Chemical and isotopic changes induced by pyrometamorphism in metasedimentary xenoliths at Tongariro volcano, New Zealand

Highlights

• Metasedimentary xenoliths immersed in arc magmas underwent compositional changes.

• Compositional changes occurred in most element concentrations and isotopic ratios

• ⁴³Nd/¹⁴⁴Nd ratios in xenoliths were relatively unaffected by pyrometamorphism.

• Pyrometamorphism induced mass losses of ~50% in xenoliths.

• Xenoliths were derived from the Mesozoic Kaweka and Waipapa terranes.

Abstract

Andesites erupted from Tongariro volcano, North Island, New Zealand contain feldspathic and quartzose xenoliths derived from basement rocks. New major oxide, trace element and Sr-Nd-Pb isotopic data indicate that both the Waipapa and Kaweka (meta)sedimentary terranes are represented in erupted xenoliths, rather than only the Kaweka terrane as previously thought. Xenolith mineral assemblages differ from their likely source materials, notably through the lack of white mica, illite, chlorite and quartz, which is reflected in contrasting chemical and isotopic compositions. Major and trace element data indicate that most xenoliths underwent bulk mass decreases of about 50% when pyrometamorphosed at temperatures of ~800–980 °C, similar to typical Tongariro magma temperatures of ~800–1000 °C. Bulk Eu concentrations were retained (in restitic plagioclase); however, other rare earth elements are commonly lower in xenoliths than in protoliths. In xenoliths, the ¹⁴³Nd/¹⁴⁴Nd ratios of protoliths were also retained, which indicates that xenoliths were derived from the Kaweka and Waipapa terranes in subequal amounts. Reductions in ⁸⁷Sr/⁸⁶Sr ratios by up to 0.003 in xenoliths, relative to their likely protoliths, were accompanied by decreases in Rb/Sr ratios from 0.1–0.8 down to <0.1, reflecting the dissolution of hydrous, Rb-rich minerals (white mica ± illite) with their radiogenic isotopic ingrowths liberated into surrounding andesitic magmas. Varied amounts of U/Pb, Th/Pb and Th/U fractionation demonstrably occurred between xenoliths and protoliths, but these are challenging to correlate with Pb isotopic fractionation that also occurred. One xenolith contains a vein of clinopyroxene, calcic plagioclase, silicic glass and graphite that formed when quartz + calcite veins were pyrometamorphosed. The vein-bearing xenolith possesses unusual chemical and isotopic features, which include a negative Ce anomaly, La_N/Yb_N ~2 and high ¹⁴³Nd/¹⁴⁴Nd (0.51284), which are also reported for xenolithic material erupted at neighbouring Ruapehu volcano.

Introduction

The presence of xenoliths in igneous rocks provides direct evidence that country rock assimilation contributes to the assembly of arc magmas (e.g. Gill, 1981). In turn, the idea that entrained country rocks are completely melted and chemically incorporated in arc magmas underpins thermochemical models of assimilation-fractional crystallisation (AFC) processes (DePaolo, 1981; Spera and Bohrson, 2004), even though intact xenoliths are abundant in arc volcanic and plutonic rocks (Gill, 1981; Graham and Hackett, 1987; Knesel and Davidson, 1999; Petcovic and Grunder, 2003; Rudnick et al., 1986; White and Chappell, 1977; Wolf et al., 2019). Some studies have quantified the percentage of each xenolith that survives the assimilation process (Cesare et al., 1997; Mariga et al., 2006), but have not examined associated changes in radiogenic isotopic compositions. For these reasons, it is unclear whether the overall assimilant composition is better approximated by the bulk composition or the composition of a partial melt derived from the country rock. This distinction is fundamentally important when modelling AFC processes.

To address this distinction, this study examines bulk chemical and isotopic relationships between basement xenoliths, their protoliths and their host magmas (as erupted volcanic rocks) from a typical andesitic arc stratovolcano: Tongariro, New Zealand. We describe 25 inclusions that were derived from country rocks (Table 1) and which are petrographically and chemically distinct from cognate xenoliths also present in the Tongariro magma system (Pure, 2020). We present chemical and isotopic data for 16 of these inclusions (Table 2, Table 3) and compare their chemical compositions with reference data for their host lavas (Hobden, 1997; Pure, 2020) and basement rocks (Price et al., 2015) exposed to the west and east of the volcano. The aims are to (1) identify the provenance of the xenoliths, relative to regional basement materials, (2) examine whether or not bulk xenolith compositions have shifted relative to their protolithic compositions, and (3) determine if the net assimilant composition (sum of all molten and unmolten components) in Tongariro magmas is better approximated by the bulk composition or a partial melt composition of the country rock.

Tongariro is a ~350 kyr old composite andesite volcano in the southern Taupō Volcanic Zone (TVZ), central North Island, New Zealand (Wilson et al., 1995; Pure et al., 2020; Fig. 1). The TVZ is a continental, rifting volcanic arc linked to subduction of the Pacific Plate beneath the Indo-Australian Plate with a convergence rate of ~43 mm/yr at the latitude of Tongariro (Nicol et al., 2007). Extension across the zone is occurring at ~15 mm/yr in the northern TVZ (offshore), reducing to ~7 mm/yr in the region of Tongariro (Beavan et al., 2016; Gómez-Vasconcelos et al., 2017; Villamor et al., 2017; Wallace et al., 2004) and terminating ~40 km farther south (Fig. 1: Villamor and Berryman, 2006).

Previous studies suggested that the metasedimentary xenoliths in eruptives from Tongariro and adjacent volcanoes Ruapehu (to the south) and Taupō (to the north) could be derived from any of three juxtaposed Mesozoic greywacke-argillite terranes (Beetham and Watters, 1985; Charlier et al., 2010; Townsend et al., 2017). From oldest to youngest depositional age, these are the Waipapa composite terrane (Jurassic), and the Kaweka (Late Jurassic) and Pahau (Early Cretaceous) terranes within the Torlesse composite terrane (Mortimer, 1994; Mortimer et al., 2014).

At the surface, Waipapa terrane rocks occur ~15 km northwest of Tongariro in an uplifted fault block, where they are locally capped by <200 m of Neogene Māui Supergroup and Oligocene Te Kuiti Group rocks (Fig. 1) (Townsend et al., 2017). Waipapa rocks have also been intercepted at shallow depths (≤200 m) at sites ~10 km west of the Tongariro edifice (Beetham and Watters, 1985; Townsend et al., 2017). Rocks of the Kaweka terrane occur ~15 km east of Tongariro in the Kaimanawa Mountains. Results of zircon provenance studies on a metasedimentary xenolith (R623 in Fig. 1) erupted in a ~27 ka rhyolite dome indicate also that Pahau terrane rocks are present beneath the Taupō area (Charlier et al., 2010; Sutton, 1995), despite the fact that the nearest outcrops are ≥100 km to the northeast of Taupō and Tongariro. A review of intersected basement greywackes from geothermal drilling studies reported that both Kaweka and Waipapa terranes form the pre-volcanic basement beneath the central TVZ (Milicich et al., 2021). However, the geometry of the Waipapa-Kaweka terrane boundary beneath Tongariro is unclear from current geophysical data and rock structure data. Geophysical studies have suggested that metasedimentary basement rocks continue to depths of 15 km beneath Tongariro (Behr et al., 2011; Harrison and White, 2006; Stern and Benson, 2011). However, these studies can only broadly identify crustal lithologies from their seismic velocities, and the lithological contrasts between the Kaweka, Waipapa and Pahau have not been distinguished in seismic profiling.

Quaternary volcanic rocks in the southern TVZ are separated from the Mesozoic metasedimentary rocks by non-metamorphosed Neogene marine rocks with thicknesses of ≤1.5 km (Robertson and Davey, 2018; Sissons and Dibble, 1981) but these are largely buried by volcaniclastic sediments of the ring plain around Tongariro (Cronin and Neall, 1997; Townsend et al., 2017). At greater depths, the Mesozoic metasediments are proposed to rest on lower-crustal meta-igneous oceanic crust; however, evidence supporting this interpretation is limited (Graham et al., 1990). No other geological units are expected to comprise basement rocks beneath Tongariro, and geophysical studies suggest that upper crustal basement lithologies are of comparable type between Tongariro and Taupō (Behr et al., 2011; Harrison and White, 2006; Robertson and Davey, 2018; Stern and Benson, 2011). For these reasons, we focus here on comparisons between our basement xenolith data versus those taken from surficial exposures of Waipapa, Kaweka and Pahau terrane rocks reported by Price et al. (2015).

Previous studies of Tongariro basement xenoliths by Graham (1987), Graham et al. (1988) and Hobden (1997) (samples from the latter re-analysed by Price et al., 2010) are limited to samples from the 1954 to 1975 pyroclastic flow deposits and lavas erupted from the Ngāuruhoe vent. In contrast, this study investigates xenoliths obtained from eruptives with ages of ~190 ka to the Holocene (Pure et al., 2020). Xenoliths from Ruapehu have been studied by Graham (1987), Graham and Hackett (1987), Graham et al. (1990) and Conway et al. (2018).

Xenolith studies by Graham, 1985, Graham, 1987, Graham et al. (1988), Hobden (1997) and Price et al. (2010) provide data for whole-rock major oxide (X ray fluorescence [XRF]) and trace element concentration (XRF, instrumental neutron activation analysis [INAA], inductively coupled plasma mass spectrometry [ICP-MS]) and Sr-Nd-Pb isotope ratios (thermal ionisation mass spectrometry [TIMS] and multi-collector [MC]-ICP-MS) for Tongariro xenoliths. Graham (1987) also reports results of electron probe microanalyses of some minerals in these xenoliths. Pb isotopic ratios of Tongariro xenoliths, analysed by TIMS, are reported by Hobden (1997) and were re-measured by MC-ICP-MS by Price et al. (2010). The latter data are preferred for comparisons here because the earlier TIMS Pb data of Hobden (1997) were externally corrected for instrumental mass fractionation and hence are of lower precision.