Throwback Thursday: Barberton Greenstone Belt
Between 3.5 and 3.2 Ga years ago, during the Paleoarchean era of the Archean Eon, the Earth didn’t look much like it does today. The atmosphere lacked oxygen meaning there was no protective ozone layer, the oceans, the crust had just formed, oceans were forming, and the first living things (cyanobacteria) were slowly creating free oxygen.
In the midst of all this the Barberton Greenstone Belt was also forming. This belt is found in South Africa east of Pretoria. It is a sequence of mafic to ultramafic lava and metasedimentary rocks (metamorphic rocks that still contain sedimentary structures) that were deposited during the Archean Eon. They are the oldest igneous rocks on earth.
The area underwent two tectonic episodes of terrane accretion, that is two periods of material being added to the tectonic plate at a subduction zone. Imagine it similar to pushing sand in sandbox to form a wall around your sandcastle. This process helped to build new continents throughout geologic time. It is hypothesized that the greenstone belts formed from a passive margin oceanic crust becoming part of a subduction undercut margin.
About 3.1 Ga, a granite batholith was intruded into the belt and partially metamorphosed into gneiss.
Why is it called the greenstone belt? Well, if you recall from earlier lessons (why is magma formed and what is it made of?) mafic and ultramafic rocks are primarily made of olivine, a green mineral. One of the main types of ultramafic rock found here are komatiites, named for the Komati River that flows through the area. It is a mantle-derived volcanic rock with an extremely high magnesium content and very low silicon, potassium and aluminum content.
Komatiites are rare because almost all of them formed during the Archean when it is though the mantle was starting to cool. During this time period, the Earth had much higher heat production simply due to all the residual primordial heat of planetary accretion. In fact, eruption temperatures were thought to be up to 1600 degrees C (2912 degrees F). The lava would have flowed with a viscosity close to water.
They are composed of forsterite (magnesium-rich olivine),
calcic and chromian pyroxenes,
anorthite (calcium feldspar)
They often take on spinifex texture meaning the crystals look like spinifex grass, long and blade-like.
Thanks for coming to today’s lesson! Tune in tomorrow to learn more about Patagotitan! Fossilize you later!
