The region comprises parts of the southern zone of the Abitibi greenstone belt and of the Bellecombe gneiss belt, and the Belleterre–Angliers greenstone belt. The stratigraphic sequence of the Abitibi Belt consists of (1) a lower ultramafic division; (2) a middle tholeiitic division; and (3) an upper diverse division; these divisions lap onto an older volcanic sequence to the north and west. In the Bellecombe Belt, sediments overlie the lower ultramafic division; these sediments coarsen toward the Abitibi Belt and toward the stratigraphic top. The volcanic rocks of the Belleterre–Angliers Belt overlie the sediments of the Bellecombe Belt.Five paleogeographic phases have been recognized. During phase I, a deep marine ultramafic lava plain covered the south part of the Abitibi Belt and the Bellecombe Belt. Faulting took place at the Duparquet–Destor break. During phase II, tholeiitic basalts formed a deep marine lava plain in the west of the Abitibi Belt, whereas flows erupted mainly from central volcanic complexes in the east. Simultaneously, sediments derived from a volcanic–plutonic hinterland to the north began to accumulate in the Bellecombe Belt. In phase III, lavas of the upper diverse division erupted from well defined central volcanic complexes in the Abitibi Belt, and sedimentation continued in the Bellecombe Belt. In phase IV, volcanic centers built above sea level, and local fans of tuff and of volcanic-derived conglomerate and sandstone interfingered with the volcanic–plutonic-derived sediments at the southern limit of the Abitibi Belt. The volcanic sequence was uplifted at the Cadillac break during phase V, with consequent progradation of coarse conglomerates southward.The stratigraphic and paleogeographic evolution of the Abitibi Belt is comparable to the evolution of immature oceanic island arcs in the Cenozoic.

Textural criteria permit distinction between the pre-Kenoran and Kenoran phases of plutonism and metamorphism. The pre-Kenoran plutons and pre-Kenoran metamorphic phases are directly related to the volcanic evolution. Synvolcanic tonalite–trondhjemite plutons and swarms of mafic and felsic dykes core central volcanic complexes. The volcanic rocks underwent three types of pre-Kenoran metamorphism, namely, a pervasive alteration, a thermal contact metamorphism that affected narrow aureoles around synvolcanic plutons, and a high-intensity hydrothermal alteration that affected cross-cutting pipes in central volcanic complexes.Synkinematic Kenoran metamorphism resulted in the growth of minerals (chlorite, actinolite, etc.) parallel to schistosities. Synkinematic metamorphic grade ranges form the pumpellyite–prehnite facies to the amphibolite facies. Late- to post-kinematic metamorphic phases resulted in the growth of minerals across schistosities. Syn- to post-kinematic plutons are not voluminous in the part of the Abitibi Belt described here, but they underlie vast areas in the Bellecombe Belt. They range from gneissose early synkinematic plutons to late-kinematic plutons that have well preserved igneous textures.The paleogeographic, tectonic, plutonic, and metamorphic histories of the Abitibi and Bellecombe belts are reviewed and we conclude that the belts are analogous to an island arc – fore-arc basin system.

In this paper, we describe the relations between the paleogeographic and tectonic evolution of the southwestern part of the Archean Abitibi and Bellecombe belts. Volcanism in the Abitibi Belt created a very thick, anisotropic plate composed of competent volcanic rocks and broken by the Duparquet–Destor break. The depocenters of the upper division of diverse volcanic rocks subsided about 10 km relative to their surroundings, and some central volcanic complexes within this division were consolidated by synvolcanic plutons and their thermal metamorphic aureole. The Cadillac break, a normal fault, separated the Abitibi and Bellecombe belts. The latter consisted of comparatively incompetent sedimentary rocks on top of a basement composed of ultramafic–mafic flows.North–south compression of the volcanic terrain during the Kenoran Orogeny produced a set of flexure folds, F1, that curve around the consolidated cores of central volcanic complexes generally in an easterly direction. Synclinoria nucleated at the deeply subsident depocenters of the upper diverse division. Further north–south flattening and subvertical stretching produced the east-trending F2 folds, their axial-plane schistosity S2, and local superposed schistosities S3 and S4. Southward verging recumbent folds suggest that the Bellecombe Belt simultaneously was pulled northward below the Abitibi Belt. During the orogeny, the Duparquet–Destor and Cadillac breaks were transformed to thrust faults; the Duparquet–Destor break also shows minor (< 3 km) right-lateral strike slip. Diapiric rise of late- to post-kinematic plutons locally distorted earlier schistosities.

No abstract

ZusammenfassungDer Faltenbau des Chibougamaugebietes im Archaischen Abitibi Belt, Quebec, ist das Resultat zweier Faltungsphasen. Die grof~en F1-Synklinalen bildeten sich zuerst als absinkende Verwerfungstr6ge, die sp~.ter in NS-Richtung eingeengt wurden und gleichzeitig dabei an die verfestigten Vulkanzentren gepref~t wurden. Die grol%n synvulkanischen Plutone konsolidierten die vulkanischen Zentren. Die Fi-Falten streichen generell OW, biegen aber lokal in NO-und SO-Richtung urn. Nord-streichende F1-Falten finden sich lokal in den Antiklinorien und werden auf das Aufsteigen der basalen Gneisse und/oder der synvulkanischen Plutone zur/ickgef/.ihrt. Die F2-Falten haben ein regelm~if~iges Ost-Streichen; ihre der Faltenachsenfl~iche paraIlele Schieferung $2 ist das deutlichsre planare Strukturelement. Die Randbriiche der Verwerfungsbecken wurden w[ihrend der F1-und F2- AbstractThe major fold pattern of the Chibougamau region in the Archean Abitibi Belt, Quebec, is the result of two fold phases. The principal Fl-synclines first formed as subsiding fault troughs, and were shortened in NS direction and molded around volcanic centers that had been consolidated by large synvolcanic plutons; they have a general easterly direction but curve locally to the NE or SE. N-trending Fl-folds are locally present in the anticlinoria and are thought to have been produced by the upwelling of basement gneiss and/or synvolcanic plutons. The F2-folds have rather regular easterly trends; their axial plane schistosity $2 is the predominant planar fabric element. Basin margin faults were transformed into east-trending thrust faults during the F1-and F2-folding.The superposed schistosities $3 and $4 are conjugate shear planes related to NS shortening. They and the kink bands produce only minor strain except at a few localities. Synkinematic and late kinematic diapiric plutons pierce the structural pattern. The Fl-folds, derived from the paleogeographic pattern during the uplift of volcanic islands and during the subsidence of fault basins, may also be interpreted as diapiric. Thus, diapirism took place during the whole period of NS compression.ENE trending left-lateral shear belts and their associated first and second order faults, displaced the Kenoran structures in late Archean and early Proterozoic time. R~sum~Les piis majeurs de la r~gion de Chibougamau dans la ceinture Arch6enne de l'Abitibi au Quebec sont le r~sultat de deux phases de plissement. Les synciinaux F1 majeurs prenaient naissance comme bassins de faille subsidents et furent comprim+s en direction NS. Ils furent moules autour des centres volcaniques qui avaient 4t4 consolid4s par des plutons synvolcaniques; leur direction g4n~rale est est-ouest mais ils s'incurvent localement vers le NE ou vers le SE.