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The first dredge haul of basement rocks obtained from the Hikurangi Plateau in the southwest Pacific Ocean consists dominantly of volcanic and volcaniclastic rocks of probable pre–Late Cretaceous age. All samples have undergone extensive seafloor weathering to phyllosilicate‐ and zeolite‐bearing assemblages. Petrography, mineral chemistry, and whole rock element concentrations and ratios of the least altered lavas (e.g., TiO2 = 1.3–1.6 wt%, Cr = 132–224 ppm, Zr/Y = 2.9–3.6, Zr/Nb = 19–29, Ti/V = 22–29) suggest a restricted igneous compositional range broadly comparable with normal to enriched mid‐ocean ridge basalt. However, isotopic compositions of leached samples (87Sr/86Sri = 0.70361–0.70374, εNdi = 5.7–6.2) are comparable with oceanic island basalt suites. Collectively, these petrological characteristics are similar to rocks from large igneous provinces of Cretaceous age in the western Pacific Ocean (e.g., Manihiki and Ontong Java Plateaus). Consequently, we interpret the Hikurangi Plateau as another of these Early Cretaceous basaltic oceanic plateaus. Speculatively, parts of the obducted Tangihua and Matakaoa igneous complexes of the North Island may have been derived from the Hikurangi Plateau as the plateau collided with the New Zealand continent in the early Miocene.

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The southern Monashee Complex is a fault-bounded exposure of upper-amphibolite-grade basement gneisses (core gneisses) and unconformably overlying mantling metasedimentary gneisses. The eastern margin is marked by the Early Eocene ductile to brittle Columbia River fault zone; the western and southern margins are marked by the Monashee Decollement and structurally higher Selkirk allochthon.The basement gneisses are exposed in the cores of large, northeast-verging nappes that subsequently have been overprinted towards the east by the Columbia River fault zone. The basement gneisses are a supracrustal sequence intruded by at least two distinct orthogneisses: (i) a biotite granite gneiss ("gray gneiss") dated by U/Pb zircon at 1874 ± 21 Ma; and (ii) a ±hornblende–biotite K-feldspar augen gneiss dated by U/Pb zircon at 1934 ± 6 Ma.The supracrustal gneisses are predominantly heterogeneous biotite–quartz–feldspar gneiss interlayered with less common pelitic schist and calc-silicate gneiss. U/Pb zircon data on detrital zircon populations from this heterogeneous supracrustal sequence give 207Pb*/206Pb* ages of less than 2.2 Ga. Whole-rock Pb isotopic data indicate an age of approximately 2.0 Ga. Whole-rock Sm/Nd model ages on the two intrusive suites indicate separate sources, the 1874 Ma gneiss having been produced from similar-age juvenile Early Proterozoic material (TDM ≈ 2.2 Ga). In contrast, Nd data from the 1934 Ma augen gneiss clearly indicate interaction with a component of older (late Archean) material (TDM ≈ 2.8 Ga). Whole-rock Sm/Nd data from the supracrustal gneisses follow this same pattern, with one group (seven samples) similar to the 1874 Ma gneiss (with TDM ≈ 2.3–2.6 Ga) and a second group (five samples) showing provenance or derivation from an Archean source (TDM = 2.8–3.3 Ga). The age of the intrusive suites, combined with the Nd data, strongly argues for a correlation with the Early Proterozoic Wopmay orogenic belt in northern Canada.

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