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Hikurangi Plateau Subduction a Trigger for Vitiaz Arc Splitting and Havre Trough Opening (Southwestern Pacific) K

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Hikurangi Plateau Subduction a Trigger for Vitiaz Arc Splitting and Havre Trough Opening (Southwestern Pacific) K https://doi.org/10.1130/G48436.1 Manuscript received 15 May 2020 Revised manuscript received 12 November 2020 Manuscript accepted 15 November 2020 © 2021 The Authors. Gold Open Access: This paper is published under the terms of the CC-BY license. Published online 30 December 2020 Hikurangi Plateau subduction a trigger for Vitiaz arc splitting and Havre Trough opening (southwestern Pacific) K. Hoernle1,2, J. Gill3, C. Timm1,4, F. Hauff1, R. Werner1, D. Garbe-Schönberg2 and M. Gutjahr1 1 GEOMAR Helmholtz Centre for Ocean Research Kiel, 24148 Kiel, Germany 2 Institute of Geosciences, Kiel University, 24118 Kiel, Germany 3 Department of Earth and Planetary Sciences, University of California, Santa Cruz, California 95064, USA 4 GNS Science, PO Box 30-368, Lower Hutt 5040, New Zealand ABSTRACT et al., 2011). It formed at ca. 125 Ma as part Splitting of the Vitiaz arc formed the Tonga-Kermadec and Lau-Colville Ridges (south- of the Ontong Java–Manihiki–Hikurangi super- western Pacific Ocean), separated by the Lau Basin in the north and Havre Trough in the plateau, which broke apart shortly after forma- south. We present new trace element and Sr-Nd-Hf-Pb isotope geochemistry for the Kermadec tion (e.g., Davy et al., 2008; Hoernle et al., and Colville Ridges extending ∼900 km north of New Zealand (36°S–28°S) in order to (1) 2010). The basement of the plateau fragments compare the composition of the arc remnants with Quaternary Kermadec arc volcanism, consists of two distinct geochemical types: (2) constrain spatial geochemical variations in the arc remnants, (3) evaluate the effect of (1) low-Ti basalts (Kroenke and Kwaimbaita Hikurangi igneous plateau subduction on the geochemistry of the older arc lavas, and (4) groups on Ontong Java) have isotopically inter- elucidate what may have caused arc splitting. Compared to the Kermadec Ridge, the Colville mediate compositions similar to that of primi- Ridge has higher more-incompatible to less-incompatible immobile element ratios and largely tive mantle, and (2) high-Ti basalts (Singallo overlapping isotope ratios, consistent with an origin through lower degrees of melting of more group on Ontong Java) have enriched mantle enriched upper mantle in the Vitiaz rear arc. Between ca. 8 and 3 Ma, both halves of the arc 1 (EM1)–type basement with unradiogenic (∼36°S–29°S) included a more enriched (EM1-type) composition (with lower 206Pb/204Pb and 206Pb/204Pb but radiogenic Sr isotope ratios 207Pb/204Pb and higher Δ8/4 Pb [deviation of the measured 208Pb/204Pb ratio from a Northern (e.g., Tejada et al., 2004; Hoernle et al., 2010; Hemisphere basalt regression line] and 87Sr/86Sr) compared to older and younger arc lavas. Timm et al., 2011; Golowin et al., 2018). Where High-Ti basalts from the Manihiki Plateau, once joined to the Hikurangi Plateau, could serve stratigraphic information is available, the high- as the enriched Vitiaz arc end member. We propose that the enriched plateau signature, seen Ti basalts overlie the low-Ti basalts. Between only in the isotope ratios of mobile elements, was transported by hydrous fluids from the ca. 117 and 79 Ma, spreading along the Osbourn western margin of the subducting Hikurangi Plateau or a Hikurangi Plateau fragment into Trough paleo–spreading center, now located at the overlying mantle wedge. Our results are consistent with plateau subduction triggering ∼25.5°S latitude, created ∼3000 km of seafloor arc splitting and backarc opening. between the Hikurangi and Manihiki Plateaus (e.g., Mortimer et al., 2019). The northern tip INTRODUCTION can et al., 1985; Wright et al., 1996; Timm et al., of the subducting Hikurangi Plateau is presently Volcanic arcs play a key role in the plate 2019; Caratori Tontini et al., 2019). Mechanisms located at ∼36°S. tectonic paradigm, being the surface expres- for triggering arc splitting, however, are con- Here we present new trace element and Sr- sion of plate convergence. Nevertheless, little troversial (Sdrolias and Müller, 2006; Wallace Nd-Hf-Pb isotopic data from 40 locations on is known about the long-term tectonic and geo- et al., 2009). the Kermadec (KR) and Colville (CR) Ridges chemical evolution of submarine remnant arc Subduction of young igneous oceanic pla- between ∼28°S and 36°S (Fig. S1 in the Supple- systems formed by arc splitting and backarc teaus is unlikely due to their buoyancy, as dem- mental Material1), recovered primarily on the basin opening, largely due to their inaccessi- onstrated by the Hikurangi Plateau when it col- R/V Sonne cruise SO255. We show that geo- bility. Contemporaneous Neogene volcanism on lided with and accreted to the Chatham Rise chemical variations along both ridges are nearly the Tonga-Kermadec and Lau-Colville Ridges at ca. 105 Ma, becoming part of the Zealandia identical, confirming that they once formed a (southwestern Pacific Ocean;Fig. 1 ) supports microcontinent. The present-day Hikurangi Pla- single arc, and that differences between the the idea that the subparallel ridges were once teau represents a rare example of an oceanic ridges reflect the KR having been the frontal part of a single volcanic arc (termed the Vitiaz plateau being subducted into Earth’s mantle arc and the CR the rear arc of the Neogene Vitiaz arc), which split at ca. 5.5–3 Ma to form the Lau beneath the North Island of New Zealand and arc. Some of the late Neogene (8–3 Ma) Vitiaz Basin and Havre Trough (e.g., Gill, 1976; Dun- the southern Kermadec arc (Fig. 1) (Reyners arc had an enriched composition distinct from 1Supplemental Material. Supplemental information about the samples and analytical methods, including Fig. S1), Table S1 (geochemical data), and Table S2 (replicates and reference materials). Please visit https://doi .org/10.1130/GEOL.S.13377182 to access the supplemental material, and contact editing@geosociety. org with any questions. CITATION: Hoernle, K., et al., 2021, Hikurangi Plateau subduction a trigger for Vitiaz arc splitting and Havre Trough opening (southwestern Pacific): Geology, v. 49, p. 536–540, https://doi.org/10.1130/G48436.1 536 www.gsapubs.org | Volume 49 | Number 5 | GEOLOGY | Geological Society of America Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/49/5/536/5273370/536.pdf by guest on 01 October 2021 ca. 8–3 Ma and depleted volcanism perhaps continuously since the early Miocene. Plots of isotope ratios versus latitude (with 1° latitude added to CR samples to compen- sate for northwest-southeast opening of the Havre Trough) show that isotopic variations are nearly identical along the KR and CR (Fig. 3), confirming that they once formed a single vol- canic arc (Gill, 1976; Timm et al., 2019; Cara- tori Tontini et al., 2019). The shift to higher Figure 1. (A) Map showing more-incompatible to less-incompatible ele- location of the Tonga-Ker- ment ratios in the CR than the KR lavas at simi- madec arc-backarc system and Hikurangi Plateau. lar isotopic composition suggests derivation Red box shows location of of CR lavas through lower degrees of melting the map in Figure S1 (see beneath the rear arc. KCR lavas with enriched footnote 1). Base map is compositions were found between 29°S and from GEBCO_2014 Grid 37°S (corrected CR; Fig. 3) with the strongest (version 20150318; http:// www.gebco.net). enriched signal being located at ∼33°S, charac- terized by the lowest 206Pb/204Pb and 207Pb/204Pb and highest 87Sr/86Sr ratios. Therefore, the enriched arc signature appears to have been limited both temporally and spatially, although more geochronology is necessary to define its exact duration. We now review possible origins of the enriched end member, beginning with the mantle wedge. Assuming corner flow, enriched intraplate mantle as found in South Fiji Basin seamounts and intraplate CR samples could have flowed from the backarc beneath the that of the Quaternary Kermadec volcanic arc, (≤18.37) and 207Pb/204Pb (≤15.53) and higher older arc. The South Fiji seamount and intra- consistent with subduction of the Hikurangi Pla- 87Sr/86Sr (≥0.7047) at similar 208Pb/204Pb, Nd, plate CR source, however, has higher 206Pb/204Pb teau or a plateau fragment. The enriched lavas and Hf isotope ratios, indicating an enriched and lower 87Sr/86Sr isotope ratios than the KCR occur along a segment of the arc where part of mantle (EM1)–type component in the source of lavas and therefore cannot explain the enriched the forearc is missing, consistent with removal the KCR lavas (between ∼29.5°S and 36.5°S), (EM1-type) signature (Fig. 2). There is also no by plateau subduction. Plateau collision and sub- not yet found in the Quaternary lavas. evidence of a plume beneath the arc, because duction is a possible mechanism for causing arc Alkalic seamounts behind and late-stage the enriched lavas show typical subduction zone splitting and backarc basin opening. cones on the CR (designated “intraplate CR”) incompatible element abundances, e.g., low have higher Nb/Th (4.3, 9.5–15.5), Ce/Pb (3–32), Ce/Pb (2.0–11.2, n = 74) and Nb/U (0.7–7.6, RESULTS Nb/U (9–50, LOI <4 wt%) and 206Pb/204Pb, and n = 74) ratios, well below typical mantle values We report analytical methods in the Supple- lower 87Sr/86Sr, suggesting a similar intraplate of 25 ± 5 and 47 ± 10, respectively, in global mental Material, trace element and isotope data source as for seamounts in the South Fiji Basin intraplate lavas (Hofmann et al., 1986). There in Table S1, and replicate materials and replicates (Mortimer et al., 2007; Todd et al., 2011). is also no geophysical (Bassett et al., 2016) or in Table S2, for lavas of the KR and CR (together geochemical evidence that continental litho- designated “KCR lavas”).
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