The Isua Greenstone Belt is a very old rock formation in southwestern Greenland, dating back between 3.7 and 3.8 billion years. It includes volcanic and sedimentary rocks that have been changed by heat and pressure in different ways. This area has the largest known section of Eoarchaean supracrustal rocks on Earth. Because of its great age and relatively low level of metamorphism compared to other Eoarchaean rocks, the Isua Greenstone Belt is an important area for studying the early development of life and the tectonic processes that shaped the Earth's surface billions of years ago.
Overview
The Isua Greenstone Belt, also called the Isua supracrustal belt because it is mainly made of supracrustal rocks, is found in southwestern Greenland, within the Isukasia terrane, near the Nuuk capital area. It is the largest supracrustal area within the Itsaq Gneiss Complex, which is mostly made of felsic orthogneisses that are 3850 to 3600 million years old (Ma). The greenstone belt includes two main groups of metamorphosed mafic volcanic and sedimentary rocks. These groups were identified using zircon uranium-lead dating. The first group is the "southern terrane," which is about 3800 Ma old, and the second group is the "northern terrane," which is about 3700 Ma old. The younger southern terrane is further split into two parts: one mainly made of boninite-like metavolcanic rocks, and the other made of tholeiitic and picritic metavolcanic rocks. The Isua Greenstone Belt is bordered to the west by the Ivinnguit Fault, which separates the Eoarchaean Itsaq Gneiss Complex from younger (Mesoarchaean) rocks in the Akia Terrane. Elsewhere, it is surrounded by felsic orthogneisses from the Itsaq Gneiss Complex. These gneisses show a similar age pattern to the supracrustal rocks in the Isua Greenstone Belt, with 3800 Ma gneisses to the south of the belt and 3700 Ma gneisses to the north.
Many geological and geochemical methods have been used to study the rocks in the Isua Greenstone Belt. These include dividing the different rock types and units in the belt using geological mapping and U-Pb zircon dating, often with sensitive high-resolution ion microprobe (SHRIMP) analysis. Other methods include analyzing major and trace element chemistry, studying rock structures, using geothermobarometry and metamorphic modeling with phase diagrams to determine metamorphic conditions, and applying a variety of stable, radiogenic, and short-lived isotope systems.
Lithologies
The Isua Greenstone Belt includes many types of rocks. The most common rocks are dark-colored volcanic rocks that have been changed by heat and pressure, with compositions ranging from those similar to boninite to tholeiites and picrites. Although some dark-colored rocks at Isua are sometimes thought to show signs of plate tectonics, these rocks are not true boninites. Other explanations, not involving plate tectonics, can also explain their formation. These volcanic rocks often include pillow-shaped lava flows and broken pieces of pillow-shaped rocks, which suggest the lava erupted underwater and that surface water existed during the very early Earth history. Lighter-colored volcanic rocks have also been found, but it is unclear if these are volcanic or sedimentary rocks. Only a few examples of andesite-like rocks have been identified, but they are heavily weathered.
The dark volcanic rock layers contain many rocks that were once ultramafic (very high in magnesium and iron) but have been changed by heat and pressure, including amphibolites, serpentinites, carbonated peridotites, and peridotite. Most of these rocks are believed to have formed from magma that solidified underground, creating layers of minerals. Some peridotite layers are thought to be pieces of Earth's mantle that were pushed up from deep underground, used as evidence that plate tectonics was active during the formation of the Isua Greenstone Belt. However, some scientists disagree, arguing that all peridotites at Isua formed from magma that solidified near the surface, connected to the volcanic layers.
Metasedimentary rocks, such as banded iron formations and quartz-rich rocks, likely formed from sedimentary rocks that were later changed by heat and pressure. These rocks are not part of the main layer of rocks above the Earth's crust but are found within and sometimes intruded by a type of igneous rock called tonalite-trondhjemite-granodiorite (TTG) orthogneisses.
Tectonics
The tectonic setting where the Isua Greenstone Belt formed is still debated. Scientists divide ideas into two main groups: plate tectonic models, which suggest the belt formed in a setting similar to modern Earth, and non-plate tectonic models, which suggest it formed in a setting different from today’s Earth. Plate tectonic models can be further split into those that claim the belt or parts of it represent an ophiolite, a piece of oceanic crust and mantle that was pushed onto land, and those that claim it represents an accretionary prism, formed in a subduction zone. Non-plate tectonic models usually suggest the belt formed due to heat from the Earth’s mantle, like a mantle plume. This debate is part of a larger discussion about when plate tectonics began on Earth and whether the early Earth had a different tectonic system.
Furnes et al. (2007) argued that pillow lavas and closely spaced dykes in the belt indicated it was an ophiolite. They believed the dykes were part of a sheeted dyke complex, a feature found in modern ophiolites. However, others disagreed, saying the dykes were actually younger and unrelated to the volcanic rocks. Other objections focused on the dykes’ composition, which did not match those in modern ophiolites.
Despite the debate over sheeted dykes, other evidence supports an ophiolite origin. This includes the geochemistry of volcanic rocks: tholeiitic amphibolites were thought to be metamorphosed island-arc tholeiites, and boninite-like amphibolites were thought to be metamorphosed boninites. Later studies, however, showed the boninite-like amphibolites were actually low-titanium basalts, not boninites. Recent research also suggests the volcanic rock compositions at Isua can be explained without requiring plate tectonics.
Another argument for an ophiolite origin is the presence of peridotite lenses in the volcanic rocks, such as "lens A" and "lens B." These were thought to be mantle rocks based on their geochemistry and mineral composition. If true, their presence would suggest the rocks were thrust to the surface, supporting an ophiolite origin. However, recent studies suggest the lenses are instead ultramafic cumulates formed in magma chambers that fed the volcanic eruptions. If correct, no thrusting was needed, and the lenses do not prove the belt is an ophiolite.
The northeastern part of the belt has been interpreted as an accretionary wedge based on small faults and repeated layers of supracrustal rocks, similar to modern subduction zones. Metamorphic gradients in this area also resemble those in subduction zones. However, this view is contested because rock types and strain are consistent across faults, and metamorphic grades are uniform throughout the belt.
Non-plate tectonic models, such as heat-pipe and mantle plume models, suggest the volcanic rocks formed from mantle-derived magma with little crustal input. In a heat-pipe model, rapid volcanic eruptions remove melt from the mantle, causing the lithosphere to sink and mafic rocks to be buried. These rocks later melt to form TTGs, which are found in the Isua Greenstone Belt. This model explains the mafic composition of sediments and the uniform metamorphism observed. However, critics argue there is no evidence that 3.7 Ga volcanic rocks or TTGs moved upward through 3.8 Ga rocks, as would be expected in a heat-pipe model.
Metamorphism
After its formation, the Isua Greenstone belt experienced two major changes in rock structure and composition. The first change occurred before the formation of the <3.5 Ga Ameralik dykes and is linked to the Eoarchaean deformation at Isua. Amphibolite-facies conditions, which involve high temperatures and pressures, were present across the belt between approximately 3.7 and 3.6 Ga. Some studies suggest higher pressure conditions in certain areas based on Ti-humite minerals found in peridotites, but the accuracy of these findings is debated. The second event also reached amphibolite-facies conditions and was a long-lasting process that occurred between about 2.9 and 2.6 Ga, followed by widespread partial reversal of changes that varied in intensity across the region. These two events make it more difficult to understand the original chemical makeup and structures in the belt.
Possible signs of very early life
The Isua Greenbelt is very old and has been studied for many years to find signs of early life on Earth. In 1996, geologist Steve Mojzsis and his team suggested that carbon-rich layers in the area contained a type of carbon that might have come from living things. They said, "Unless a non-living process can create this type of carbon and then place it in certain rock grains, our findings show that life on Earth existed at least 3,800 million years ago."
In August 2016, a research team from Australia found evidence that the Isua Greenstone Belt may contain the remains of stromatolite microbial colonies, which are layered rock structures formed by ancient microorganisms. These stromatolites are estimated to be about 3.7 billion years old. However, scientists disagree about whether these structures are truly stromatolites. If they are, they would be older than the previously known oldest stromatolites, which were found in the Dresser Formation in western Australia, by about 220 million years.
If the stromatolites at Isua are real, their complex shapes suggest that life on Earth was already advanced and strong by the time they formed. This idea is supported by the fact that Earth's surface was unstable 3.7 billion years ago, with frequent asteroid impacts. The possibility that fossils from this time were preserved shows that life may have developed early and widely on Earth.
The stromatolite fossils found at Isua are wavy and dome-shaped, usually 1–4 cm (0.4–1.6 in) tall. They were discovered in rocks rich in iron and magnesium, which had recently been exposed by melting snow. The surrounding rocks suggest the stromatolites may have formed in a shallow ocean environment. While most rocks in the Isua Greenstone Belt are too changed by heat and pressure to preserve fossils, the area where the stromatolites were found may have kept original sedimentary rocks and the fossils inside them. However, some scientists believe the structures formed from changes to the original rock over time.
The sedimentary layers in Isua that may contain stromatolites are found above volcanic rocks dated to 3.709 billion years old. These layers are covered by dolomite and banded iron formations with zircons dated to 3.695 ± 0.4 billion years old. All layers, including those near the stromatolites, were changed by heat and pressure after forming, but temperatures never reached more than 550 °C (1,000 °F).
Scientists are unsure if the features in Isua are stromatolites because similar structures can form through non-living processes. Some researchers believe the textures above the possible stromatolites are caused by sand collecting against them during their formation, suggesting the structures formed during sedimentary processes rather than later changes. Others argue that the rocks are too altered to support sedimentary interpretations.
In 2016, geologist Abigail Allwood said finding stromatolites in Isua makes it more likely that life could have developed on other planets, like Mars, soon after they formed. However, in 2018, she and other scientists published a study that questioned whether the structures formed from biological processes or from changes to the rock. Because of this, the stromatolites in Isua remain a topic of continued scientific study.