Detrital zircons and the magmatic history of Viti Levu, Fiji

Abstract

We integrate the existing detrital zircon data from multiple modern river sediment samples on Viti Levu, Fiji, with the most current available geological and topographic mapping of the respective river drainage basins to compare detrital populations with potential bedrock sources. The temporal and spatial variations in zircon geochemistry supplement what is known from igneous rocks and confirm the petrological differences between plutonic and volcanic rocks from the Eocene to early Oligocene (Yavuna age, >30 Ma), middle Oligocene to middle Miocene (Wainimala age, 30–12.5 Ma), late Miocene (Colo age, 12.5–6.5 Ma) and latest Miocene (Namosi age, 6.5–5 Ma). The >30 Ma Yavuna-age zircons are restricted to areas that drain the previously mapped Yavuna Group. The 30–12.5 Ma zircons are found across central Viti Levu from west to east, and the 30–15 Ma zircons have distinctively low U/Yb and high Dy/Yb ratios. They are the best radiometric evidence of widespread early to middle Miocene arc magmatism in Fiji that was relatively U-poor. Peak deconvolution of the Colo age zircons from individual basins suggests the following ages for undated or poorly dated plutons from central Viti Levu. The large Mavuvu pluton is probably composed of multiple intrusions in the 12–10 Ma range, the Waiqa pluton is probably ca 10 Ma, and the Noikoro pluton is probably ca 9 Ma. There are zircons from unknown plutonic or volcanic sources between 8 and 7 Ma in western Viti Levu that have distinct Eu/Eu* ratios. We attribute the highest U/Yb ratios in some Colo age zircons to crustal anatexis. Namosi-age zircons are abundant in the Medrausucu Group and can be found scattered across Viti Levu.

KEY POINTS

1. Detrital zircon samples have aided geological mapping of the tropical and poorly exposed terrain in Fiji and elucidated the magmatic history.

2. Detrital zircons indicate widespread early to middle Miocene arc magmatism across central Viti Levu.

3. Detrital zircon ages have provided further constraints on the ages of 12-9 Ma plutons in central Viti Levu.

4. Detrital zircon ages indicate that there is an unknown igneous source of 8-7 Ma zircons in western Viti Levu.

Keywords:

• Fiji
• Viti Levu
• Southwest Pacific
• detrital zircon
• arc plutonism
• geochemistry

Acknowledgements

We appreciated correspondence with Cluzel and Meffre about the data in their stimulating paper. We are also grateful to D. Clemens-Knot, C. Holm-Denoma, J. Gooley and three anonymous reviewers for their constructive comments. Any use of trade, firm or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Data availability statement

Data sharing is not applicable to this article, as no new data were created or analysed in this study.