Abstracts
Abstract
The Avalonlan terrane around Saint John exhibits basement of tonalltlc gneiss veneered by a shelf type metasedimentary sequence. These units are intruded and overlain by volcanic-plutonic complexes of Late Proterozoic age emplaced in three stages. Mafic magmatlsm accompanied by submarine slumping heated and rentobillzed older rocks about 770-830 Ha. Calc-alkaline magnetism. mainly subaerial with minor submarine sedimentation occurred about 600 Ma, and subsequently Eocambrlan basaltic volcanlsm and volcanlclastic sedimentation passed into Camhro-Qrdovlclan sedimentation. Silurian rocks occur only as a fault-bounded slice and Devonian rocks are absent. Coarse clastic rocks of Carboniferous and Trlasslc age accumulated during deformation.
Major deformation and metsmorphlsm occurred In Late Proterozoic time, but most mappable structures appear to be of Carboniferous age. Carboniferous deformation resulted mainly from transcurrent motion on curved or stepped faults which locally produced spectacular thrust allochthons.
Comparison of the Saint John terrane with other Avalonlan terrenes suggests that some of the Avalon tectonostratigraphic zone Is floored by basement of Grenvllllan aspect. The western side of this zone was strongly affected by Late Ordoviclan-Early Silurian lMgimrM«m which reflects the amalgamation of this zone with more westerly zones. These observations suggest that Avalon terranes may be of relatively local, rather than exotic, origin, and suggest a history involving repeated breakup and rewelding of a continental edge.
RESUME
Autour de Saint John, la laniere avalonienne englobe un socle de gneiss tonalltlque qu'est venu recouvrlr un tegument metasedimentaire a cachet neritique. Ces unites sont recoupees et recouvertes par des complexes volcano-plutoniques d'age tardl-proterozoique emplaces en trols etapes. On pmgMHnmP mafique et des gllssements synsedimentaires sous-marins ont rechauffe et remobilise les rocbes preexlstantes 11 y a environ 770-830 Ma. Des venues magmatlques calco-alcallnes, en grande partie subaeriennes et associees a une falble sedementation sous-marine, ont prls place autour de 600 Ma. Le volcanlsme basaltique et la sedimentation volcanlclastlque eocambrlens ont debouche sur une sedimentation cambro-ordovlcienne. Les rochea siluriennes se limltent a une ecaille bordee par des falllea alors que les rocbes devoniennes font defaut. On apport detrltlque grossler a eu lieu pendant la deformation au Carbonlfere et au Trlaslque.
Une deformation et un metamorphlsme Importants furent presents au tardi-Proterozoique blen que la plupart des structures cartographiques semblent etre d'age carbonlfere. La deformation carbonlfere est liee surtout faux coullssages affectant des failles courbes ou en gradlns ce qui, par chevauchement, a prodult localement des allochtones spectaculaires.
La comparalson entre la laniere de Saint John et d'autres lanleres avalonlennes suggere que la zone tectonostratlgraphlque d'Avalon repose en partie sur un socle d'aspect grenvllllen. L*amalgamation de celle-cl avec d'autres zones plus a l'ouest y a imprlme son cachet sur le flanc ouest comme en fait foi un BWflBHtl pay prononce tardl-ordovlclen ou eo-sUurlen. De ces observations, on conclut que les lanleres d'Aval on seraient d'une orlglne relatlvement locale et non paa exotlque et que leur histoire seralt infeodee au jeu repete du morcellement et de la restauration de la marge continentale.
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