The Shickshock Sud fault has a history of Ordovician (Taconian), Silurian (Salinic), and Devonian (Acadian) movements. Taconian deformation involving ductile dextral oblique-slip faulting is recorded in Cambrian rocks in the footwall of the Shickshock Sud fault. Metabasalt and metaarkose at amphibolite grade are converted into phyllonite and mylonitic schist. Shear bands, asymmetric garnet porphyroclasts, C–S fabrics, and mica-fish textures indicate dextral shearing. The regional sense of shear is top to west and southwest on generally southeast dipping shear zones. Hornblende of metabasalt yielded an 40Ar/39Ar age of 455.9 ± 2 Ma, and muscovite from the mylonitic schist yielded an 40Ar/39Ar age of 454.3 ± 0.9 Ma, which indicate metamorphism and deformation during the Taconian orogeny. Evidence for Silurian activity is indicated by the Salinic unconformity to the south related to normal block-faulting. Deformation features in the Ordovician and Silurian–Devonian rocks in the hanging wall were predominantly brittle and involved dextral transpression. Kinematic indicators point to predominantly dextral strike-slip movement. Kinematic analysis of brittle fault-slip data indicates that the shortening axis direction during strike-slip deformation was northwest–southeast and subhorizontal, which is essentially coaxial to the average pole of Acadian cleavage. Deformation in the hanging wall of the Shickshock Sud fault is Acadian-related. The irregular geometry of the Laurentian margin, including the Grenville basement, might be the cause for Taconian and Acadian transpression in the Gaspé Appalachians.

Variolitic lavas from Archean tholeiitic series north and south of Rouyn-Noranda (Abitibi Metavolcanic belt, Canada) contain large, sharply defined, spheroidal to subspheroidal felsic varioles (up to 5 cm in diameter) set in a ferruginous matrix of more mafic composition. Quench texture and flow differentiation studies indicate that the variolites were produced by rapid cooling of a two-liquid magma, and that these liquids were in contact and chemically discrete prior to extrusion. Physical mixing models do not adequately account for these contiguous magmas, yet a liquid immiscible model demonstrably satisfies almost all variolite field, microscopic, microprobe, and chemical data. We conclude Archean variolites are formed by immiscible splitting of a magma of tholeiitic composition.Dans la ceinture metavolcanique d'Abitibi, des laves variolitiques de series tholeiitiques archeennes au Nord et au Sud de Rouyn-Noranda, contiennent de larges varioles felsiques nettement definies, spheroi'des a subspheroi'des, dont le diametre peut atteindre 5 cm, et assemblees dans une matrice ferrugineuse de composition mafique. Les etudes sur la texture squelettique de refroidissement rapide ('quench') et sur la differenciation des coulees, montrent que les variolites ont ete produites par refroidissement rapide d'un magma adeux phases liquides, et qu'avant I'effusion, ces liquides etaient en contact tout en gardant leur chimisme propre. Des modeles de melanges physiques ne tiennent pas compte de la possibilite de la coexistence de deux magmas contigus, alors qu'un modele d'immiscibilite de liquides satisfait presque toutes les donnees de terrain, les donnees microscopiques, chimiques, ainsi que les donnees obtenues par la microsonde sur les variolites. En conclusion, nous disons que les variolites de I'Archeen sont formees par processus d'immiscibilite sur un magma de composition tholeiitique.[Traduit par le journal] IntroductionThe concept of liquid immiscibility as a process of igneous differentiation is not new. Recently, the application of the theory to silicate melts has been forcefully reviewed, and the combination of convincing experimental data (e.g. Gtlinas 1974) suggest that immiscibility has indeed been operative in certain silicate systems at temperatures that are of geological interest.In view of this renewed interest, it is not surprising that other phenomena that texturally resemble globules produced by liquid immisci-

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A clast containing sapphirine has been found in the Troie Complex in the Archean Minto Subprovince, in the northeastern part of the Superior Province, in northern Quebec. The area is dominated by synmagmatically deformed felsic plutonic rocks that host volcano-sedimentary slivers and rafts. The Douglas Harbour Domain contains the Faribault-Thury tonalite-trondhjemite complex, intruded by the 2740-2726 Ma enderbitic Troie Complex. Slivers and enclaves of volcano-sedimentary rocks in the Troie Complex are metamorphosed to granulite grade. A breccia in the core of the Troie Complex contains a heterogeneous population of xenoliths in an enderbitic matrix. The clast contains sapphirine as symplectitic rims with plagioclase or K-feldspar (or with both) developed around sillimanite and cordierite. Sapphirine also forms a corona around green hercynite, with Kfeldspar rimming the sapphirine. Reaction textures, annealing and polygonization indicate localized equilibration, with mosaic equilibrium preserved on the thin-section scale. Various geothermobarometers yield estimates of high temperatures (755-1260°C) and pressures (7.5-14 kbar). The existence of localized equilibria is difficult to interpret in terms of simple prograde or retrograde metamorphism linked with orogenesis. Either static P-T conditions for a protracted period, or minimal attainment of granulitefacies conditions are required to produce and preserve these mosaic textures.

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