Detailed ⁴⁰Ar/³⁹Ar Geochronology of the Loyalty and Three Kings Ridges Clarifies the Extent and Sequential Development of Eocene to Miocene Southwest Pacific Remnant Volcanic Arcs

Abstract

The 2015 VESPA voyage (Volcanic Evolution of South Pacific Arcs) was a seismic and rock dredging expedition to the Loyalty and Three Kings Ridges and South Fiji Basin. In this paper we present 33 40Ar/39Ar, 22 micropaleontological, and two U/Pb ages for igneous and sedimentary rocks from 33 dredge sites in this little-studied part of the southwest Pacific Ocean. Igneous rocks include basalts, dolerites, basaltic andesites, trachyandesites, and a granite. Successful Ar/Ar dating of altered and/or low-K basalts was achieved through careful sample selection and processing, detailed petrographic and element mapping of groundmass, and incremental heating experiments on both phenocryst and groundmass separates to interpret the complex spectra produced by samples having multiple K reservoirs. The 40Ar/39Ar ages of most of the sampled lavas, irrespective of composition, are latest Oligocene to earliest Miocene (25-22 Ma); two are Eocene (39-36 Ma). The granite has a U/Pb zircon age of 23.6 ± 0.3 Ma. 40Ar/39Ar lava ages are corroborated by microfossil ages from associated sedimentary rocks. The VESPA lavas are part of a >3000 km long disrupted belt of Eocene to Miocene subduction-related volcanic rocks. The belt includes arc rocks in Northland New Zealand, Northland Plateau, Three Kings Ridge, and Loyalty Ridge and, speculatively, D'Entrecasteaux Ridge. This belt is the product of superimposed Eocene and Oligocene-Miocene remnant volcanic arcs that were stranded along and near the edge of Zealandia while still-active arc belts migrated east with the Pacific trench.Please note that this is an author-produced PDF of an article accepted for publication following peer review. The definitive publisher-authenticated version is available on the publisher Web site. Key Points► A major pulse of 25-22 Ma volcanism is documented on the Loyalty and Three Kings Ridges, southwest Pacific Ocean ► The ridges are part of a more than 3000 km long belt of Eocene to Miocene remnant volcanic arcs, stranded along the edge of Zealandia ► With care in sample selection, and petrological work, meaningful Ar/Ar ages can be obtained from altered and/or very low-K submarine basalts Plain Language SummarySamples of lava from the seabed between New Zealand and New Caledonia have been dated using atomic clocks and fossils. Most lavas erupted in a big pulse of volcanic activity between 25 and 22 million years ago. They are part of a belt of now-extinct undersea volcanoes that stretches for more than 3000 km between New Zealand and the Solomon Islands. These volcanoes were formed by subduction of the Pacific Plate under the Australian Plate.

Reconnaissance Basement Geology and Tectonics of North Zealandia

“…Ar‐Ar dating was done at the United States Geological Survey, Menlo Park following procedures in Calvert and Lanphere (2006); Ar‐Ar ages in this paper are reported relative to a Fish Canyon sanidine age of 28.198 Ma (i.e., they are compatible with Gans et al. (2023)).…”

“…Xenoliths of probable quartz veins were found in a lava dredged from the northern Norfolk Ridge (Mortimer et al., 2021). Detrital zircon grains of Permian to Cretaceous age, likely sourced from Eastern Province basement, have been obtained from Late Oligocene sedimentary rocks on the Three Kings and Loyalty Ridges (Figures 1 and 9, Crawford, 2004; Gans et al., 2023; Meffre et al., 2006).…”

“…An Eocene subduction‐related volcanic arc is inferred to underlie the Loyalty Ridge (Collot et al., 2011; Maurizot et al., 2020b and references therein). Linking these two areas, a belt of arc and backmost‐arc volcanic rocks of Eocene to Miocene age has recently been sampled and mapped along most of the eastern edge of North Zealandia (Gans et al., 2023) and is shown in Figure 9. The spatial coincidence of the volcanic arc with the ophiolitic allochthons is probably no coincidence, the two both being responses to Cenozoic subduction initiation along the northeastern edge of Zealandia (Sutherland et al., 2020).…”

The Norfolk Ridge: A Proximal Record of the Tonga‐Kermadec Subduction Initiation

“…VESPA DR42 targeted a scarp identified on seismic data where deep parts of the stratigraphy of the ridge are emergent at the seafloor (Patriat et al, 2015). This dredge recovered limestones with predominant shallow-water benthic foraminifera of probable Late Oligocene age (Lawrence et al, 2019), hard-ground laminated silicified limestone of undefined age and an Early Miocene lower bathyal calcareous mudstone (Crundwell et al, 2016;Gans et al, 2022). Farther east in the Norfolk Basin, several fossils found within a sandstone and conglomerate, including a fossil leaf, dredged at site SS01/2003 DR28 are interpreted as the relicts of a large latest Eocene to earliest Miocene island (Meffre et al, 2006).…”

“…The Norfolk Basin is located immediately east of Norfolk Ridge and is thought to have opened as the back‐arc of westward‐dipping and eastward retreating Tonga Kermadec subduction, but the basin may contain hyper‐extended continental fragments that were previously contiguous with the Norfolk Ridge (DiCaprio et al., 2009). Three Kings Ridge (Figure 1) is a relic Oligocene to early Miocene arc (Agranier et al., 2023; Gans et al., 2022; Mortimer et al., 1998), and is offset from its northern equivalent, Loyalty Ridge, by the Cook Fracture Zone (Figure 2). Magnetic anomaly and seafloor fabric interpretation combined with the age of dredged rocks show that Cook Fracture Zone activity was synchronous with the opening of the South Fiji and Norfolk basins during the interval 34–15 Ma (Herzer et al., 2011).…”