The Rocks That Link Australia, New Zealand, Antarctica & New Caledonia

There is a rock that links New Zealand, Australia, New Caledonia, and Antarctica. This is the fascinating story that takes us across the oceans to find the hidden link that tells of a time when these landmasses were joined during the Permian period when a subduction event occurred, forming an extensive volcanic arc system.

The Permian Arc Connection

During the Permian period, approximately 280–250 million years ago, a vast oceanic volcanic arc system formed along the eastern margin of Gondwana, the ancient supercontinent that once included most of the modern southern hemisphere. This system was created by subduction, a process where one tectonic plate—usually a dense oceanic plate—is forced beneath another, typically a lighter continental plate, sinking into the Earth's mantle where it melts and generates magma. This molten rock then rises to the surface, forming a chain of volcanoes known as an island arc.

Among these ancient arcs, the Brook Street Terrane of New Zealand, the Gympie Terrane of Queensland, Australia, the Teremba Terrane of New Caledonia, and parts of northern Victoria Land in Antarctica share striking geological similarities, suggesting they were once part of the same volcanic arc system before continental drift separated them.

These terranes primarily consist of volcaniclastic sequences (rock fragments produced by volcanic eruptions), mafic-ultramafic cumulates (dense, magnesium- and iron-rich rocks that crystallized from magma deep within the crust), and intrusive rocks (magma that cooled and solidified below the Earth's surface). Their widespread distribution suggests that they were once part of a single, continuous arc system along the edge of Gondwana, which was later broken apart by geological forces.

A Vision of the Permian Volcanic Arc

Imagine standing at the edge of a vast, turbulent ocean during the Permian period. Before you, an unstable chain of volcanic islands rises from the depths, roaring with eruptions of fiery magma and billowing plumes of volcanic ash. The skies are often darkened by clouds of sulfur and fine ash, while the surrounding waters churn with pyroclastic flows—scorching avalanches of gas and rock that race down the flanks of the volcanoes, hissing violently as they meet the sea.

Unlike the stable continental landmasses of Gondwana, this arc was separate, floating in the vast Paleo-Pacific Ocean. It was a world of fire and water, where violent subduction processes continuously fed the growing volcanic chain. Massive earthquakes rumbled beneath the waves, and molten lava poured into the sea, creating new land bit by bit. These volcanoes were the first stepping stones in the gradual assembly of what would later become parts of New Zealand, Australia, and Antarctica.

The Brook Street Terrane: A Permian Island Arc in New Zealand

The Brook Street Terrane (BST), found predominantly in New Zealand’s South Island, is a crucial geological remnant of this Permian arc. It consists of clinopyroxene-phyric basalts (volcanic rocks with large, greenish-black crystals of pyroxene minerals), high-MgO ankaramite dikes (dark, magnesium-rich rocks formed from magma intrusions), and basaltic to andesitic volcaniclastic and sedimentary rocks. Dacites and rhyolites, more silica-rich volcanic rocks, are also present but rare, suggesting that the volcanic arc was still in a primitive stage of development.

Intrusive bodies, such as trondhjemite plutons (light-colored granitic rocks formed from deep-seated magma chambers) and dolerite dikes (narrow vertical sheets of basaltic rock that cut across older formations), provide further insight into the magmatic evolution of the terrane. Field observations and petrological studies indicate that many of these rocks are the products of upper crustal magmatic differentiation, a process where molten rock slowly cools and crystallizes, forming distinct layers of mineral-rich rock at different depths.

The BST is unique in its widespread occurrence of high-Mg ankaramite and primitive island arc tholeiites, which suggest that arc parental magmas may have been more primitive than previously thought. This indicates that deep mantle sources, rather than crustal recycling, played a key role in the terrane’s magmatic evolution.

Some sections, such as the Bluff Complex, appear to have formed in a back-arc or arc-rift environment, where tectonic forces pulled the volcanic arc apart, creating basins filled with new volcanic activity. These regions provide evidence of geochemical variation within the arc system, suggesting different stages of volcanic growth and differentiation.

The Australian Connection: The Gympie Terrane

Across the Tasman Sea, in Queensland, Australia, the Gympie Terrane provides another crucial link in this geological puzzle. Like the BST, it consists of Permian volcanic and sedimentary sequences, including high-Mg basalts, ankaramites, and arc-derived volcaniclastic sediments.

Geochemical studies of the Gympie Terrane confirm strong similarities with the BST, particularly in its mineralogy and isotopic composition. Fossil evidence from sedimentary layers interbedded with volcanic rocks further supports the hypothesis that these terranes were once part of the same volcanic arc system, active along the eastern Gondwanan margin before it was dissected by plate movements and later accreted onto their respective landmasses.

How the Volcanic Arc Was Torn Apart and Its Mineral Deposits

As Gondwana began to fragment during the Jurassic and Cretaceous periods, powerful rifting events and the opening of the Tasman Sea and Southern Ocean pulled apart what was once a contiguous island arc system. The geological forces responsible for this fragmentation included rifting, subduction rollback, and plate divergence, which drove the landmasses in different directions. As Zealandia—a now largely submerged continent—drifted eastward, it carried pieces of the volcanic arc with it, while New Caledonia and parts of eastern Australia remained attached to the larger Gondwanan landmass.

The volcanic arc's violent history also left behind significant mineral deposits, particularly in the form of platinum-group elements (PGEs), chromite, and nickel-rich ultramafic cumulates. These deposits are most notable in the Greenhills Complex of New Zealand, where layered mafic intrusions have yielded platinum-rich horizons. Additionally, copper, gold, and base metal sulfides are associated with hydrothermal systems that developed in the volcanic arc's subaqueous environment. In New Caledonia, the terranes host significant nickel laterite deposits, a result of prolonged weathering of the ultramafic rocks derived from the Permian island arc.

Conclusion

The Brook Street Terrane is a vital geological link that connects the ancient histories of New Zealand, Australia, New Caledonia, and Antarctica. Once part of a vast Permian volcanic arc, these terranes now exist as fragmented pieces of a lost tectonic past, separated by millions of years of continental drift and oceanic spreading.

By studying these terranes, geologists continue to piece together the story of eastern Gondwana’s tectonic evolution, revealing the deep connections between seemingly distant landmasses. Through careful geochemical and petrological analysis, we can trace their shared origin, bringing to light the fascinating history of the ancient island arcs that helped shape the continents we see today.