T165 Evolution of the Precambrian Earth

Thursday, 8 November 2001
8:00 AM - 12:00 PM

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

Paper number: T165-18
Program: Technical Sessions
Time: 8:00 AM – 12:00 PM

Authors

Abstract

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.