Contrasting Old and Young Volcanism from Aitutaki, Cook Islands: Implications for the Origins of the Cook–Austral Volcanic Chain

Abstract

Abstract The Cook–Austral volcanic lineament is thought to be the product of three overlapping South Pacific hotspot tracks: Macdonald, Arago, and Rarotonga. However, the entire Rarotonga hotspot track consists of volcanism on just two islands, Rarotonga and Aitutaki, making it difficult to evaluate a hotspot origin for the young volcanism on these islands. We present new Sr–Nd–Hf–Pb–He isotopic and major and trace element data on Aitutaki—which has two periods of volcanism, older (9·39 Ma) and younger (1·382–1·941 Ma)—and Atiu in the Cook Islands to evaluate a hotspot origin. Like Atiu, the ‘older volcanic series’ at Aitutaki was tentatively linked with the Arago hotspot based on its age, but this link has never been tested with geochemistry. New geochemical data on the older Aitutaki volcanic series show clear affinities to the Arago hotspot volcano Atiu. In contrast, the younger Aitutaki volcanic series, erupted on a near-atoll, exhibits evidence for a rejuvenated, non-plume volcanic origin. If Rarotonga (1·157–1·697 Ma) and the young series at Aitutaki share a common heritage and mechanism for formation as previously proposed—they erupted only 260 km apart over an overlapping time period and exhibit overlapping radiogenic isotopic compositions—then Rarotonga lavas may also represent a subaerial rejuvenated volcanic stage capping an older volcano related to a different hotspot, consistent with no direct involvement from a mantle plume in the origin of subaerial lavas at Rarotonga. Rarotonga, Aitutaki, and Atiu exhibit some of the most extreme enriched mantle (EM) signatures in the Cook–Austral volcanic lineament. There are three geographically separated geochemical domains along the Cook–Austral volcanic lineament—a northern geochemical domain with EM geochemical signatures is separated from a southern EM domain by a 1000 km long central HIMU (‘high μ’, or high 238U/204Pb) domain—and Rarotonga, Aitutaki, and Atiu are part of the northernmost geochemical domain. There is a possible role for lithospheric thickness in controlling geochemical compositions of melt extracted from the mantle along the Cook–Austral volcanic lineament, particularly across the Austral Fracture Zone. However, this is complicated by the lineament’s juxtaposition with the South Pacific Superswell, which may modify lithospheric behavior and complicate a simple lithospheric thickness model describing variable hotspot volcano compositions in the region.