A Department of Geology Seminar presented by Alan Cooper and James Scott
Although the South Island is not usually thought to have been particularly volcanically active, there are actually three large (but extinct) volcanic fields: the ~34 Ma Waiareka-Deborah-Volcanic Formation, the ~25-20 Ma Oligocene-Miocene Alpine Dyke Swarm and the 24-9 Ma Dunedin Volcanic Group.
The Waiareka-Deborah field erupted on the Otago submarine continental shelf near Oamaru and comprises tuff, pillow lavas and large sills. The field was built from many small overlapping volcanoes that probably breached the sea surface but were rapidly eroded below wave base. Magmas were, unusually, mainly a sub-alkaline, although some eruptions, including the spectacular mantle xenolith-bearing Kakanui Mineral Breccia, show that some magmatism was alkaline.
The Alpine Dyke Swarm (ADS) intruded Haast Schist in the Southern Alps at ~25 Ma during inception of the Alpine Fault plate boundary. At Haast River, magmas, enriched in volatiles, LILE, and HFSE (including REE), evolved by fractional crystallisation from primitive lamprophyres to phonolites, and then by liquid immiscibility to carbonatite. Highly sodic carbonatitic fluids metasomatised quartzofeldspathic schist to aegirine-albite fenites.
The Dunedin Volcanic Group (DVG), at 7,800 km2, is one of the largest intraplate provinces in New Zealand. Eruptions began near Middlemarch and Milton, before becoming regionally widespread. The field is dominated by lavas, with occasional diatremes; aside from the initiation of the Dunedin Volcano, little appears to have been submarine. DVG magmas, although also undergoing extensive fractionation like the ADS, failed to achieve the extreme alkali enrichment necessary for silicate melt-carbonatite immiscibility.
Isotopically, the Waiareka-Deborah volcanic field is distinct from the DVG or ADS and this means that the intraplate volcanism in Otago had multiple mantle sources. We suspect the mantle lithosphere has a major role in the DVG and ADS, whereas melting in the asthenosphere led to the Deborah-Waiareka volcanism.
|Date||Tuesday, 11 August 2020|
|Time||1:00pm - 2:00pm|
|Location||Benson Common Room (Gn9), Department of Geology, Dunedin Campus, University of Otago|