The Lau Backarc Basin and neighboring North Fiji Backarc Basin are located about half-way between Hawaii and New Zealand. The Lau Basin is formed by westward subduction of Pacific plate into the Tonga trench. The active portion of the Samoan hotspot is located ∼100 km north of the northern terminus of the Tonga trench. The traces of the older portions of several hotspot tracks---including the Louisville, Rurutu and Rarotonga hotpots---intersect with the Tonga trench. This study explores how these different hotspots have contributed to the geochemical diversity in the Lau Basin as well as the adjacent North Fiji Basin.

Lavas from the Lau and North Fiji Basins show an array of geochemical signatures that suggest incorporation of enriched mantle source material often associated with intraplate hotspots. However, the origin and extent of these geochemical signatures remain uncertain. In this presentation I will integrate three geochemical datasets from lavas in the Lau and North Fiji Basins with tectonic plate reconstructions to better constrain the extent and processes through which enriched hotspot material is integrated into the region. The first set of results extends the western limit of enriched geochemical observations in the Basins and improves the resolution of North-South gradients in geochemical signatures across the backarc basins.

We find that the origin of the enriched signatures in both backarc Basins can be explained by a model in which Samoan-plume material and ambient depleted mantle undergo decompression melting during upwelling after transiting from beneath the thick Pacific lithosphere to the thin lithosphere of the backarc basins. The second study suggests that lavas in the northeast Lau Basin (NELB) have unique enriched geochemical signatures that are likely linked to the subduction of seamounts associated with the Cook-Austral volcanic lineament. Using a plate reconstruction model, we show that older portions of the traces of two Cook-Austral hotspots that contributed volcanism to the Cook-Austral volcanic lineament---the Rarotonga and Rurutu hotspots---were potentially subducted in the Tonga trench beneath the NELB. The geochemical signatures of the Rarotonga, Rurutu, and Samoan hotspots provide a compelling match to the extreme geochemical components observed in the new NELB lavas.

Finally, a third dataset examines a suite of submarine dredge samples that provide the first complete transect across the northern Lau and North Fiji Basins. This new dataset, combined with previous geochemical datasets on lavas from the region, provides unprecedented spatial resolution of geochemical patterns in the backarc basins that can be used to investigate mantle flow patterns around a subducting slab and geochemical contributions from subducted hotspot tracks.