DEFINITION OF LATE EXTENSIONAL COLLAPSE, DIAPIRISM, DENUDATION

It is widely recognized that in Phanerozoic orogens, late stages

of orogenic development are characterized by extensional collapse

of structurally over-thickened crust. In granite-greenstone

terrains, late stages of orogenesis are characterized by

the intrusion of abundant, locally diapiric granitic magmas,

with chemical signatures indicative of crustal anatexis. The

intrusion of late granitic plutons in greenstone terrains may

be related to rapid uplift and crustal melting accompanying

extension in the upper crust. Early plutons of the tonalitetrondhjemite-

gabbro-granodiorite suite are generated in an

island arc setting and are in turn intruded by continental

margin arc magmas after the primitive arcs collide and form

larger continental fragments. When plate collision causes further

crustal thickening, adiabatic melting produces thin

diapiric plutons that rise partway through the crust but will

crystallize before rising very far, as they do not contain

enough heat to melt their way through the crust. The crustal

sections in these collisional orogens gravitationally collapse

when the strength of quartz and olivine can no longer support

the topography. Decompression in the upper mantle and

lower crust related to upper crustal extension generates significant

quantities of basaltic melts. These basaltic melts rise

and partially melt the lower or middle crust, becoming more

silicic by assimilating crustal material. The hybrid magmas

thus formed intrude the middle and upper crust, forming the

late to post-kinematic granitoid suite so common in Archean

granite-greenstone terrains. If the time interval between

crustal thickening and gravitational collapse is short, then

magmas related to decompressional melting may quickly rise

up the partially solidified crystal/mush pathways provided by

the earlier plutons generated during the crustal thickening

phases of orogenesis. Such temporal and spatial relationships

easily account for the common occurrence of composite and

compositionally zoned plutons in Precambrian and younger

orogenic belts.

See also ARCHEAN; CONTINENTAL CRUST; CRATONS.

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