Further Reading

Dawson, J. B. Kimberlites and theirXenoliths.

New York: Springer-Verlag, 1980. Gemological Institute of America homepage. Available online. URL: www.gia.edu. Accessed January 14, 2009. Mitchell, R. H. Kimberlites, Mineralogy, Geochemistry, and Petrology. New York: Plenum Press, 1989.

ophiolite has structurally disrupted (faulted) pillow lavas, mafic flows, breccia, and chert overlying a mixed dike and gabbro section that grades down into layered gabbro, cumulate ultramafics, and mantle peridotites. High-temperature mantle fabrics and ophiolitic mantle podiform chromitites have also been documented from the Dongwanzi ophiolite, and it has ophiolitic mélange intruded by arc magmas.

Dismembered ophiolites appear to be a widespread component of greenstone belts in Archean cratons, and many of these apparently formed as the upper parts of Archean oceanic crust. Most of these are interpreted to have been accreted within forearc and intra-arc tectonic settings. The observation that Archean greenstone belts have such an abundance of accreted ophiolitic fragments compared to Phanero-zoic orogens suggests 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.

OCEANIC PLATEAUx ACCRETION

oceanic plateaux are thicker than normal oceanic crust formed at midocean ridges; they are more buoyant and relatively unsubductable, forming potential sources of accreted oceanic material to the continental crust at convergent plate boundaries. Accretion of oceanic plateaux has been proposed as a mechanism of crustal growth in a number of orogenic belts, including Archean, Proterozoic, and Phanerozoic examples. oceanic plateaux are interpreted to form from plumes or plume heads that come from the lower mantle (D") or the 415-mile (670-km) discontinuity, and they may occur either within the interior of plates or interact with the upper mantle con-vective/magmatic system and occur along midocean ridges. oceanic plateaux may be sites of komatiite formation preserved in Phanerozoic through Archean mountain belts, based on a correlation of allochtho-nous komatiites and high-Mgo lavas of Gorgona Island, Curaçao, and the Romeral fault zone, with the Cretaceous Caribbean oceanic plateau.

Portions of several komatiite-bearing Archean greenstone belts have been interpreted as pieces of dismembered Archean oceanic plateaux. For instance, parts of several greenstone belts in the southern Zimbabwe craton are allochthonous and show a similar magmatic sequence, including a lower komatiitic unit overlain by several kilometers of tho-leiitic pillow basalts. These may represent a circa 2.7-Ga oceanic plateau dismembered during a collision between the passive margin sequence developed on the southern margin of the Zimbabwe craton and an exotic crustal fragment preserved south of the suturelike umtali line.

The accretion of oceanic plateaux and normal oceanic crust in arc environments may cause a back-stepping of the subduction zone. As the accretionary complex grows, it is overprinted by calc-alkaline magmatism as the arc migrates through the former subduction complex. Further magmatic and structural events can be caused by late-ridge subduction and strike-slip segmentation of the arc. Average geo-chemical compositions of the continental crust, however, are not consistent with ocean plateau accretion alone.

Parts of many Archean, Proterozoic, and Pha-nerozoic greenstone belts interpreted as oceanic plateau fragments are overprinted by arc magmatism, suggesting that they either formed the basement of intraoceanic island arcs or they have been intruded by arc magmas following their accretion. Perhaps the upper and lower continental crusts have grown through the accretion of oceanic island arcs and ocean plateaus, respectively. Accreted oceanic plateaux may form a significant component of the continental crust, although most are structurally disrupted and overprinted by arc magmatism.

BACK ARC BASIN ACCRETION

The formation, closure, and preservation of back arc basin sequences has proven to be a popular model for the evolution of some greenstone belts. Paradoxically, the dominance of buoyant subduction styles in the Archean should have led to dominantly compres-sional arc systems, but many workers suggest back arc basins (which form in extensional arcs) as a modern analog for Archean greenstone belts.

ARC-TRENCH MIGRATION AND ACCRETIONARY OROGENS; A PARADIGM FOR THE ARCHEAN

Turkic-type accretionary orogens are large, subcontinent-size accretionary complexes built on one or two of the colliding continents before collision, through which magmatic arc axes have migrated, and are later displaced by strike-slip faulting. These accretionary wedges are typically built of belts of flysch, disrupted flysch and mélange, and accreted ophiolites, plateaux, and juvenile island arcs. In 1996 A. m. Celal Sengor and Boris Natal'n reviewed the geology of several Phanerozoic and Precambrian orogens and concluded that Turkic or accretion-ary-type orogeny is one of the principal builders of continental crust with time. The record of Archean granite-greenstone terranes typically shows important early accretionary phases followed by intrusion by arc magmatism, possibly related to the migration of magmatic fronts through large accretionary complexes. In examples like the superior Province, many subparallel belts of accreted material are located between continental fragments separated by many hundreds of miles, and thus may represent large accretionary complexes that formed prior to a "Turkic-type" collision. Late-stage strike-slip faulting is important in these Archean orogens, as in the Altaids and Nipponides, and may be partly responsible for the complexity and repetition of belts of similar character across these orogens.

Turkic or accretionary-type of orogeny provides a good paradigm for continental growth. These orogenic belts possess very large sutures (up to several hundred kilometers wide) characterized by subduction-accretion complexes and arc-derived granitoid intrusions, similar to the Circum-Pacific accreted terranes (e.g., Alaska, Japan). These sub-duction-accretion complexes are composed of tec-tonically juxtaposed fragments of island arcs, back arc basins, ocean islands/plateaux, trench turbidites, and microcontinents. Turkic or accretionary-type orogens may also experience late-stage extension associated with gravitational collapse of the orogen, especially in association with late collisional events that thicken the crust in the internal parts of the orogen. In the Archean slightly higher mantle temperatures may have reduced the possible height that mountains would have reached before the strength of deep-seated rocks was exceeded, so that extensional collapse would have occurred at crustal thickness lower than those of the younger geological record. Another important feature of these orogens is the common occurrence of orogen parallel strike-slip fault systems, which resulted in lateral stacking and bifurcating lithological domains. In these respects the accretionary-type orogeny may be considered as a unified accretionary model for the growth of the continental crust.

LATE-STAGE GRANITES AND CRATONIZATION

Archean cratons are ubiquitously intruded by late- to post-kinematic granitoid plutons, which may play a role in or be the result of some process that has led to the stabilization or "cratonization" of these terranes and their preservation as continental crust. Most cratons also have a thick mantle root or tec-tosphere, characterized by a refractory composition (depleted in a basaltic component), relatively cold temperatures, high flexural rigidity, and high shear wave velocities.

Outward growth and accretion in granite-greenstone terranes provides a framework for the successive underplating of the lower parts of depleted slabs of oceanic lithosphere, particularly if some of the upper sections of oceanic crust are offscraped and accreted, to be preserved as greenstone belts or eroded to form belts of greywacke turbidites. These underplated slabs of depleted oceanic lithosphere will be cold and compositionally buoyant compared with surrounding asthenosphere (providing that the basalt is offscraped and not subducted and converted to eclogite) and may contribute to the formation of cratonic roots. one of the major differences between Archean and younger accretionary orogens is that Archean subducted slabs were domi-nantly buoyant relative to the dense mantle, whereas younger slabs were not. This may be a result of the changing igneous stratigraphy of oceanic lithosphere, resulting from a reduction in heat flow with time,

Idealized cross section of craton, showing thick mantle root

perhaps explaining why Archean cratons have thick roots and are relatively undeformable compared with their younger counterparts. Geometric aspects of underplating these slabs predict that they will trap suprasubduction mantle wedges of more fertile and hydrated mantle, from which later generations of basalt can be generated.

Many granites in Archean terranes appear to be associated with crustal thickening and anatexis during late stages of collision. However, some late-stage granitoids may directly result from decompressional melting associated with upper-crustal extensional collapse of Archean orogens thickened beyond their limit to support thick crustal sections, as determined by the strength of deep-seated rocks. Decompressional melting generates, from the trapped wedges of fertile mantle, basaltic melts that intrude and partially melt the lower crust. The melts assimilate lower crust, become more silicic in composition, and migrate upward to solidify in the mid to upper crust, as the late to postkinematic granitoid suite. In this model the tectosphere (or mantle root) becomes less dense (compositionally buoyant) and colder than surrounding asthenosphere, and this makes it a stable cratonic root that shields the crust from further deformation.

Late-stage strike-slip faults that cut many Archean cratons may also play an important role in craton stabilization. Specifically the steep shear zones may provide conduits for massive fluid remobiliza-tion and escape from the subcontinental lithospheric mantle, which would both stabilize the cratonic roots of the craton and initiate large-scale granite emplacement into the mid and upper crust.

See also Archean; greenstone belts; orogeny; Precambrian.

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