably were parallel to, and close to, New Caledonia, but were moved to the north-east by the opening of the South Fiji Basin (36–25 Ma) By the Middle Miocene, the Lau–Tonga arc, Fiji, and Vanuatu arcs formed a single east-facing arc above subducted Pacific plate This simple arc, sometimes referred to as the Vitiaz Arc, was disrupted in the Late Miocene with the onset of back-arc extension in the North Fiji Basin This extension caused the clockwise rotation of Vanuatu and the anticlockwise rotation of the Fiji platform In the Pliocene, spreading extended east and then south, causing separation of the Lau and Tonga ridges and the oblique subduction of South Fiji Basin lithosphere at the Kadavu Trench Rotation of the Fiji platform caused local compression and extension As Fiji moved away from sites of active subduction, volcanism became mainly alkali basalt in composition Tonga The Tongan archipelago is a north–north-easttrending double-island chain that straddles the northern end of the active convergent plate boundary that separates the Pacific and Australian plates A welldefined Benioff zone dips westward at 43–45 beneath Tonga and extends to a depth of 700 km The larger islands of the archipelago form the eastern chain, 130–150 km west of the Tonga–Kermadec Trench These are low-lying islands made up mainly of coraline limestone On Eua, the southernmost of this eastern chain, the stratigraphic sequence has been described as consisting of coraline limestone, Miocene tuffs and tuffaceous limestones, Late Eocene tuffaceous marlstone (thickness, 60–80 m), Late Eocene foraminiferal and algal limestone, and Middle Eocene (46 Ma) basaltic and intermediate lava, volcanic breccia, and tuffs cut by dykes Although the Eocene and Miocene rocks are not exposed elsewhere in Tonga, seismic profiles indicate that they form the basement of the Tonga Ridge The crust beneath the ridge is 12–18 km thick Development of the Tonga Trench and uplift of the frontal arc occurred in Middle to Late Oligocene The Miocene tuffs are thought to represent the initial stages of arc volcanism The islands of the western chain, which lie 150–200 km west of the trench, comprise the Tonga or Tofua volcanic arc The volcanoes have a history of intense modern activity, with more than 35 eruptions occurring during the past 200 years, including both subaerial and submarine events The volcanic products are mainly basalt and basaltic andesite with subordinate dacite, as is to be expected from an intraoceanic arc system West of the Tonga or Tofua arc is
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