HR: 08:35h
AN: T21E-03 INVITED     [Abstracts]
TI: Archaean Greenstone Belt Architecture and Stratigraphy: are Comparisons With Ophiolites and Oceanic Plateaux Valid?
AU: * Bedard, J H
EM: jean.bedard@nrcan.gc.ca
AF: Geological Survey of Canada, 490 de la Couronne, Quebec, PQ G1K9A9 Canada
AU: Bleeker, W
T21E-03 AF: Geological Survey of Canada, 601 Booth St., Ottawa, Ont K1A 0E9 Canada
AU: Leclerc, F
T21E-03 AF: INRS-ETE, 490 de la Couronne, Quebec, PQ G1K9A9 Canada
AB: Archaean greenstone belts and coeval plutonic belts (dominated by TTGs, tonalite-tronhjemite-granodiorite), are commonly interpreted to represent assembled fragments of oceanic crust, oceanic plateaux or juvenile arc terranes, variably reworked by Archaean orogenic processes related to the operation of plate tectonics. However, many of the lava successions that have been interpreted to represent accreted oceanic plateaux are demonstrably ensialic, can be correlated over long distances along-strike, have depositional contacts onto older continental crustal rocks, show tholeiitic to calc-alkaline cyclicity, and have isotopic signatures indicating assimilation of older felsic crust. Inferred Archaean ophiolites do not have sheeted dyke complexes or associated mantle rocks, and cannot be proven to be oceanic terranes formed by seafloor-spreading. Archaean supracrustal sequences are typically dominated by tholeiitic to komatiitic lavas, typically interpreted to represent the products of decompression melting of mantle plumes. Subordinate proportions of andesites, dacites and rhyolites also occur, and these, together with the coeval TTGs, are generally interpreted to represent arc magmas. In the context of uniformitarian interpretations, the coeval emplacement of putative arc- and plume-related magmas requires extremely complex geodynamic scenarios. However, the relative rarity of the archetypal convergent margin magma type (andesite) in Archaean sequences, and the absence of Archaean blueschists, ultra-high-pressure terranes, thrust and fold belts, core complexes and ophiolites, along with theoretical arguments against Archaean subduction, together imply that Archaean cratonic crust was not formed through uniformitarian plate-tectonic processes. A simpler interpretation involves soft intraoceanic collisions of thick (30-50km), plume-related, basaltic-komatiitic oceanic plateaux, with ongoing mafic magmatism leading to anatexis of the hydrated plateau base to generate TTGs, and eruption to form the platformal greenstone successions. The negative Nb-Ta-Ti anomalies and steep REE profiles of TTGs would result from the presence of residual garnet and rutile, and do not require the existence of subduction zones. Alternative models involving vertical tectonic processes, lower crustal restite delamination and multi-stage catalytic mantle and crustal melting are preferred.
DE: 0416 Biogeophysics
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
DE: 8137 Hotspots, large igneous provinces, and flood basalt volcanism
DE: 8140 Ophiolites (3042)
DE: 8178 Tectonics and magmatism
SC: Tectonophysics [T]
MN: Fall Meeting 2005