Mapping, petrological studies, and U-Pb dating o f volcanic and plutonic units have provided a new under standing o f the stratigraphy and paleotectonic setting of rocks in the Guysborough area. From base to top, the stratigraphic sequence is interpreted to consist of: (1) volcanic flows and pyroclastic rocks, with minor interlayered sedimentary rocks (Sunnyville Formation), (2) varied conglomerate (Glenkeen Formation), (3) quartz wacke with minor interlayered quartz arenite and conglomerate, and (4) massive to shaly or laminated siltstone. These volca nic and sedimentary units are intruded by small gabbroic plutons, sills, and dykes. All the units are early Middle Devonian, based on a U-Pb (zircon) age of 389 ± 2 Ma for rhyolitic tuff from the Sunnyville Formation, and a preliminary U-Pb (baddeleyite) age o f ca. 385 Ma for one of the gabbroic plutons in the siltstone unit.Based on whole-rock chemistry, the mafic volcanic and gabbroic rocks are similar, and formed in a continen tal within-plate setting. However, the rocks in the eastern part of the map area are more alkalic than those in the west, which are dominantly tholeiitic. The mafic magmas are interpreted to have formed by partial melting of the subcontinental upper mantle and to have evolved by crystal fractionation processes; the more alkalic magmas in the east may represent lesser amounts o f partial melting in that area. This interpretation is consistent with the presence of felsic volcanic rocks only in the western part of the area. They may represent crustal melts formed as a result of mafic magma underplating of the crust.The regional tectonic significance of these igneous rocks is not yet resolved, but they are clearly older than Late Devonian to Carboniferous igneous units elsewhere in northern mainland Nova Scotia and in Cape Breton Island with which they were previously assumed to be correlative.Des travaux de cartographic, des dtudes petrologiques et la datation au U-Pb d'unites volcaniques et plutoniques ont permis une meilleure comprehension de la stratigraphie et du cadre pateotectonique des roches du secteur de Guysborough. On interprete la composition de la sequence stratigraphique comme suit, de sa base a son sommet : 1) des ecoulements volcaniques et des roches pyroclastiques, avec une proportion mineure de roches s£dimentaires intercalees (Formation Sunnyville); 2) un conglomerat diversify (Formation Glenkeen); 3) de la wacke qurtzeuse avec une presence mineure de conglomdrat et de quartzite sedimentaire interstratifies; et 4) des siltstones massifs a schisteux ou feuilletes. Ces unites volcaniques et sedimentaires sont penetrees par de petits dykes, filonscouches et plutons gabbrolques. Toutes les unitds remontent au debut du Ddvonien moyen d 'apres une datation au U-Pb (zircon) situant & 389+2 Ma le tu f rhyolitique de la Formation Sunnyville ainsi qu'une datation au U-Pb (baddeleyite) situant provisoirement l'un des plutons gabbroi'ques de l'unite de siltstones k environ 385 Ma. D 'aptes la chimie des roches, les roches volcanomafi...
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