Old rocks make Zealandia’s case stronger

Evidence of billion-year-old rocks under New Zealand is considered further proof that the largely underwater expanse of continental crust known as Zealandia is a genuine continent.

Researchers were able to determine the age of the rocks by analysing microscopic grains of the mineral zircon in granite rocks found at the surface in Fiordland and Rakiura/Stewart Island.

Study lead author Dr Rose Turnbull of GNS Science said the sampled granites were basically crystallised magma that formed deep in the Earth’s crust and had then been brought to the surface.

The findings showed some granite rocks in Fiordland and Rakiura/Stewart Island were likely linked to a supercontinent called Rodinia that existed a billion years ago.

Previous GNS Science-led work had been able to justify for various reasons why Zealandia should be considered a continent, Turnbull said.

But one thing previously thought to be missing, that some people in the world of geology might consider to be necessary for a continent, was old rocks.

Until now, New Zealand’s oldest rocks – in the Tasman District

– were dated about 500 million years old and linked to the supercontinent Gondwana that came after Rodinia, Turnbull said. ‘‘This new study ticks that final continental box. There is no longer any doubt that we live on top of a continent.’’

Zircon grains found in the studied granite showed there were 1-billion-year-old rocks deep in the crust that were formed as part of the Rodinia supercontinent, Turnbull said.

Zircon is a durable mineral that crystallises in most magmas. It can survive many geologic events over billions of years and, in spite of being exposed to extreme conditions, the core of the mineral remains unchanged and it preserves the chemical (isotopic) characteristics of the magma in which it originally crystallised. ‘‘This work sheds new light on Zealandia’s ancient past – a geological past that has now been shown to be over 1 billion years old.’’

It was hard to know how far the ancient rocks were under the surface, but they would be at least 40km down. The research has been published in leading geological journal Geology, and is expected to attract widespread interest because of the possible implications of the work for understanding the make-up of Rodinia.