Eoarchean geology
Major well-preserved rock units dated to this era are known from three localities, the Isua Greenstone Belt in Southwest Greenland, the Acasta Gneiss in the Slave Craton in Canada, and the Nuvvuagittuq Greenstone Belt in the eastern coast of Hudson Bay in Quebec.
From the dating of rocks in these three regions, scientists suggest that the beginning of plate tectonics could have started as far back as early as the Eoarchean.
[3] The earth's condition during Archean to Proterozoic (including Eoarchean era) serves as a crucial linkage between Hadean magma ocean to present-day plate tectonics.
Most of the evidences shows a probability that pre-plate tectonics dominantly involved intense surface volcanism, active magmatism and crustal recycling.
[9] Between these two chronologically different regions, a thin sedimentary unit lying in the Isua Supracrustal Belt is the dividing boundary.
[13] Measured ages from the tonalites in the northern terrane are between 3720 and 3690 Ma,[2] which is 100 million years younger than those in the southern region.
[9] The Isua Greenstone Belt is currently under heavy investigation as it provides a unique opportunity to study early earth's tectonics.
There is no single widely accepted tectonic explanation for the formation of the Isua supracrustal belt and the adjacent area, although some viable models have been proposed.
These TTGs are between 3720 and 3710 Ma old, with the composition of these relatively juvenile igneous rocks showing that it is sourced from partial melting of eclogitized mafic material, with high magnesium but low silica content.
[11] In addition, the material found in lateral transport thrusts has been recorded from both plume-related volcanic centers and in impact centres.
After the initial documentation of very ancient zircons present in the Acasta River area,[15] a significant scientific debate regarding the true age of these important rocks was born.
[18] This debate culminated in a series of papers and comments regarding the discrepancy between zircon age information and whole-rock data.
[1] Mafic rocks are distributed within the entire Acasta Gneiss Complex as minor blocks such as enclaves and bands.
First, the oldest rocks in the Acasta region, the Idiwhaa Tonalitic Gneiss, shows a distinctive geochemistry of high Fe but low Mg content, and a relatively flat REE pattern.
This led to the idea that the earliest phase of crust formation in the Acasta region occurred by petrologic processes similar to modern Iceland, that is, shallow intrusion of dry basalts and partial melting at low pressures.
[26] Other authors, using the Thorium-to-niobium ratio in the amphibolites, suggested that subduction occurred much earlier, closer to 4.0 Ga.[1] The Nuvvuagittuq Greenstone Belt (NGB) is located in Northern Quebec, covering approximately 8 km2 of the Hudson Bay.
[29] The Nuvvuagituq Greenstone Belt is divided into three lithological units:[30][31] The Ujaraaluk unit is an amphibolite enriched in cummingtonite, plagioclase and biotite, and depleted in hornblende, Gabbroic and ultramafic intrusive bodies, and a chemical sedimentary protolith, i.e. banded iron formations (BIF) and banded silicate formations (BSF).
[33] Surrounding tonalites, trondhjemites and granodiorites (TTGs) are the product of partial melting of Hadean Mafic lithologies, which was similar to the informally-named Ujaraaluk unit.
[33] Crustal recycling produced the TTGs surrounding the Nuvvuagituq Belt from arc-like source rocks, i.e. the Ujaraaluk Unit.
A large scale simultaneous accumulation of TTGs and subsequent partial melting only occurs in particular tectonic settings.
[28] This resembles a subduction system in modern plate tectonics, but the geologic evidence is still insufficient to draw direct parallels.
