GSA Annual Meeting, November 5-8, 2001

Paper No. 0
Presentation Time: 11:15 AM

IS CRATONIZATION THE RESULT OF HORIZONTAL TERRANE ACCRETION OR PARTIAL CONVECTIVE OVERTURN? EVIDENCE FROM TONALITE AND TRONDHJEMITE PLUTONS IN THE NORTHERN SUPERIOR PROVINCE


BÉDARD, Jean H.1, MADORE, Louis2, BROUILLETTE, Pierre1 and BERCLAZ, Alain3, (1)Centre Geoscientifique de Quebec, Geol Survey of Canada, CGQ, CP7500, 880 Ste-Foy, Ste-Foy, QC G1V 4C7, Canada, (2)Géologie Québec, Ministère des Ressources naturelles du Québec, 400 boulevard Lamaque, Val d'Or, QC J9P 3L4, Canada, (3)Géologie Québec, Ministère des Ressources naturelles du Québec, 545 Crémazie Est, Montréal, QC H2M 2V1, Canada, jbedard@rncan.gc.ca

Archaean tonalite/trondhjemites (TT), enderbites (E), and granodiorite/granites (GG) of the Minto (northeastern Superior) have been interpreted as Andean marginal and intra-oceanic arc roots; with terrane assembly between ~2.73 and 2.69 GA producing steep N-NW foliations and lineations during a series of 'barrovian' tectono-metamorphic events. However, Minto TT-E-GG commonly preserve igneous textures such as interstitial quartz. Tonalites have hornblende with rare clinopyroxene cores, biotite, titanite, Fe-Ti-oxides, apatite, zircon, epidote, allanite ± muscovite. Trondhjemites lack hornblende. Muscovites (high-Fe-Ti) and magmatic epidotes (Ps23-25) have resorbed edges against quartz + plagioclase; suggesting ascent of magma as crystal slurries from higher pressures; an interpretation compatible with phase equilibrium studies. Tonalites have flat heavy-REE profiles, while trondhjemites are HREE-depleted with positive Eu-Sr anomalies; precluding large-scale plagioclase fractionation. Extraction of ~18% hornblende and trace phases can account for trace element variations; suggesting either high-pressure fractionation, or localized segregation from mafic crystal slurries after emplacement. Enderbites (orthopyroxene, ± clinopyroxene, biotite, Fe-Ti-oxides, apatite,) geochemically resemble the TTs, suggesting a similar, albeit dehydrated, source. Thermobarometric data for plutons indicate recrystallization at depths ~4-6Kb during deformation at near-solidus temperatures higher than recorded in widely distributed platformal supracrustal slivers, which display complex, sheath-fold geometries. Mineral assemblages in supracrustals seem to represent partial equilibration with the dominant granitoids. The data are incompatible with a regional tectono-metamorphic 'overprint' on TT-E-GGs, most of which preserve synmagmatic deformation fabrics. Furthermore, the broad uniformity of these high-temperature fabrics seems inconsistent with accretion of disparate older terranes. We propose instead that dense intra-platformal greenstones subsided as cold fingers, while TT-E-GG terranes ascend both by focused synkinematic intrusion of crystal slurries, and as broad upwarps, in an overall extensional regime, with ongoing mafic magmatism and crustal anatexis.