Compositional maps of orthopyroxene and garnet of contrasting grain size and in contact with different minerals were made from two paragneiss granulites from the Minto terrane of northern Quebec. The compositional maps provide clear evidence of late exchange of Fe/(Fe + Mg) after Ca in garnet and A1 in orthopyroxene had been quenched-in. The extent of late Fe-Mg exchange was controlled by neighbouring minerals, with negligible Fe-Mg gradients against plagioclase and quartz, and substantial gradients against exchangeable Fe-Mg minerals. Cores of grains in contact with exchangeable Fe-Mg neighbours are progressively more reset in Fe/(Fe + Mg) as grain size decreases, whereas cores of even small grains surrounded by only plagioclase and quartz are not significantly different in Fe/(Fe + Mg) than cores of the largest grains. Gradients of Ca in garnet and of Al in orthopyroxene in grains of uniform Fe/(Fe + Mg) preserve a high-temperature retrograde history during which intergranular exchange effected compositional uniformity of mineral rims and intragranular Fe-Mg diffusion in garnet and orthopyroxene was rapid enough to homogenize Fe/(Fe + Mg). The transition from efficient intergranular exchange at relatively high temperatures to local Fe-Mg exchange at lower temperatures may have been controlled by loss of an intergranular exchange medium in the rock, possibly an internally generated dehydration melt phase. Implications for geothermometry of granulites include the following (numerical values are particular to this study): (1) core compositions of garnet and orthopyroxene grains in contact with exchangeable neighbours may be reset in Fe/(Fe + Mg) relative to the most refractory compositions by an amount equivalent to 120°C; (2) Fe-Mg exchange thermometry using even the most refractory Fe/(Fe + Mg) compositions may not record peak granulite conditions, possibly recording instead the temperature at which an intergranular exchange medium was lost from the rock; and (3) temperaturesensitive net transfer equilibria involving Al solubility in orthopyroxene yield temperatures up to 150" C higher than maximum Fe-Mg exchange temperatures, even in grains with flat Fe/(Fe + Mg) compositional profiles, making them a better means of estimating peak granulite temperatures than Fe-Mg exchange thermometry.

The Ashuanipi complex and Minto block of the Superior Province are large regions that have been classified as "high-grade gneiss" terranes on the basis of the presence of orthopyroxene-bearing units. Like the granite-greenstone and metasedimentary subprovinces of the southern Superior Province, the two terranes consist predominantly of intrusive rocks, but are distinguished by their primary magmatic orthopyroxene. Both "high-grade" and "gneiss" are misnomers because granulite-facies gneisses are only sparingly present and the regions consist dominantly of massive, unmetamorphosed plutonic rock.The Ashuanipi complex consists of a deformed, metamorphosed package of metasedimentary rocks and primitive, early tonalite cut by widespread orthopyroxene + garnet granodiorite (diatexite), as well as plutons of tonalite, granite, and syenite.Based on its lithological and chronological similarity and on-strike position, the complex appears to be the continuation of metasedimentary subprovinces such as the Quetico. Its evolution involved deposition of immature greywacke in an accretionary prism, early arc (tonalitic) magmatism and deformation, followed by widespread intracrustal magmatism in the period 2700-2670 Ma. Both metamorphic and igneous rocks record equilibration under granulite-facies conditions (700-835°C; 0.35-0.65 GPa; a,,,, -0.3) and indicate exposure levels of -20 km. The Minto block at the latitude of Leaf River consists of several north-northwest-trending domains of similar scale and diversity to the east-trending subprovinces of the southern Superior Province. The central Goudalie domain is dominantly amphibolite-facies tonalitic rocks including some with ages > 3 Ga, with small belts of volcanic and sedimentary origin. Lake Minto domain contains poorly preserved supracrustal remnants in a plutonic complex comprising hornblende granodiorite, clinopyroxene + orthopyroxene granodiorite, orthopyroxene-biotite diatexite, and granite. The hornblende granodiorite suite constitutes most of the Utsalik and Tikkerutuk domains and is thought to represent continental arc magmatism. On the basis of their distinct aeromagnetic and lithological character, two additional domains are evident north of the Leaf River area, the Inukjuak domain in the west and the Douglas Harbour domain in the east.The northerly grain of the Minto block appears to have been established in situ with respect to the easterly belts of the southern Superior Province (i.e., no large-scale block rotation) during the same interval of time (3.0-2.7 Ga). Modification of the tectonic framework for the Superior Province is required to explain Minto arc magmatism. In the interval -2730-2690 Ma ago, a continental magmatic arc built the Berens River and Bienville subprovinces and Minto block on the southern and eastern edges, respectively, of a northern protocratonic foundation. In the same period, primitive volcanic arcs and accretionary prisms developed outboard on oceanic crust and were accreted to form a southern tectonic regime.Le complexe d'Ashuanipi et...

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RESUMERegional-scale mapping of index-mineral isograds in mafic units of the early Proterozoic Cape Smith Thrust Belt (northern Quebec) has revealed contrasting pressure-temperature regimes associated with two distinct structural domains. In the southern domain, crustal thickening was accomplished by early, piggy-back thrust faults. Isograds cross-cut the thrusts, indicating that thermal-peak mineral growth outlasted deformation associated with early imbrication. Mineral zones are: (1) actinolite (Act) + albite (Alb); (2) hornblende (Hbl) + Act + Alb; (3) Hbl + Act + oligoclase (Oh); (4) Hbl + Oli; and (5) garnet (Grt) or clinopyroxene + Hbl + Oli-andesine. The oligoclase isograd occurs at higher grade than the hornblende isograd, a sequence typical of medium-pressure terranes (5-7 kbar). An Hbl-Alb bathograd, calibrated from mixed-volatile equilibria in the NCMASH-CO, model system, suggests minimum pressures of about 5.4 kbar.Metamorphism in the northern domain was a consequence of re-imbrication, by means of out-ofsequence thrust faults active during and after peak metamorphic conditions. Mineral growth was coeval with thrusting, as documented by the syn-kinematic garnet porphyroblasts. Compared to the southern domain, a different sequence of isograds in mafic rocks shows that the albite-oligoclase transition takes place in the garnet zone. Based on thermobarometry in garnet-hornblende rocks, the oligoclase isograd occurs in a temperature range of 525-600" C, typical of high-pressure terranes (7-10 kbar). Calibrated bathograds for the Hbl-Ms-Alb and Grt-Alb bathozonal assemblages, respectively in the KNCMASH-CO, and NCMASH model systems, indicate minimum pressures in the northern domain of 6.7 and 8.5 kbar. Higher-pressure series for this domain are explained by out-of-sequence thrusts exposing deeper crustal levels. For similar structural levels, only minor amounts of syn-deformational uplift (1-2 kbar and 50-75" C) are recorded in metabasites of this domain, compared to results in adjacent metapelites of the area (essentially isothermal uplift of 3-5 kbar).La bande du Cap Smith (nord du Quebec) est une ceinture de chevauchement d'8ge proterozoique infkrieur, dominee par des roches mafiques. La cartographie d'isogrades minCraux indicateurs dans les unites mafiques de la ceinture a rCvClC deux regimes contrastes de pression-temperature, chacun associk a des Cpisodes distincts d'epaississement crustal. Dans le domaine sud, des failles de chevauchement en-drie sont responsables pour I'empilement tectonique. Les isogrades recoupent les failles, indiquant que I'apogCe thermale a suivi I'emplacement initial des nappes de charriage. Les zones minerales sont: (1) actinote (Act) + albite (Alb); 2) hornblende (Hbl) + Act + Alb; (3) Hbl + Act + oligoclase (Oh); 4) Hbl + Oli; et (5) grenat (Grt) oij clinopyroxene + Hbl + Oh-andesine. L'isograde d'oligoclase apparait B plus haute temperature que I'isograde d'hornblende, une sequence typique des terrains de pressions moyennes. Un bathograde Hbl-Alb, calibre a partir d'equil...