The first continental components are composed of three types of granite rocks: tonalite, trondjemite, and granodiorite (TTG).Credit: Jaana Hara
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The first continental components are composed of three types of granite rocks: tonalite, trondjemite, and granodiorite (TTG).Credit: Jaana Hara
Earth scientists have discovered a missing piece in the mysterious story of how the continents developed. This is a modified origin story that does not require external factors to explain the beginning of plate tectonics or the formation of continents. Instead, it was discovered that published last week nature communications It relies solely on internal geological forces that occurred within the oceanic plateaus that formed during the first hundreds of millions of years of Earth’s history.
A major hurdle in understanding how continents formed during the Archean Era (4 billion to 2.5 billion years ago) was identifying the components of Earth’s early crust. Much of the “new” Archean crust that formed during this period was composed of a very distinct combination of three types of granitic rocks: tonalite, trondjemite, and granodiorite (TTG).
Understanding what happened to form TTG and the magma from which it formed is difficult because so many geological processes occurred between initial melting and final crystallization. did. Previous researchers focused on the trace element composition of these rocks in hopes of finding clues about TTG magma and its origin.
“We tracked a specific set of trace elements that were unaffected by alteration and preserved intact traces from the original magma that formed the new TTG crust,” said Dr. said Dr. Matthijs Smit, Canada Research Chair. UBC) Department of Earth, Ocean, and Atmospheric Sciences. “Thanks to these elements, we were able to trace back the chemical changes that the TTG magma undergoes and trace the melt composition back to its initial state and source, probably a type of gabbro.”
“Interestingly, many people use this type of stone as countertops in their kitchens,” says Dr. Smit. “In a sense, many of us are preparing our dinner on the types of rocks that were involved in the formation of modern continents.”
Much of the continental crust that formed during the Archean era is still part of the continent today, as seen in the Rocky Mountains of Banff National Park in Canada.Credit: Golgo
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Much of the continental crust that formed during the Archean era is still part of the continent today, as seen in the Rocky Mountains of Banff National Park in Canada.Credit: Golgo
The Archean TTG crust is still part of the continent today. For example, in North America, it occupies most of the interior of Canada between the Cordillera Mountain Range in the west and the Grenville and Appalachian Mountains in the east. Most of Ontario, Quebec, Manitoba, Saskatchewan, the Northwest Territories, and Nunavut are composed of Archean crustal debris dominated by TTG and its slightly younger, more evolved granitic counterpart. Masu.
“All of these rocks, and especially their combinations, can be explained by the model we have presented,” Dr. Smit said. “Our model is a simple one in which TTGs and the young rocks with which they are typically associated probably arose through slow burial, thickening, and melting of a precursor crust similar to an oceanic plateau, whereas continental crust , was destined to develop its method, because it had no choice but to continue to be buried further and the rocks at its base to melt. In doing so, they created a winning recipe for the continent’s survival and growth. We created TTG that proved that.”
UBC researchers’ discovery of an independent “intracrustal” mechanism producing TTGs dispels long-held theories that Archean TTGs formed in Earth’s first subduction zones and marked the beginning of plate tectonics. It is.
“There has always been a chicken-and-egg question of which came first: the beginning of plate tectonics or the TTG magmatism that created new continental crust,” Smit says. “We show that these things may not actually be directly related. Recognizing the source rock type makes this leap possible, and it also helps that meteorites There is also no need to consider other mechanisms to explain the growth of the first real continents, such as collisions.
The study by Dr. Smit and his UBC-based team revealed that all TTG samples analyzed to date, from fragments of Archean cratons exposed worldwide by researchers over the past 30 years, Sample data was used. This allowed Dr. Smit and his team to rule out local anomalies and analytical problems and see the actual trends in composition that the rocks were capturing. The study used the vast amount of data currently available in the open-source geochemical data repository Geochemistry of Rocks of the Oceans and Continents hosted by the Georg-August University of Göttingen.
For more information:
Matthijs A. Smit et al, Archean continental crust formed from mafic sediments, nature communications (2024). DOI: 10.1038/s41467-024-44849-4