Evolution from subduction initiation to mature island arc volcanism in the Upper Eocene to Middle Miocene Vitiaz Arc, SW Pacific: Evidence from Malekula Island (Vanuatu)

The oldest volcanic rocks exposed on Malekula Island, now belonging to the New Hebrides Island Arc, formed in the Upper Eocene to Middle Miocene Vitiaz Island Arc, Southwest Pacific. They are thought to have formed contemporaneously with Fiji and the Izu-Bonin-Mariana (IBM) arc during westward subduction initiation of the Pacific beneath the Indo-Australian Plate [e.g., 1]. To test this hypothesis with regard to the mantle source compositions and contributions from the subducting slab, we provide major- and trace element data combined with Hf, Nd, and Pb isotopes for twenty-seven volcanic rocks of Malekula Island. Our results show that Malekula lavas display similar magma types, i.e., boninite-series rocks, island arc basalts, and MORB-type tholeiites, to the earliest volcanic rocks of Fiji and the IBM arc resembling the sequential stratigraphy of the IBM system [2], rather than the interlayered stratigraphy of early arc rocks on Fiji [3]. Moreover, Malekula lavas display a change in Hf-Nd isotope composition from isotopically ‘Indian’, similar to the IBM arc [4, 5], to mainly ‘Pacific’, like on Fiji [3]. We interpret this progressive change in mantle source composition to reflect the propagation of ‘Pacific’ South Fiji Basin spreading into the Vitiaz Arc. Hence, the Malekula lava succession provides a link between subduction initiation in the Northwest and Southwest Pacific. [1] Hall (2002) J Asian Earth Sci 20. [2] Ishizuka et al. (2011) EPSL 306. [3] Todd et al. (2012) EPSL 335-336. [4] Reagan et al. (2010) G311(3). [5] Li et al. (2019) EPSL 518.