General characteristics of greenstone belts:

• North American, South African, Australian and Indian cratons all display greenstone belts.
• they often occur in arrays of parallel greenstone belts, separated by tonalite-metamorphic realms.
• they are mostly Archean in age, but with some Proterozoic versions.
• average dimensions - 20-100 km wide, and up to 100s of kms long.
• structurally they are primarily synclinoriums with steep foliations.
• contacts are usually synkinematic plutons with a dynamic intrusive style, but in S. Africa see that part of the greenstone sequence was deposited on basement rocks.
• polyphase deformation common. Some argue that with more detailed mapping can work out internal thrust stacks, suggesting significant horizontal shortening.
• filled with volcanics and sediments in manner suggesting they represent a long term basin, one deep enough to receive turbidites.
• greenschist (hence the color and the name) to amphibolite metamorphic facies, with a decrease of grade into the interior of the belt.
• while an individual greenstone belt may not be unique to the Precambrian, the formation of parallel arrays of them is!

Internal stratigraphy of greenstone belts:

• 10-20 km thick sections described (real or due to tectonic repetition?).
• 2 common units:
  • upper sedimentary unit: usually 15-30% of total section. Made up of graywackes, cherts, argillites, BIFs, with some arkose and cg at very top.
  • lower volcanic unit: see an overall evolution in the volcanic pile with a lower bimodal suite and an upper calc-alkaline suite. The lower suite often has pillows and can have komatiites, with minor rhyolites. Higher up andesites and dacites exist. Evidence of cyclicity.

Some examples and images of components:

Barberton belt in S. Africa is a classic and well studied example of a greenstone belt. Deformation and metamorphism or less intense here so the earlier basin history is clearer.

Eli greenstone belt of Minnesota: see below images and text.

This is an image of a hypabyssal and phenocrystic dacitic intrusion in part of the Eli greenstone belt. To the left are the fine-grained metasediments the dacite intruded into. Note the small xenoliths in the dacite. Intermediate composition volcanics (dacites and andesites) are a common rock type in the area. Metamorphism is relatively mild and original igneous textures are preserved.

This is a glaciated flatrock exposure from the Eli greenstone belt literally just down the road from the above outcrop. Exposed are graded volcanic wackes layers intercalated with muds, suggesting these are turbidites in a basin next to a volcanic chain. They are upright here and elsewhere show evidence of both soft-sediment and tectonic deformation. One can also see a nice branching fault termination here.

Models for greenstone belts

Are they collapsed impact basins?

Are they accreted arcs?

• volc. signature is estimated at 57% tholeiitic, 38% calc-alkaline, and 5% alkalic on average.
• perhaps a primitive arc? Modern arcs and greenstone belts do have similar REE pattern.
• doesn't explain initiation and the long history as a basin.

Are they collapsed rear-arc basins?

• Mesozoic Rocas Verdes may represent younger analog in southern tip of S. America.
• Carolina terrane in the southern Appalachians somewhat similar.
• would explain the basin and tholeiite aspects.
• would also explain the collapse and subsequent tonalite/granodiorite intrusions as collapse of basin and reestablishment of arc magmatism.
• why not more common in the Paleozoic? Why in parallel arrays, common age?

Are they collapsed continental rifts?

• idea would be they are rifts but on thinner, hotter continental crust and in a permobile regime so that collapse and subsequent intrusion would cause the pervasive deformation that is seen. The craton would thicken, cool and rigidify during and after this.
• would explain the basin history, but not the calc-alkaline volcanic component.

Are they polygenetic?

Course materials for Plate Tectonics, GEOL 3700, University of Nebraska at Omaha. Instructor: H. D. Maher Jr., copyright. This material may be used for non-profit educational purposes with appropriate attribution of authorship. Otherwise please contact author.