Caracteristiques sedimentologiques, volcanologiques et structurales du Bassin de Granada dans la ceinture de roches vertes de l'Abitibi (Quebec).

摘要

The Granada Basin, correspond to the stratigraphic unit known as the Granada Formation. This unit lies in the southernmost part of the Southern Volcanic Zone of the greenstone dominated Abitibi Subprovince in Quebec.;Deposits of the Granada Basin are transitional facies separated by intra-basin unconformities, which are characteristic of sedimentary basins formed along fault margins. The volcano-sedimentary deposits of the Granada Basin have characteristics of sedimentation influenced by tectonisme such as: i) local and basinwide lateral and vertical facies variations; ii) alternating fining and coarsening upwards sequences; iii) high rate of accumulation of sediments; iv) some conglomerate boulders are over 1 m in size and the thickness of conglomerate deposits; v) interdigitation of conglomerates and sandstones that denotes a progradation; vi) intra-basin unconformities; vii) the local source of conglomerates; and viii) associated alkaline magmatism. Tectonism would be ongoing throughout the development of the basin which would have induced several phases of source renewal areas for the sediments coupled with intra-basin unconformities. The earliest formed sedimentary deposits of the Granada Basin were affected by uplift and subsequently eroded and recycled into later basin fill along intra-basin unconformities. Gradually as the basin filled the sedimentary deposits became less and less shallower which is a characteristic of piggyback-type basins. The decrease in the depth is a consequence of an increase in topography due to repeated uplift along faults.;Sedimentary rocks of the Granada Basin were intruded by alkalic mature volcanic arc-type sills, stocks and porphyritic dikes. These intrusive rocks are similar to Timiskaming-type porphyry intrusions and volcanic rocks found in the Kirkland Lake area of Ontario.;The structural style of the Granada Basin, consisting of overturned southward verging tight to isoclinal folds, is characteristic of thrust basins. Two major east-west trending synclinal axial traces are distinguishable and their juxtaposition in a back to back relationship is compatible with there being an unconformity between them. This unconformity is further demonstrated by the presence of sandstone clasts derived from the Granada syncline in conglomerates occurring along the southern margin of the basin.;Facies analysis in the Granada Basin has identified clastic and volcaniclastic rocks deposited in an environment evolving from relatively deep marine to fluvial. The earliest sediments were mainly turbidites (facies 1a and 1b) deposited in extensional sub-basins. These sediments were overlain by a regressive sequence that evolved from shallow marine wave base dominated sediments (facies 2a) to coastal deposits (facies 2b). These deposits represent the transition between marine and continental depositional environments. The transition zone is marked by deposits of sandstone-argillites-stratified pebble conglomerate (facies 2b) transported by waves and tides between the subtidal and intertidal zone. Facies 3a and 3b of the felsic volcaniclastic lithofacies are interbedded with facies 1a, 1b and 2a.;Repeated uplift during shortening and continuous compression resulted in a gradual progradation of the sedimentary deposits and marine regression. Asymmetric folds and progradation of sediments are two of the major characteristics used to define piggyback basins. A late exhumation phase is responsible for the deposition of facies 5 consisting predominantly of conglomerates that formed a molasse basin. The late extensional phase is manifested in the deformation zone found along the southern margin of the Granada Basin where it is in contact with the Pontiac Subprovince. The normal movement observed in this area is compatible with uplift of the Pontiac in the south, producing a likely source for facies 5 conglomerates.;The Piche Group is a lithological unit associated with the Cadillac Fault Zone. It consists of mafic to ultramafic metavolcanic rocks, volcaniclastic rocks and conglomerates containing porphyry clasts. It outcrops between the Granada Basin and metasedimentary rocks assigned to the La Bruyere Formation. Rocks of the Pontiac Subprovince near the Granada Basin are composed of finely bedded greywackes containing lithic clasts.;The geodynamic evolution of the Granada Basin is identical in its architecture and in the nature of its sedimentary deposits to that of a peripheral foreland basin involving arc subduction or terranes accretion. There were three phases of evolution: i) the flysch phase characterized by deep marine facies and probably developed in an extensional regime; ii) the early molasse phase that records the effects of shortening that generated folding and thrusting of the basin created the uplift responsible for the generation of relief that fed sedimentation and the massive conglomerates deposited in deltaic fans; and iii) the late molasse phase developed in a late-tectonic extensional regime that allowed for the exhumation of the Pontiac Subprovince to the south along the deformation zone marking the southern edge of the Granada Basin.;Key worlds: Sedimentary basin, Granada, Abitibi, Timiskaming, structural evolution, piggyback, progradation, thrusting, extension.