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Feature

New Zealand: An Australian Export?!

As the old saying says, one man's cast-offs may be another man's legacy. New geological evidence suggests that New Zealand, as a piece of real estate, largely came from Queensland!

Hamish Campbell

For more than a century, geologists in New Zealand have been grappling with a major geological mystery -- the whereabouts of the source rocks that produced all the sediment that makes up our greywacke. Where did all that sediment come from?

The extent of greywacke in New Zealand is vast. Undoubtedly it is most conspicuous as aggregate in roading, railway tracks, airports and concrete construction, and as gravels and sands in our river beds and on our beaches.

But it is much more than that. In the North Island, it forms the hard basement rock of all the major ranges from Northland to Wellington: Hunua, Urewera, Kaimanawa, Kaweka, Ruahine, Tararua, Aorangi -- all of them!

It also forms the main parent rock to the soft grey, mainly mudstone and sandstone "papa" of the Taranaki, Wanganui, Mana-watu, central and east coast North Island areas.

It also underlies the active North Island volcanoes -- they punch right through the greywacke and have mantled it with lava and ash.

In the South Island, greywacke gravels make up the entire Canterbury Plains, formed by erosion of mountains of greywacke inland. Almost all of Marlborough, Canterbury and Otago provinces are greywacke. This includes all the main ranges and the Southern Alps. Mount Cook itself is greywacke!

Even the schist rocks of Otago are really greywacke -- they are just more metamorphosed, more deeply buried and cooked. Greywacke also forms much of the basement rock of the West Coast through which the much younger granite rocks have been intruded.

As if this wasn't enough, we also know that much of the submerged continental shelves adjacent to the New Zealand landmass are also underlain by basement greywacke: the Campbell Plateau, the Chatham Rise and even the Chatham Islands. New Zealand's eastern-most islands, the Forty Fours, are also greywacke.

New Zealand: An Australian Export?! Figure A (3KB)
Greywacke (in black) -- a firm foundation for New Zealand

What is Greywacke?

As a geological term, "greywacke" is rather non-specific and refers to rather common, nondescript rock. The term is of German derivation, from "grau-wacke" meaning grey sandstone. Major scientific symposia have been devoted to its definition and description. It is lots of things to lots of people but, by common assent, geologists agree that "greywacke" is best defined simply as a hard, grey, sedimentary rock.

The important thing is that it is sedimentary. Originally it was composed of gravel, sand, silt and mud that was deposited in water and, almost invariably, sea water. Most of it was muddy sand, and most greywacke is now muddy sandstone. By definition, this sediment had to have been eroded from some pre-existing rock on some landmass.

So what kind of rock was this parent rock? Where was it? What was the landmass?

The answer to the first question is easy. Many decades of detailed geological study by many researchers have established beyond all doubt that the parent rock to the New Zealand greywacke was granitic or granitoid. Only granitic rocks could produce sediment with the distinctive mineral and chemical composition observed in the greywacke. For such vast amounts of sediment to be produced, a large granite terrane with dimensions at least comparable to that of the New Zealand landmass itself, must have existed. Almost certainly, this would have to be a continental landmass.

The hunt has been focused on Antarctica for the last three decades but no compelling parent rock has been found. During this time, many other explanations have emerged, including claims that the sediment was derived from a now lost continent.

Now, there is a bold and exciting new hypothesis suggesting that the source was northeast Queensland. This will come as a shocking news to many New Zealanders. How could such a preposterous statement be made -- Australia! Worse still, Queensland!!

Despite any emotive and/or nationalistic feelings, the evidence behind this hypothesis is very compelling and is largely based on age determination of detrital minerals within the greywacke.

A distinction is made here between detrital minerals and metamorphic or "new" minerals that have formed as a consequence of burial and lithification of the original sediment. Detrital minerals are the original sediment that accumulated as sand and silt grains of mineral or rock detritus.

In the case of greywacke, the most useful detrital minerals are those that are durable and not greatly affected by metamorphism, yet readily datable using conventional techniques of radiometric dating: potassium-argon (K-Ar), argon-argon (Ar-Ar) and uranium-lead (U-Pb). For dating, minerals such as potassium-bearing micas and uranium-bearing zircons are best. The idea behind this age analysis is simple: the age distribution pattern of detrital minerals will determine the age range of the rock types that they were derived from, and this in turn will greatly constrain where to search for source rocks of matching age.

Accordingly, with this objective in mind, New Zealand's foremost geochronologist, Dr Christopher Adams of the Institute of Geological and Nuclear Sciences, has undertaken many years of painstaking research on the greywackes of New Zealand.

This has involved innumerable sampling expeditions, months of laboratory work, and collaboration with some of the world's most sophisticated dating facilities, including our own. The results are now published and represent the culmination of almost 20 years of sustained effort.

The results are fascinating. They indicate derivation of sediment from a granitic source area of predominantly Permian-Triassic age (around 200-300 million years ago) with a minor mica-rich Ordovician (c. 425-450 Mya) component, perhaps a mica schist.

Geological Detective Work

Armed with this information, all that we now need do is look at the geology of our nearest neighbours and search for a suitable rock terrane with the same composition and age as the detrital minerals in the greywacke. Simple detective work!

New Zealand can be ruled out immediately. Simply on logistical grounds, the volume of greywacke in New Zealand vastly exceeds the volume of all other rocks in New Zealand, and besides, these other rocks are all of the wrong composition, wrong age or both.

Similarly, there are no known rocks of appropriate composition and age in Antarctica (the Antarctic Peninsula, Marie-Byrd Land, North Victoria Land), Tasmania, southeast Australia, central east Australia or northern-most east Australia. However, there are appropriate rocks of the right composition and age in northeast Queensland.

Adams is adamant that Queensland is the most plausible source, and his detrital mineral age data is further supported by isotopic data. Strontium isotopic composition is another powerful method used in distinguishing and finger-printing large bodies of sedimentary rocks, and then sorting out the most likely parent rocks. Queensland fits best.

The only outstanding question is how did New Zealand get here if it came from Queensland?

The two areas are separated by 2,000-3,000 km. The answer is simple -- a combination of sediment transport by rivers and sea, crustal transport by plate tectonics, and plenty of time to do it in.

A major implication of a Queensland source for the New Zealand greywacke is that there has been a systematic 2,000-3,000 km shift of crustal blocks from northwest to southeast, around the ancient Gondwanaland margin. Greywacke rock of eastern Australia may be derived from Papua New Guinea, and greywacke rocks in the Antarctic Peninsula may be derived from the basement rocks of southern New Zealand and the Campbell Plateau.

The new hypothesis is very exciting and is likely to significantly change geological thinking in New Zealand. As a scientific hypothesis it will, of course, be subject to the utmost scrutiny.

You can read the article by C.J. Adams and S. Kelly "Provenance of Permian-Triassic and Ordovician metagreywacke terranes in New Zealand: evidence from 40Ar/39Ar dating of detrital micas" in the Geological Society of America Bulletin 110: 422-432.

Dr Hamish Campbell works at the Institute of Geological and Nuclear Sciences Ltd in Lower Hutt.