The Dunedin Volcanic Group and a revised model for Zealandia's alkaline intraplate volcanism. Issue 4 (1st October 2020) - Scott, James M.
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.