18.2: It was different in the Archean

We don’t find blueschist in Archean rocks. We don’t find paired metamorphic belts in Archean rocks. It appears, therefore, that plate tectonics was not operating on Earth in that early time. Instead, our oldest rocks appear distinctively different, in both rock type and structure, and they therefore imply a different tectonic regime for Archean Earth.

So what was that “style” of tectonics, and what is the evidence for it? The oldest coherent features we see that can answer this question are called greenstone belts. These are linear or branching zones of distinctive low-grade metasedimentary and metavolcanic rocks. Notable greenstone belts on Earth include examples from the oldest parts of the cratons of Australia, South Africa, Europe, Siberia, Brazil, Antarctica, India, Greenland, China, and northcentral North America. All are included in otherwise fairly homogenous felsic crust of similarly ancient age. We call each of these “continental nuclei” by the term craton. Where the craton’s rock is exposed at Earth’s surface, we call it a “shield.” Areas where the craton is present, but buried beneath a sedimentary blanket, are called “platforms.”

Figure \(\PageIndex{1}\): Map showing the distribution of Precambrian (Archean + Proterozoic) crust on Earth. (Callan Bentley graphic)

Examples of cratons include the Pilbara Craton of Western Australia, the Kaapvaal Craton of South Africa, and the Superior Craton of the U.S. and Canada. The greenstone belts are typically sandwiched between large bodies of crustal rocks such as granite and gneiss. Not all greenstone belts are Archean; some are also known from the Proterozoic. But the Proterozoic’s tectonic record is more diverse, so for the sake of this case study, we will focus on the Archean.

The greenstone belts are interpreted as the remains of ancient ocean basins, which were squished between ancient proto-continental terranes, in the same sense as how in modern plate tectonics, ophiolites mark the position of former ocean basins. The process of partial melting had repeatedly “distilled” the silicate crust of the planet into small mobile blobs, roughly the size of modern U.S. counties or states, and once solidified, these resisted subduction due to their low density. They drifted about, approaching other similar granitic terranes, with the intervening ocean basin sagging out of the way, straight down below. The more sagging occurred, the more room there was for sediments to accumulate atop the subsiding region, until the neighboring granitic blobs got close enough to compress the rocks between them. Once they collided, they often stayed stuck together, resulting in a granite/greenstone belt/granite horizontal sequence.

Let’s now examine them in more detail, starting with the roster of rocks we find in greenstone belts, and progressing to an overview of their large-scale structure.

Did I Get It? - Quiz

Exercise \(\PageIndex{1}\)

What word describes linear or branching zones of distinctive low-grade metasedimentary and metavolcanic rocks that are found sandwiched between granite "proto-continents" in Archean aged rocks?

a. paired metamorphic belts

b. greenstone belts

c. mountain belts

Answer

b. greenstone belts