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
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.