The Dunedin Volcanic Group (DVG) is a long-lived (25 to 9 Ma) and dominantly alkaline intraplate field in New Zealand’s South Island. From a compilation of 92 dates, I will show that it initiated as small eruptions in the west of the field prior 20 Ma before becoming widely distributed over > 7,800 km2 without a pattern over the next 11 Ma.
The best analogue is a shark attack; unexpectedly something will rise up from the depths and attack somebody at the surface.
A compilation of 550 analyses shows that although most centers erupted basanite, the largest center, the emergent Dunedin Volcano, discharged basanite and basalt through to phonolitic and trachyte repeatedly between 16 and 11 Ma. 85 analyses show that the source to most DVG components was isotopically light δ26Mg (-0.20 to -0.47) mantle reservoir with 87Sr/86Sr ~0.7029, 143Nd/144Nd = ~0.5129, 206Pb/204Pb = ~20.0, 207Pb/204Pb = ~15.65 and 208Pb/204Pb = 39.5. These data overlap with those from metasomatised peridotite mantle xenoliths that can be found in numerous flows or plugs.
However, a subtly different mantle source is required for a cluster of distinct potassic basaltic flows in the northwest that has elevated 207Pb/204Pb and extends to more radiogenic-Sr.
I present a hypothesis that, like making a cake, DVG magmatism results from a two-stage process of preparation and then cooling. First, the mantle lithosphere requires ingredients being added. A slightly isotopically heterogeneous amphibole-bearing metasoma-rich lower mantle is formed during the cessation of Early Cretaceous subduction. Disturbance of the 1100oC isotherm in the lithosphere promoted metasome melting and results in alkaline magmatism, with the extent of the perturbation influencing the volume of magma generated.
This two-stage process could explain all Zealandia intraplate basalts and also explain the striking isotopic similarity of lithospheric mantle with the intraplate magmas.
|Date||Wednesday, 11 September 2019|
|Time||1:00pm - 2:00pm|
|Location||Benson Common Room (Gs9), Geology Building|