#acasta

Some of the oldest rocks on Earth - The Acasta Gneiss

The Acasta Gneiss complex contains some of the oldest known continental crust on the planet, dated at approximately 4.03 billion years old. Since we know the age of the earth to be approximately 4.5 billion years old, this means that there are no known preserved rocks from the first 500 million years of the Earth’s history. As such, these rocks are one of the only clues that geologists have to study the geology and crustal evolution of the early Earth. Discovered in 1980’s, the Acasta Gneiss complex is named for the nearby Acasta river. It is exposed over approximately 40 square km of the foreland fold and thrust belt of the Wopmay orogen in geological Slave Province in the Northwest Territories, Canada. This location within the relatively tectonically stable Canadian Shield is what has allowed these rocks to be preserved for such an incredibly long period of time. The Acasta Gneiss consists of polydeformed, metamorphosed and migmatized tonalitic and granitic gneisses. Many detailed zircon U-Pb geochronology studies have given a maximum igneous crystallization age of 4.03 billion years, and show evidence of subsequent metamorphism at 3.75, 3.6, and 1.7 billion years.

Detailed study of the Acasta Gneiss has had many implications on our understanding of the early geological history of our planet. Its granitic composition provides evidence for differentiated crustal rocks in the early Archean. Granitic rocks like the Acasta Gneiss cannot form from direct crystallization of primary (ultramafic) mantle magmas. A felsic magma must first be created and this occurs as a result of magmatic differentiation processes such as fractional crystallization and partial melting. Magmatic differentiation is defined as a process which modifies the bulk chemistry of a primary magma. These processes can create felsic magmas by selective removal of components from the primary magma via mineral crystallization (fractional crystallization) or by the addition of components to the magma by melting existing continental crust (partial melting). These processes can occur because different minerals will crystallize/melt at different temperatures. Partial melting of continental rocks occurs in places where it is subjected to high heat flow, such as at hotspots or during subduction. Chemical analysis has indicated that partial melting of older continental rock is at least partially responsible for creating the magma from which the Acasta Gneiss crystallized. It has been suggested that this reworking of older crustal material may have occurred as a result of continental collision or subduction, which would imply that plate tectonics were active 4 billion years ago. However, exactly when plate tectonics began is still being debated amongst researchers.

In 2008 a team of researchers may have discovered rocks older than the Acasta Gneiss in the Nuvvuagittuq greenstone belt in northern Quebec, Canada. For more about these rocks, see this previous Earth Story post (https://tmblr.co/Zyv2Js25wVGSp). This discovery recently gained a lot of traction in the geologic community with a paper published in Science (http://bit.ly/2q8wLMT). These faux-amphibolite rocks give a Sm-Nd age of 4.28 billion years old. It should be noted that there is some discussion over whether this age corresponds to the true crystallization age of the rock or the age of the magma the rock crystallized from. Either way, all of these rocks give us a rare glimpse into early geological processes on Earth.

-CD