18.3: Rocks found in greenstone belts

Rock types in greenstone belts are of both igneous and sedimentary varieties. In general, both tend to be lightly metamorphosed (greenschist facies). Here we will examine the igneous ones first, both volcanic and plutonic, and then move on to the sedimentary ones.

IGNEOUS

Basalt and komatiite

Figure \(\PageIndex{1}\): Archean pillow “basalts” near Ely, Minnesota, showing a greenschist overprint (including a prominent tectonic cleavage). It’s a palimpsest rock! (Callan Bentley photo)

Volcanism and plutonism have been themes in Earth history from very early times. In the fact that they show evidence of the production of magma and lava, Archean greenstone belts are not that different from modern oceanic ridge systems.

Basalt was a common rock then as it is today. Greenstone belts get their very name from this prominent rock, which served as protolith for the low-grade metamorphic rock called greenstone. Then, as now, basalt erupted as mafic lava onto or near Earth’s surface, cooling rapidly to make a fine-grained igneous rock. We know there must have been at least some water present, because greenstone belt basalts include pillow structures, features that form when basaltic lava erupts underwater. Other primary volcanic structures include vesicles, the “fossil gas bubbles” that formed as exsolved gases were trapped in solidifying lava.

Figure \(\PageIndex{2}\): A vesicle “train” within a pillow structure from greenstone (meta-basalt) from the Superior Craton, near Ely, Minnesota. Pen for scale. (Callan Bentley photo)

While basalts in greenstone belts looks more or less like basalts forming today, that’s not true for komatiite, an ultramafic volcanic rock. It is named for the Komati Valley of South Africa, a river valley in the area of the Barberton Greenstone Belt. Komatiite is often cited as an example of an “extinct rock,” since it hardly ever forms on Earth any more. The reason for its formation in the Archean but not today is temperature. Komatiite is dominated by the mineral olivine, and olivine has a very high crystallization temperature, around 1500 \(^{\circ}\)C. Lavas that hot don’t erupt on our planet today; a more typical temperature for a modern mafic lava (like the Kilauea volcano in Hawaii) would be around 1100-1200 \(^{\circ}\)C.

Figure \(\PageIndex{3}\): Heat production was much higher during the Hadean and Archean eons than it is today. There were many more unstable radioactive isotopes then, decaying and releasing heat energy into the Earth system. (Callan Bentley graphic)

The fact that we see komatiites in Archean-aged deposits is strong evidence that the early Earth was hot. At least internally, it was hotter than today. Why was this? A few reasons: (1) First, less time had passed since the planet was assembled from an unthinkable number of meteorite impacts, events that instantaneously transformed a very high kinetic energy into heat energy. So it was “still hot,” like a pie recently removed from the oven. It may be cool on the outside, but you’ll burn your mouth if you take a bite! (2) Second, the rate of new heat production was much higher in early Earth history. Less than a billion years after the nebula-forming supernova of Earth’s predecessor star, there were a lot more unstable (radioactive) isotopes around. Remember that [link to numerical dating discussion here] half of a population of radioactive atoms fall apart in the first half-life, followed by a quarter of them in the second half-life, followed by an eighth in the half-life after that, and so on. Totaling all known radioactive isotope sources suggests something like five times as many radioactive decay events in the Archean as today. So there was a much higher level of heat production in the first eons of Earth history than there was in subsequent eons.

Figure \(\PageIndex{4}\): A modern lava lake, or the Hadean Earth? Uniformitarianism says, “Yes.” (From a video by the USGS.)

In summary, not only was the planet hot from its initial smash-up mode of formation, but radiation also kept it “on a hotplate” for its initial stages. Only gradually did that heat naturally get “turned down” over time, as there were fewer and fewer unstable isotopes left to decay and make new heat. The result was that the mantle, and the crust it produced, were both much hotter than they are today. Things were hot enough that ultramafic magma stayed molten all the way to Earth’s surface, and was erupted hot to flow as lava.

Examine this pair of komatiite samples and note two features: (1) the prominent brownish-orange weathering rind, indicating the high iron content of the rock, and (2) the long needle like crystals of olivine, a feature called “spinifex texture.”

Callan Bentley GigaPan

Though komatiite is a volcanic rock (and thus we would expect a mostly fine-grained aphanitic texture), these large crystals of spinifex olivine are a common sight in komatiite outcrops, particularly near the top of the lava flows. Apparently, as the ultramafic lava lost heat, it began to crystallize most rapidly from the top down. Crystals of olivine nucleated on the chilly upper surface of the flow and then grew downward into its molten interior, hanging into the molten rock like chandeliers on the ceiling. Meanwhile, chunkier olivine crystals nucleated within the flow, free floating and untethered. These settled to the floor of the komatiite flow, building up a cumulate layer there.

Figure \(\PageIndex{5}\): Spinifex-shaped olivines nucleate on the underside of a solidified “crust” on the top of an ultramafic lava flow, growing downward into the flow interior, expelling a residual melt. This diagram also shows the development of a cumulate layer of chunkier-shaped olivines at the base of the lava flow. (Callan Bentley cartoon)

Did I Get It? - Quiz

Exercise \(\PageIndex{1}\)

Komatiites are an indication of what aspect of the Archean Earth?

a. The Archean Earth must have been much hotter (internally) than today.

b. The Archean Earth's atmosphere was devoid of free oxygen, unlike today.

c. The Archean Earth was host to many more meteorite impacts than today.

Answer

a. The Archean Earth must have been much hotter (internally) than today.

Exercise \(\PageIndex{2}\)

What is spinifex structure, and how does it form?

a. It's long needle-like growths of olivine that formed on the "ceiling" of komatiitic lava flows.

b. It's fossilized spinifex grass, a common spiky grass that still exists on the modern Earth.

c. It's a description of the overall structure of greenstone belts, with thin "keel" like projections of sedimentary and volcanic rocks between dome-like bodies of granite-like plutonic rocks.

Answer

a. It's long needle-like growths of olivine that formed on the "ceiling" of komatiitic lava flows.

Tonalite, trondjhemite, granodiorite suites

Granite is a coarse-grained, felsic igneous rock. Its coarse grained (phaneritic) texture indicates it cooled slowly, which implies it cooled underground – that is to say, it is intrusive. Granite is a common rock of the modern continental crust. There are granite-like rocks in Archean cratons, too. Technically, these are not precisely granite, but instead a suite of three similar plutonic rocks: tonalite, trondjhemite, and granodiorite. Collectively, this trio of rocks are often called the “TTG” suite. The distinction between TTGs and a true granite is the absence of the mineral potassium feldspar in the TTGs.

Here is an example of one such unit: an outcrop of the Kaap Valley Tonalite, a 3.2 Ga pluton from South Africa’s Kaapvaal Craton. Explore it and and count how many different textural or compositional aspects to the pluton you can find:

Callan Bentley GigaPan

Overall, we think that the TTG plutons originated the way modern granitic plutons originate: through the partial melting of other rocks, “sweating out” a magma of the easiest-to-melt minerals (those at the bottom of Bowen’s reaction series). Those minerals are nonferromagnesian and silica-rich, and the result is a felsic magma. Then as now, those magmas cooled slowly underground, developing a coarse-grained texture.

Did I Get It? - Quiz

Exercise \(\PageIndex{1}\)

What rock type indicates the Archean Earth was hotter than the modern Earth?

a. Tonalite

b. Komatiite

c. Basalt

d. Blueschist

Answer

b. Komatiite

Exercise \(\PageIndex{2}\)

Which rock suite is similar to granite, but lacks potassium feldspar?

a. Banded iron formation

b. Blueschist

c. Komatiite and basalt

d. Tonalite, trondjhemite, and granodiorite

Answer

d. Tonalite, trondjhemite, and granodiorite

SEDIMENTARY

Chert

Interestingly, the oldest Archean sedimentary rocks are not silicliclastic. There are no sandstones. There are no conglomerates. There are no shales. This implies, quite strongly, that there was no land exposed above sea level – no rock exposures available for weathering and erosion. But underwater, there were still chemical sedimentary rocks forming due to mineral precipitation from the ocean. Among the oldest Archean sedimentary rocks is chert.

The Barberton Greenstone Belt’s Buck Reef Chert is one of the most extensive chert deposits in the Archean geologic record. It is thick and covers a wide area, and records an extended period of chemical precipitation of silica from seawater, sometime around 3.4 billion years ago. Examine this boulder of the Buck Reef Chert, and consider its alternating black and white layers, subsequent faulting, veining, and pressure solution, and a modern surface veneer of lichens.

Callan Bentley GigaPan

Did I Get It? - Quiz

Exercise \(\PageIndex{1}\)

What does the (a) presence of chemical chert and (b) the absence of clastic sedimentary rocks imply about conditions during the Archean?

a. There was no land.

b. There were frequent meteorite impacts.

c. Water was shallow.

d. It was hot.

Answer

a. There was no land.

Exercise \(\PageIndex{2}\)

Which of the following features are *primary* aspects of the Buck Reef Chert in the Barberton Greenstone Belt?

a. bedding

b. faults

c. lichens

d. veins

Answer

a. bedding

Banded iron formation

Figure \(\PageIndex{6}\): Deformed banded iron formation, Soudan, Minnesota (Callan Bentley photo)

Banded iron formations (‘BIF’s) are another distinctive kind of chemical sedimentary rocks, known only* from the Archean Eon and subsequent Paleoproterozoic Era. They are chemical sedimentary rocks composed of alternating layers of iron-rich oxides (usually hematite or magnetite) and silica precipitates (chert or jasper). Banded iron formations are the main ore of metallic iron for human use, but in the context of Historical Geology, the thing that’s really interesting about BIFs is that they are a signature of radically different ocean chemistry, and therefore also imply a radically different atmospheric chemistry.

Specifically, BIFs imply a world without widespread free oxygen. This is hard for us to imagine, considering that oxygen makes up around 21% of Earth’s modern atmosphere. But oxygen is a highly reactive element, and it likes to form bonds with other atoms. After all, about half of Earth’s crust is oxygen, it’s just bonded to silicon and other elements to make up minerals.

Callan Bentley GigaPan

As to the rhythmic nature of the alteration in BIF’s internal composition, several hypotheses have been proposed. One possibility, the straightforward one, is that these are primary sedimentary layers: bedding, in other words. In that case, a rhythmic alteration in sediment source is required. This sedimentary “rhythm” is potentially tidal, seasonal, or represents fluctuations in the position of the chemocline (a boundary within the water column separating anoxic, iron-rich water below from oxygenated, iron-free water above). However, another possibility is that the original sedimentation was more or less homogeneous, and only later during diagenesis did the silica-rich and iron-rich components separate out from each other into layers.

Regardless of the details, BIF’s mode of formation must have been a chemical compromise between the upwelling waters loaded with dissolved iron and shallow surface waters, pumped full of oxygen by the photosynthetic action of microbes, including those microbial mats we today call stromatolites.

Figure \(\PageIndex{7}\): A simple model for the formation of iron oxide chemical sedimentary deposits (BIF) in a largely anoxic ocean, given inputs of dissolved oxygen as a waste product of photosynthesis in the sunlit shallows. (Callan Bentley cartoon)

Did I Get It? - Quiz

Exercise \(\PageIndex{1}\)

Why are banded iron formations important in geologic history?

a. They imply very cold conditions.

b. They imply a very hot early Earth.

c. They imply an early atmosphere and ocean devoid of free oxygen.

d. They imply violent meteorite impacts.

Answer

c. They imply an early atmosphere and ocean devoid of free oxygen.

Turbidites of graywacke and shale

Figure \(\PageIndex{8}\): Graded bedding in Lake Vermilion Formation graywacke, Minnesota (Callan Bentley photo)

Later in the Archean, enough land had risen above sea level that significant quantities of siliciclastic sediments were produced: mud, sand, and pebbles. Turbidity currents flowed into deep-water marine basins of the Archean, adjacent to significant sources of clastic sediment such as proto-continental terranes. Then, as now, these turbidity currents transported a slurry of sediments of different sizes. As the current slowed, its capacity dropped, and sedimentary grains settled out, in order of their weight. The biggest ones tended to be the heaviest ones, and settled out first, followed by progressively more and more lightweight grains (which tended to be smaller). The result was a graded bed: coarse at the bottom, fine at the top. Using this pattern, can you tell which way is stratigraphic “up” in the graded bedding photo here?

Shallow-water sandstone with microbially-induced sedimentary structures

In shallower water, in river deltas and nearshore settings, there were better-sorted, more mature sand deposits, dominated by quartz sand. These sand deposits record evidence of widespread microbial mats, preserved as thin, sediment-binding dark layers called “microbially-induced sedimentary structures” (MISS). These are essentially the siliciclastic version of stromatolites. Here’s an example from Barberton, seen in cross-section:

Callan Bentley GigaPan

Figure \(\PageIndex{9}\): Taphonomy of a microbially-induced sedimentary structure: a 3 frame sequence showing (1) a modern mat, freshly ripped, rotated a bit, and wrinkled, then (2) the same structure a week later after desiccation, and finally (3) a fossil example from the Mesoarchean Pongola Supergroup. All three examples are about 40 cm in diameter. (Photographs by Nora Noffke; reproduced with permission.)

Next, consider an example from the Pongola Supergroup in Africa, as viewed on the bedding plane, and compared with a modern analogue. Many MISS show soft-sediment (and soft-mat!) deformational features such as rips and tears, wrinkles, and “elephant skin texture.” Sometimes they are rolled up, or folded back on themselves.

MISS show us that microbial mats were a key feature in Archean sedimentary systems, helping to bind sediment in place. They also show us that at least one aspect of the biology on Earth has been consistent for billions of years.

Did I Get It? - Quiz

Exercise \(\PageIndex{1}\)

The appearance of clastic sedimentary rocks in younger Archean strata implies which of the following?

a. The presence of sufficient land exposing rocks that can be weathered.

b. Frequent, violent meteorite impacts.

c. Very low levels of free oxygen in the early Earth atmosphere.

d. High internal temperatures for the early Earth.

Answer

a. The presence of sufficient land exposing rocks that can be weathered.

Exercise \(\PageIndex{2}\)

Are there any fossils in Archean clastic sedimentary rocks?

a. Yes, there are dinosaur fossils in Archean sedimentary rocks.

b. Yes: microbially-induced sedimentary structures show microbial mats binding sedimentary grains together.

c. No, there are no fossils in Archean rocks at all.

Answer

b. Yes: microbially-induced sedimentary structures show microbial mats binding sedimentary grains together.

Conglomerates

Figure \(\PageIndex{10}\): Strained metaconglomerate of the Seine Group, Ontario. Note the different styles of deformation exhibited by different clasts in this smeared-out rock. (Callan Bentley photo)

Some Archean sediments are very similar to modern day conglomerates – full of cobbles and pebbles and sand, that were deposited by fast-moving river currents. Based on their size, sorting, and the rounding of the cobbles in them, we think they accumulated in modestly-sized wrench basins formed in releasing bends along the edges of transpressional embryonic orogens. The clasts that make up these conglomerates tend to be volcanic in origin, as one might expect for a hot, young, immature planet. As small proto-continental masses were colliding with one another along what we today call greenstone belts, dilational wrench basins opened up, ready to receive the sloughed-off clastic detritus of the primordial mountains.

Some Archean conglomerates contain sand grains and pebbles of pyrite. These detrital pyrites are primary sedimentary grains, not some later manifestation of metamorphism. While pyrite exists as a mineral in the modern world, we never see it as a major constituent of sediment, because it rapidly rusts (oxidizes) in the planet’s current oxygen-rich atmosphere. The fact that pyrite grains could tumble along, getting physically rounded but not chemically weathered is a surefire signal that the atmosphere of the Archean Earth was decidedly oxygen-poor. In this way, the “testimony” of the detrital pyrites corroborates the testimony of the banded iron formations we discussed earlier.

Figure \(\PageIndex{11}\): Detrital pyrite in conglomerate of the Steyn Reef Placer, Witswatersrand Supergroup of South Africa. (Photo by Frances Deegan; reproduced with permission.)

Meteorite impact strata

Figure \(\PageIndex{12}\): Meteorite impact melt hosting inclusions of pre-melt granite, from the 2.02 Ga Vredefort Impact Structure of South Africa. (Callan Bentley photo)

Another unusual aspect of really old sedimentary rocks is that they record a fair number of violent meteorite impact related rocks. Theoretically, we would expect more meteorite impacts in early Earth history than today, since then the Archean was much closer in time to the planet’s accretionary formation. Since then, we’ve had more than three billion additional years of the planets’ gravity cleaning stray chunks of space rock out of our solar system. in other words, we expect the number of meteorite impacts to decrease through time.

But during the early Archean, even though the Late Heavy Bombardment had concluded, impacts were still fairly frequent events. Fortunately, we can now begin to read the story of their impact because unlike the Hadean, a rock record still persists from the Archean. The record of impacts is recorded in rocks that come in several varieties: ejecta and fallout as well as jumbled and disturbed sedimentary layers deposited by tsunamis, rocks that record perturbations in ocean chemistry due to the impact, and also rock melted directly by the collision, called suevite.

A famous example of suevite can be found in the Vredefort Impact Structure in South Africa, and and even older example occurs in the Sudbury Impact Structure of southern Ontario in Canada. In both cases, clasts of pre-impact basement rocks (granites, mostly) are found ripped up and “floating” in the glassy suevite.

Events energetic enough to melt rock also blasted rock high into the sky, generating energetic clouds of suspended ash and water vapor, violently twirling around (generating static electricity). Clumps of ashy particles formed, and grew layer upon layer in the manner that hailstones do. We call these odd little sphere-shaped structures “spherules” and they are present in sedimentary layers of the time. Violent volcanic eruptions (of ultramafic komatiitic composition) also produces little “volcanic hailstones” in the same fashion, though it didn’t take a meteorite impact to make them happen. Their similar-shaped (and sized) structures are called “accretionary lapilli” rather than spherules, but they are essentially the same thing. Compare the two (both Archean, from the Barberton Greenstone Belt) below:

Callan Bentley GigaPans

Meteorite impacts, being violent events on an Archean planet without true continents, were very likely to happen in the oceans, which generated tsunamis. These high-energy events ripped up pre-existing strata and mixed them with spherules, redepositing the mix as energy conditions abated and the waters calmed, as seen in the virtual sample below. See if you can find a foliated chunk of basement rock, an angular chunk of chert, and a spherule...

Callan Bentley GigaPan

Another consquence of crust-piercing, ocean-churning impacts was to alter the chemistry of the oceans radically. One interesting feature associated with impact-origin strata are odd chemical sedimentary deposits of barite, a mineral rich in the element barium. Note that these strata show bedding-perpendicular oriented crystal forms that are interpreted as nucleating on the ocean floor and growing ~vertically upward into the water column. Ocean chemistry must have been seriously weird to permit such odd layers:

Callan Bentley GigaPan

Did I Get It? - Quiz

Exercise \(\PageIndex{1}\)

Which of the following is cited as evidence for frequent large meteorite impacts during the Archean?

a. Barite layers

b. Tsunami deposits with angular chert chunks, sand, and spherules

c. Spherules (accretionary lapilli)

d. All of these answers are correct

e. Circular- or ring- shaped deposits of suevite including chunks of basement rocks

Answer

d. All of these answers are correct

METAMORPHIC

Greenstone

Figure \(\PageIndex{13}\): A plot showing the various metamorphic facies that result from different combinations of temperature and pressure. Note that blueschist and eclogite result from relatively low-temperature but relatively high-pressure metamorphism: conditions we would expect in a subduction zone. Greenschist reflects relatively low-temperature/low-pressure conditions. (Callan Bentley graphic)

As their name implies, greenstone belts have seen relatively light levels of metamorphism: merely “greenschist facies,” which is sufficiently hot and high-pressure to transform basalt to greenstone (or its foliated equivalent, greenschist) but this allows certain primary features such as vesicles, pillows, and cooling columns to persist. There are no high temperature+pressure metamorphic rocks (e.g., amphibolite, granulite), nor subduction related blueschists (high pressure and relatively low temperature).

Because the volcanic and sedimentary packages of greenstone belts were protected by the surrounding buffers of “granitic” material (see next section), they were never subjected to proper modern-style mountain-building or subduction, and this is a good thing from the perspective of the historical geologist, as it has allowed subtle, delicate primary features to survive, relatively undisturbed, for billions of years! If this hadn’t been the case, we would today find completely recrystallized metamorphic rocks, and instead we can see primary features such as these pillow basalts in Michigan:

Ron Schott GigaPan

Did I Get It? - Quiz

Exercise \(\PageIndex{1}\)

True or false: "Greenstone belts are sites of high-grade metamorphism."

a. True

b. False

Answer

b. False