Abstract
The intraplate rocks of the Dunedin Volcanic Group (DVG) in New Zealand's South Island erupted in two discrete areas between 25 and 21 Ma before becoming distributed over > 7,800 km
2
until ∼9 Ma. Although most eruptive centres were of small volume and mainly vented alkaline basanite, the largest centre-the 16-11 Ma composite Dunedin Volcano-discharged basanite and basalt through to trachyte and phonolite. DVG components were mainly derived from mantle sources with
87
Sr/
86
Sr = ∼0.7029,
143
Nd/
144
Nd = ∼0.5129,
206
Pb/
204
Pb = ∼20.0,
207
Pb/
204
Pb = ∼15.65,
208
Pb/
204
Pb = 39.5 and εHf = +3.5 to + 10.1 that extended to anomalously light δ
26
Mg (−0.47). Exceptions are some potassic basalts in NW of the field with elevated
207
Pb/
204
Pb and more radiogenic Sr. The DVG Sr-Nd-Pb isotopes mostly overlap with metasomatised anhydrous mantle peridotite xenoliths but have less radiogenic Hf, meaning that equivalent anhydrous mantle rock-types cannot be the sole magma sources. Although there is debate regarding whether DVG was derived from the lithospheric or asthenospheric mantle, intermittent melting of a middle lithospheric mantle metasomatised by hydrous asthenosphere-derived melts could account for: (1) the widely distributed magmatism for ∼16 Myr during which time Otago lithosphere shifted NW ∼ 870 km over the asthenosphere; (2) the small chemical range of the least evolved magmas; (3) the Sr-Nd-Pb-Hf isotopic range; and (4) an absence of lower lithosphere mantle xenoliths. This process could account for other occurrences of isotopically restricted Zealandia alkaline intraplate volcanism.