Zircon search leads to new discoveries

The project aim to

• use the isotopic composition of zircon grains to help image the structure of the deep crust of Zealandia to help us find old, deep, long-lived faults

To achieve these objectives, we

• collected samples of granite rock from Stewart Island, Fiordland, Westland and NW Nelson to extract zircon grains
• analysed thousands of zircon grains to determine their isotopic composition (oxygen, hafnium) and their age
• produced models that told us what the age and composition of rock buried deep below Zealandia is – these ancient rocks were melted to form the younger granites we sampled at the surface

Zealandia – the eighth continent

For a long time, scientists debated whether Zealandia was a real continent. They thought New Zealand and New Caledonia were merely isolated remnants of the great continent.

In 2017, GNS Science unveiled a new and largely underwater continent called Zealandia. There was unprecedented worldwide media interest when a paper was published in Geology magazine(external link).

This ground-breaking study revealed that:

• Zealandia is an intact continent of some 4.9Mkm², nearly the same size as Australia. The islands of New Zealand and New Caledonia are all that remain above sea level
• it is the youngest, thinnest and most submerged of all the continents, meaning the crust is thin rather than thick

However, unlike the other continents, Zealandia was missing something – ancient rocks over  1 billion years old,  a feature of all the other seven continents. The discovery that there are 1-billion-year-old rocks still concealed deep in the crust beneath Fiordland and Rakiura/Stewart Island ticks the final continental box – there is no longer any doubt that Zealandia is a continent.

This new information has enabled scientists to place Zealandia in the ‘family tree’ of continents descended from Rodinia.

How we found Zealandia and Rodinia 

GNS Science’s Rose Turnbull and her team headed to the remotest locations in southern New Zealand by boat, helicopter and on foot to collect granite rock samples. Samples of rock were chipped off from the mountainous tops of Southern Fiordland, the impressive rock walls of Milford Sound, and small dark creeks on Rakiura/Stewart Island. Other samples came from New Zealand’s National Petrology Reference Collection. This gave them a good spread of sites across the South Island and Rakiura/ Stewart Island.

Back in the lab, the rocks were crushed into a fine sand, from which tiny zircon grains were separated. The team began analysing the zircon grains using sophisticated laboratory equipment in Germany and Australia. They used the results to model the age when the zircon grains crystallized, as well as the age and composition of the rocks that were melted to form them.

And that’s when the story got exciting. The zircons were telling a tale of a time when new continents were being formed and then destroyed. A time one billion years ago when magmas were crystallizing into hard rock and forming the crust of the supercontinent Rodinia. This ancient crust now sits hidden beneath Zealandia, and was itself melted during the construction of the Gondwana supercontinent to form the granites that now dot the landscape of southern New Zealand.