The research, by geologist Associate Prof James Scott and colleagues, has also resulted in a piece of peridotite mantle rock from Wanaka being dated at more than 2.7billion years old.
This record-breaking rock is more than five times the age of the age of the country’s previously oldest-known rocks, 500million-year-old trilobite rocks from Nelson.
The Wanaka rock was found with the help of his father, Yfore Scott, of Alexandra, and his jet-boat, in 2011, at an extinct volcano on Lake Wanaka’s western shoreline.
"We took his jet-boat across the lake and used a rock drill to excavate the mantle rocks," Prof Scott said.
The small piece of Wanaka rock had been carried to the surface by volcanic magma about 23million years ago, initially from a depth of about 30km-70km below the surface.
Peridotite is a green magnesium-rich igneous rock from Earth’s mantle, which is the hot layer below the continental crust, the latter about 30km deep in much of Otago, including Dunedin, these days.
Peridotite was rare even in Otago, but because of the ancient volcanism, it was the best region in New Zealand to find the ancient rock, he said.
Nevertheless, the rock was usually difficult to find, often requiring special efforts to reach remote locations.
Collecting peridotites from the sub-Antarctic Auckland Islands in 2012 was definitely a highlight of his rock-hunting quest.
The New Zealand naval vessel HMNZS Wellington dropped him and a colleague from GNS Science, Dr Ian Turnbull, in Port Ross, on Auckland Island.
"We camped for three or four days amongst the flowering southern rata and bellbirds, and walked around on the tops [about 415m high].
"The peridotites were on a small knob known as Mt Eden, and some of these rocks reached about 40 centimetres in diameter.
He had also collected the rocks on the Chatham Islands, after a local farmer took him on the back of a quad bike to one site, in 2011. The rocks also occurred in Westland, where he went collecting with two geology honours students, after kayaking across Lake Brunner.
He had collected bits of peridotite from Fiordland, and East and West Otago too.
Such rocks were also among the volcanic rocks around Dunedin, but were not particularly common.
His interest in the rocks was part of a larger research programme to study the composition, age and evolution of the roots of the Zealandia continent.
He was studying the lithosphere mantle rocks below the crust, ranging, on average, from about 30km to about 70km below the surface.
This sort of “what lies beneath?” study of the the world’s eighth continent had never been done before.
He and his research group had now studied about 500 peridotites and gathered age data on more than 100 of them, from most areas of southern and central Zealandia.
"It’s a pretty special collection."
"This will help understand Zealandia.
"This continent is not only crust.
"Knowing the composition of the mantle helps to inform on the deep processes that have helped to make Zealandia."
Much of New Zealand was relatively young in geological terms, but some parts below the surface were "vastly older — over 2billion years older — than previously thought", he said.
