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Australian researchers have uncovered the origin of pink garnet sand found on South Australian beaches, tracing it back to ancient mountains in Antarctica.
Their findings have shed light on the timing of the Australian tectonic plate’s subduction beneath the Pacific plate and unveiled previously unknown ancient mountains in Antarctica.
Pink Sand Composition on Australian Beaches
The pink sand is primarily composed of garnet, a mineral that is rare in beach sand due to its susceptibility to destruction by prolonged exposure to ocean waves and currents. Garnet formation requires high temperatures and is typically associated with the development of large mountain belts. In Australia, garnet formed locally during significant geological events such as the Delamerian orogeny, which created the Adelaide Fold Belt around 514–490 million years ago, and during the formation of the Gawler Craton in western South Australia between 3.3–1.4 billion years ago.
Potential Sources of Garnet in Australia
Kimberlite Pipes
Garnets can surface through carrot-shaped volcanic structures known as kimberlite pipes, found in South Australia, such as at Eurelia. However, these deposits are not abundant, are located far from the coast, and are only around 170–190 million years old, making them an unlikely source of the beach garnets.
Erosion of the Adelaide Fold Belt
Garnet forms in greater volumes in areas with thick crust, such as under mountains. Erosion of such mountains can reveal garnet as a record of the former mountain belt. The erosion of the Adelaide Fold Belt, stretching north from Adelaide and developing between 514–490 million years ago, could be another potential source of garnet.
Gawler Craton
The Gawler Craton is a vast slab of ancient rock beneath South Australia, with outcrops in the Adelaide Fold Belt. It contains abundant garnet formed during several episodes between 3.3 billion and 1.4 billion years ago.
Findings from the New Research
The recent study discovered that most garnets in the beach sand formed around 590 million years ago. This timing suggests the garnets were too young to originate from the Gawler Craton and too old to come from the eroding Adelaide Fold Belt. Similar-aged garnet has been found in the Transantarctic Mountains in East Antarctica. Ice-flow indicators along South Australian shorelines suggest that garnet-rich glacial sands were transported to Australia by a north-west moving ice sheet during the Late Palaeozoic Ice Age, a period when Australia and Antarctica were part of the supercontinent Gondwana.
