POLYBARIC SYN-KINEMATIC TONALITE-TRONDJHEMITE PLUTONISM IN THE ARCHAEAN MINTO BLOCK, SUPERIOR PROVINCE: FORMATION AND EVOLUTION OF CONTINENTAL CRUST

Archaean tonalite/trondhjemites (TT), enderbites (E), and granodiorite/granites (GG) of the Minto (northeastern Superior Province) have been interpreted as the roots of Andean margin and intra-oceanic arcs; with terrane assembly at ~2.7GA producing near-pervasive steep N-NW foliations and lineations. New mapping and geochronological work (MRNQ and GSC) show that the TT tend to be older (up to 3GA), with E and GG dominating after 2.73GA. Widely distributed supracrustal belts (tholeiite, komatiite, calc-alkaline tuffs, metasediment) display sheath-fold geometries. Minto TTE are dominated by plagioclase (typically antiperthitic, An25-29), quartz, and subordinate K-feldspar. Quartz is generally deformed, though it may preserve an interstitial habit. Feldspar typically has serrated high-energy grain boundaries, suggesting fairly low strains (despite well-developed macroscopic fabrics). Tonalites contain hornblende (rare clinopyroxene cores), biotite, titanite, Fe-Ti-oxides, apatite and zircon. Many contain euhedral allanite with epidote rims, and muscovite. Dioritic facies have less quartz, more clinopyroxene, and lack epidote or muscovite. Trondhjemites have little hornblende, low mafic contents, and more quartz. Tonalites have flat heavy-REE profiles, while trondhjemites have depleted profiles. Zircons generally contain older relict cores. Combined with Nd isotopic data, this suggest near-complete reworking of an older tonalitic protolith, itself derived from a garnet-metabasite source. Magmatic epidotes (Ps23-25), are enriched in incompatible elements, and are partly resorbed when not armoured by mica. There is no association with hornblende. Muscovites (high-Fe-Ti) show evidence of marginal resorption. These textures imply ascent of magma as crystal slurries from pressures >8Kb. Thermobarometry suggests partial recrystallization during near-solidus deformation at depths ~4-6Kb. Fractional crystallization of trace phases can account for trace element variations among TT rocks. Enderbitic facies contain orthopyroxene, biotite, Fe-Ti-oxides, apatite, ± clinopyroxene. Geochemically they resemble the tonalite/trondjhemite suite. The pervasive synmagmatic deformation, and uniform fabrics of Minto TTs are inconsistent with accretion of disparate older terranes. We propose instead an overall extensional regime, with ongoing mafic magmatism, crustal anatexis, and locally focussed synkinematic emplacement of crystal slurries.