Rock from Wanaka oldest found in NZ by far

A piece of peridotite rock found by geologist Associate Prof James Scott in Wanaka is 2.7 billion years old, more than five times the age of the previously oldest-known New Zealand rocks.

The previously oldest-known rocks are trilobite rocks from Nelson that are about 500 million years old.

"Yes, it has rewritten the standard account, although most people still think that the 500 million-year-old trilobite rocks are the oldest bits of Zealandia," Prof Scott, of the university geology department, said yesterday.

Associate Prof James Scott, of the University of Otago geology department, examines a small piece...
Associate Prof James Scott, of the University of Otago geology department, examines a small piece of magnesium-rich peridotite which has been found to be 2.7 billion years old, more than five times older than the previously oldest-known rocks in the country, trilobite rocks from the Nelson area. PHOTO: PETER MCINTOSH

"However, this is only true if you consider [the continent of] Zealandia as being the [outer] crust," he said.

Geologists were starting to appreciate that the underlying mantle played an important role in preserving continents.

"We now know that Zealandia actually comprises crust and mantle and together these are called the lithosphere."

The Wanaka rock was found by drilling a few centimetres into an extinct volcano on the western shoreline of Lake Wanaka near Mt Albert Station.

This ancient rock — known as a peridotite — was plucked and carried to the surface by volcanic magma about 23 million years ago.

Peridotite is a green magnesium-rich rock from Earth's mantle, which is underneath the continental crust.

The rock was recovered by Prof Scott and his father Yfore Scott, of Alexandra, in 2011.

Prof Scott had collected pieces of ancient peridotite, including a lump of ancient rock from near the Pigroot (State Highway 85), as part of a larger research programme to study the roots of the Zealandia continent.

Much of New Zealand was relatively young in geological terms, he said.

However, parts of the continent were "vastly older — over 2 billion years older — than previously thought".

He had also collected bits of mantle peridotite from the Chatham Islands, the sub-Antarctic Auckland Islands, Fiordland, through East and West Otago, and in Westland.

The isotope dating analyses for the Wanaka find were undertaken at the University of Alberta in Edmonton, Canada, in a collaboration between Dr Candace Martin, also of Otago geology, and Canadian researchers.

Peridotite was rare even in Otago, but because of the region’s history of ancient volcanism, it was the best overall region in New Zealand to find the ancient rock, Prof Scott said.

These "striking green rocks" were rare because they had travelled to the surface from a much lower level — from 30km to about 60km below the Earth’s surface.

The continent of Zealandia was like an iceberg, with only a small amount of it above sea level, but a much thicker root beneath it.

"These small rocks give a unique insight into what lies deep beneath our feet."

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