DEFINITION OF GREENSTONE BELTS

Elongate accumulations of generally

mafic volcanic and plutonic rocks, typically associated with

immature graywacke types of sedimentary rocks, banded iron

formations, and less commonly carbonates and mature sedimentary

rocks. Most greenstone belts are Archean or at least

Precambrian in age, although similar sequences are known

from orogenic belts of all ages. Most greenstone belts are

metamorphosed to greenschist through amphibolite facies

and intruded by a variety of granitoid rocks. Older quartzofeldspathic

gneisses are found associated with some greenstone

belts, although most of these are in fault contact with

the greenstones.

Until recently, few complete Phanerozoic-like ophiolite

sequences were recognized in Archean greenstone belts, leading

some workers to the conclusion that no Archean ophiolites

or oceanic crustal fragments are preserved. These ideas

were recently challenged by research documenting partial dismembered

ophiolites in several greenstone belts, and a complete

ophiolite sequence in the North China craton. Archean

oceanic crust was possibly thicker than Proterozoic and

Phanerozoic counterparts, resulting in accretion predominantly

of the upper basaltic section of oceanic crust. The crustal

thickness of Archean oceanic crust may in fact have resembled

modern oceanic plateaux. If this were the case, complete

Phanerozoic-like Mid-Ocean Ridge Basalt (MORB)–type

ophiolite sequences would have been very unlikely to be

accreted or obducted during Archean orogenies. In contrast,

only the upper, pillow lava-dominated sections would likely

be accreted. Archean greenstone belts have an abundance of

accreted ophiolitic fragments compared to Phanerozoic orogens,

suggesting that thick, relatively buoyant, young Archean

oceanic lithosphere may have had a rheological structure

favoring delamination of the uppermost parts during subduction

and collisional events.

Greenstone belts display a wide variety of shapes and

sizes and are distributed asymmetrically across Archean cratons,

in a manner reminiscent of tectonostratigraphic zonations

in Phanerozoic orogens. For instance, the Yilgarn

craton has mostly granitic gneisses in the southwest, mostly

circa 2.9 Ga greenstones throughout the central craton, and

circa 2.7 Ga greenstones in the east. The Slave Province contains

remnants of a circa 4.2–2.9 Ga gneissic terrain in the

western part of the province, dominantly mafic greenstone

belts in the center, and circa 2.68 Ga mixed mafic, intermediate,

and felsic calc-alkaline volcanic rocks in the eastern part

of the province. Other cratons are also asymmetric in this

respect; for example, the Zimbabwe craton has mostly

granitic rocks in the east, and more greenstones in the west.

The Superior Province contains numerous subparallel belts,

up to thousands of kilometers long, which are distinct from

each other, but similar in scale and lithology to “terranes” in

Phanerozoic orogens. These distributions of rock types are

analogous to asymmetric tectonostratigraphic zonations,

which are products of plate tectonics in younger orogenic

belts, and emphasize that greenstone belts are perhaps only

parts of once larger orogenic systems.

There are three significantly different end-member regional

outcrop patterns reflecting the distribution of greenstone

belts within cratons. These include broad domal granitoids

with interdomal greenstones, broad greenstone terrains with

internally bifurcating lithological domains and irregular granitoid

contacts, and long, narrow, and straight greenstone belts.

The first pattern includes mostly granitoid domes with synformal

greenstone belts, which result from either interference

folding or domal/diapiric granitoids. The second pattern

includes many of the terrains with thrust belt patterns, including

much of the Yilgarn Province in Western Australia and the

Slave Province of Canada. Contacts with granitoids are typically

intrusive. The third pattern includes transpressional belts

dominated by late strike-slip shear zones along one or more

sides of the belt. Granite-greenstone contacts are typically a

fault or shear zone.

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