Zircon search leads to new discoveries

27601 Lloyd Homer GNS Science

Using zircons found in rocks throughout New Zealand we uncovered two geological secrets – Te Riu-a-Māui/Zealandia is older than we thought, and Zealandia was once part of an even older supercontinent called Rodinia.

Overview

This is one of those “cool” science stories. Zircons are used worldwide to precisely date extinction events, the age of volcanic eruptions, the timing of precious metal formation, and to determine the age and evolution of Earth’s continents.

Our search began with a hypothesis: Can we 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? Finding such faults is useful for understanding Zealandia’s tectonic and magmatic history, and it may also lead to the discovery of important mineral deposits near these old fault systems.

As often happens with science, we found something quite different from what we originally intended to discover, including a piece of an ancient supercontinent that’s existence was hidden in the composition of tiny zircon grains just waiting to be revealed.

  • the final piece of evidence that Zealandia is a real continent
  • underneath Zealandia we have uncovered an ancient piece of the supercontinent Rodinia
  • this discovery places Zealandia at the centre of the debate on Rodinia reconstructions

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

The project

Finding billion-year-old rock

Geologist Dr Rose Turnbull and her team found the faults they were seeking, but they also found two things they weren’t looking for – Te Riu-a-Maui/Zealandia is much older than we thought, and it’s linked to an even older supercontinent called Rodinia.

Until now, the oldest rocks in New Zealand are 500-million-year-old rocks in the Tasman District that formed when Zealandia was part of the Gondwana supercontinent. The isotopic signature of zircon grains we analysed in this study tell us there are 1-billion-year-old rocks still concealed deep in the crust beneath Fiordland and Rakiura/Stewart Island – rocks that were formed as part of the Rodinia supercontinent.

The tiny zircon grains taken from granite rocks collected in Fiordland and Rakiura/Stewart Island have led to a potential breakthrough in ancient continental reconstruction. And that’s linked to another GNS discovery. Back in 2017, GNS scientist Dr Nick Mortimer and his team set the geological world alight: Zealandia is a real continent, the eighth continent. New Zealand and New Caledonia are all that remain above water.

Those granite rocks in Fiordland and Rakiura/Stewart Island took us even further back in time – to a 1 billion-year-old supercontinent called Rodinia.

All of today’s eight continents have older ancestors such as Gondwana, Laurasia, and Pangea. Now, we can place Zealandia in the ‘family tree’ of continents descended from Rodinia. And that raises important new questions. Where was the continent of Zealandia within Rodinia? Is Zealandia a ‘missing link’ between South China, Australia, and North America? What does this say about the position of South China and Zealandia within Rodinia?

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.

Read more about Rodinia 

The name Rodinia comes from a Russian word meaning “homeland”. It’s rather appropriate for the first dominant landmass on earth we can identify with any certainty.

Our continents are locked into a 3-billion-year-old mash-up of forms from Rodinia, Laurentia and Pangaea to Gondwana and Te Riu-a-Maui/Zealandia.

Research project details

Collaborators: University of Western Australia, California State University, Northridge, USA, Curtin University, Australia, Heidelberg University, Germany, The University of Vermont, USA

Duration

2018–2022

Funding platform

Marsden Fund Fast-Start Grant

Status
Programme leader

Rose Turnbull, GNS Science

Associate Professor Marco Fiorentini, University of Western Australia
Dr Joshua Schwartz, California State University Northridge

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