Home / Our Science / Earth Science / Regional Geology / Geological Origins

Geological Origins

Gondwana

The development of the New Zealand landmass we see today has occurred over many hundreds of millions of years. New Zealand’s foundations are between 500 and 100 million years old. These rocks occur within terranes and batholiths and understanding their origins and assembly is a major focus of our research.

  • Zealandia
  • Sedimentary basins
  • Gondwana
  • Plutons and batholiths
  • Sedimentary and volcanic terranes

Assembling the batholiths and terranes

New Zealand’s geological basement comprises metasedimentary, metavolcanic and plutonic rocks between 500 and 100 million years old. These rocks occur within more than eight major tectonostratigraphic terranes and several batholiths, and the origins and assembly of these is a major focus of our research.

Click here to enlarge image.

We undertake detailed petrographic, geochemical and geochronological studies to understand the tectonic setting, timing and emplacement controls of plutons and sedimentary basins.

Because of the New Zealand region’s chequered tectonic history along Gondwana’s southern edge, there is a complex structural and metamorphic history to understand as well.

Example: Unravelling Fiordland’s intrusion and metamorphic history

Magmatic, metamorphic and tectonic events in Fiordland

Click here to enlarge image.

The QMAP Fiordland geological mapping was closely supported by detailed petrology, geochemistry and geochronology and revealed a complex history of multiple igneous intrusions and metamorphic events in a variety of tectonic settings. One major recent discovery has been of New Zealand’s oldest pluton, the Pandora Orthogneiss, U-Pb dated at c. 500 million years (Late Cambrian).

Example: Tracking Australian connections

Zircon ages from greywacke

Range of U-Pb ages from zircon crystals from 63 greywacke samples.

Click here to enlarge image.

The source areas of New Zealand’s Cambrian to Cretaceous sandstones are being determined using various analytical techniques. U-Pb geochronology of zircon, a durable mineral that commonly crystallises within felsic igneous rocks such as granite and rhyolite, has been a very successful technique for constraining source areas and also depositional age. Our research the shows rocks present in Queensland today may well have been eroding in the Permian-Triassic 260 to 200 million years ago with the detritus washing into basins and trenches along the Gondwana margin. These sediments were subsequently lithified and were uplifted as the greywacke rocks that dominate the New Zealand landmass.

Example: Development of the Eastern Province Accretionary Wedge

Schist wedge Development of the Eastern Province Accretionary Wedge

Click here to enlarge image.

One of the currently favoured models for the development of the New Zealand's Eastern Province greywacke and schist terranes is that they formed as a Jurassic-Early Cretaceous accretionary wedge adjacent to the Median Batholith magmatic arc. Tests of this model involve detrital provenance linkages between the arc and the wedge, and also demonstrating that the arc, forearc and wedge share a related structural-metamorphic history. The numerous K-Ar and Ar-Ar white mica cooling ages from the Otago Schist have recently been supplemented by Ar-Ar ages on detrital K-feldspars. The results show plausible thermo-tectonic links across the entire convergent margin probably by the Early Cretaceous and possibly by the Middle Jurassic. The K-feldspar argon thermochronometer can provide useful information on the timing of geological events in sub-greenschist facies rocks.

Example: Structural and metamorphic framework of Greenland Group

South Westland basement rocks

Click here to enlarge image.

Early Paleozoic basement rocks in South Westland consist of regionally metamorphosed Greenland Group of probable Ordovician depositional age. Field, petrographic and electron microprobe data reveal a progressive, regional, south-eastward increase in metamorphic grade and define new chlorite, biotite-albite, biotite-calcic plagioclase and sillimanite-microcline zones. Minerals such as cordierite and andalusite, along with semi-schistose textures, indicate a low P/T Buchan style of regional metamorphism spanning the lower greenschist to upper amphibolite facies. This contrasts with medium to high P/T metamorphism in the Haast Schist east of the Alpine Fault. In South Westland. Peak Greenland Group metamorphism is loosely constrained as Devonian to Carboniferous (330-375 Ma) in age; the present day isograd pattern was mostly established by the Late Cretaceous and has not been strongly influenced by Alpine Fault deformation.