GRANULITE-FACIES METAMORPHISM AND CRUSTAL MAGMATISM IN THE ASHUANIPI COMPLEX, QUEBEC LABRADOR, CANADA

The high-grade Archean Ashuanipi complex contains an older sequence of granulite-facies migmatitic paragneiss and tonalite cut by abundant orthopyroxene-bearing, enclave-laden granitoid bodies (diatexite) of strongly peraluminous (garnet-bearing) and mildly peraluminous (garnet-absent) granodioritic composition, inferred to be magmatic in origin. Temperature estimates for garnet-orthopyroxene-biotite-plagioclase-quartz assemblages in both metamorphic and igneous rock types are mainly in the range 700-835-degrees-C, but apparent pressures are higher (0.6 - 0.65 GPa) in a wide belt of paragneiss and associated tonalite than in the enclosing diatexites (0.35 - 0.55 GPa), possibly owing to fluid-enhanced retrograde re-equilibration within the crystallizing igneous assemblages. Paragneiss has bulk compositions typical of Archean greywacke (58-68 wt.% SiO2), including high Cr (110-250 ppm), Ni (20-100 ppm), and LREE [(70-100) x chondrites]. Garnet-bearing diatexites have compositions virtually identical to paragneiss whereas garnet-absent diatexites are characterized by marked HREE depletion. High degrees of fusion of a source such as paragneiss, with entrainment of crystalline phases such as garnet and orthopyroxene, are required to explain the composition of garnet-bearing diatexites, whereas lower amounts of melting, leaving residual garnet, may account for the origin of the garnet-absent varieties. CO2 may have been a melt component in diatexite, based on several observations: (1) the high degrees of fusion implied in the genesis of diatexite require either extreme temperatures (> 1000-degrees-C), for which there is no mineralogical evidence, or some fluxing agent other than H2O (cf. Peterson & Newton, 1990); (2) some xenoliths have orthopyroxene-rich (dehydration) margins, implying relatively anhydrous melt conditions; and (3) orthopyroxene is unaltered, suggesting that low alpha(H2O) conditions persisted during crystallization. U-Pb zircon geochronology constrains the time for heating and magma production to < 18 Ma (2700 Ma for detrital zircon in paragneiss; 2682 Ma for crystallization of igneous zircon in diatexite). Combined with the evidence for high crustal temperatures and possible CO2 involvement, the rapid heating implies that underplated basaltic magmas played a key role as heat and fluid sources driving high-grade metamorphism and granitoid melt production.