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TitleFossil mantle rift controls on deformation and mineralization in the Quebec Abitibi and on transverse structures in Grenville Province basement
AuthorHarris, L B; Bédard, J H; Dufréchou, G
SourceAssociation de l'exploration minière du Québec 2012, résumés; 2012 p. 1
Alt SeriesEarth Sciences Sector, Contribution Series 20130024
MeetingAssociation de l'exploration minière du Québec 2012 / Quebec Mineral Exploration Association 2012; Québec, QC; CA; November 19-22, 2012
ProgramTargeted Geoscience Initiative (TGI-3), 2005-2010
Released2012 11 01
AbstractIn the Abitibi Subprovince of the Superior Province in Quebec, extrusive sequences were formed in a volcanic plateau-like setting during plume-related rifting of older cratonic lithosphere at ca. 2.78-2.75 Ga. Mantle plume activity led to focussed thermal erosion, destruction, and assimilation of ancient lithosphere and formation of isotopically juvenile crust that now constitutes the Abitibi belt. 3D images of S-wave seismic tomographic data of the Superior Province illustrate that the Abitibi Subprovince overlies a symmetrical rift in the sub-crustal lithospheric mantle (SCLM) of older Archaean lithosphere (N Superior Province and Minnesota River Valley domain). Whilst generally E-W-trending, the Neoarchaean rift changes to a NW-SE orientation in the easternmost Abitibi and Grenville Province parautochthon. Seismic tomography shows no evidence for ¿fossil¿ Archaean subduction zones. Enhanced aeromagnetic images of the central-northern Abitibi illustrate features indicative of penetrative E-W dextral ductile shearing that preceded formation of discrete ductile to brittle-ductile conjugate transcurrent and E-W reverse ± dextral shear zones. Offset and ductile deflection of dense, mafic crust along regional transcurrent shear zones in the Abitibi Subprovince and its continuation in the Grenville Province parautochthon is apparent on short wavelength Bouguer gravity images. The NE segment of an Archaean sinistral, NE-striking shear zone (the proto-Grenville shear zone) identified along the SE margin of the Abitibi and Opatica subprovinces may be controlled by an aulacogen in the rifted mantle. The displacement history and geometry of reverse and strike-slip shear zones is similar to that of structures developed during progressive lateral escape and indentation during impingement of a rigid body. We propose that southward migration of the old cratonic nucleus (N Superior Craton) in response to mantle flow acting upon its deep lithospheric keel, and not subduction-related processes, led to progressive southward accretion of crustal fragments and oceanic plateaux-like segments like the Abitibi. Terrane accretion led to shortening, and rift inversion in the Abitibi. Major epigenetic gold deposits are located above rift-bounding faults in the SCLM, suggesting that early rift structures localized subsequent deformation and hydrothermal fluid flow during N-S shortening in the ca. 2.7 Ga Shebandowanian orogeny. The 1.1 Ga Desmaraisville and 0.55 Ga Otish kimberlite clusters also coincide with interpreted rift structures in SCLM. NW-SE-striking structures in Archaean basement to the Grenville Province (i.e. transverse to the Grenville orogen) formed during Palaeoproterozoic rifting and reactivated during the Mesoproterozoic Grenville orogenic cycle also correlate with margins to the Neoarchaean SCLM rift. Our observations highlight the important role of ancient mantle structures on localizing deformation, hydrothermal fluid flow, and emplacement of igneous bodies in the overlying crust.
Summary(Plain Language Summary, not published)
Geophysical data are interpreted to show the existence of a rift structure in the mantle beneath the Abitibi greenstone belt. The geometry is inconsistent with a subduction origin and implies an extensional event, which we attribute to a mantle plume. Later reactivation of rift-bounding crustal faults during subsequent compression localized fluid flow and controlled the location of gold deposits.

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