Abstracts
Abstract
The eastern Caledonian Highlands of southern New Brunswick consist mainly of greenschist-facies or subgreenschist-facics volcanic, sedimentary, and plutonic rocks generally considered to be typical of Late Precambrian sequences in the Avalon Terrane of the northern Appalachian Orogen. As aresult of regional mapping combined with petro logical studies and radiometric dating, these rocks in the eastern Caledonian Highlands have been divided into two contrasting groups. The older group, apparently ranging in age from about 600 to 630 Ma, consists of metatuffaceous rocks ranging from mafic to felsic in composition, fine-grained volcanogenic melasedimentary rocks (slate and phyllite), arkosic metasedimentary rocks, and dioritic to granitic plutons. The volcanic rocks appear to be calc-alkalic, and both the volcanic and plutonic rocks were formed in a subduction-related tectonic setting.
In contrast, the other group of rocks appears to be about 550 Ma in age and consists of arkosic sedimentary rocks, subaerial rhyolite and basalt flows, laminated siltstone, felsic lapilli tuff, volcaniclastic conglomerate, and plutons of mainly gabbrotc and syenogranitic composition. These units typically are much less deformed and metamorphosed than die older group of units. The volcanic and plutonic rocks appear to be oogenetic bimodal suites, with petrochemical characteristics suggesting that they formed in a rifting environment -within an older volcanic arc.
RESUME
Les Monts Cal^doniens orientaux (Nouveau-Brunswick meridional) sont constitue's surtout de roches volcaniques, s£dimeniaires et plutoniques melamorphis£es dans le fades des schistes verts (ou presque), g£n£ralement considerees comme typiques des series lardiprecambriennes de la Laniere d'Avalon de l'Orogene appalachien septentrional. A la suite d'un lever regional combing a des dtudes p^trologiques et des datations radiome'triques, on a divise" ces roches des Monts Caledoniens orientaux en deux groupes distincts. Le plus ancien groupe, qui s'echelorme vraisemblablement d'environ 600 a 630 Ma, comprend des roches a mdtatufs de composition mafique a felsique, des roches me"tas£dimentaires volcanoge^iiques a grain fin (ardoise et phyllade), des roches mctasedimentaires arkosiques ct des plutons dioritiques a granitiques. Les volcanites scmblent ctre calco alcalines. Les volcanites comme les plutonites furent engendrdes dans un contexte tectonique lie" a une subduct ion.
Par contre, ]'autre groupe de roches semble dater d'environ 550 Ma et comprend des roches s^dimentaires arkosiques, des dpanchements subaeriens de basalte et de rhyolite, des siltstones lamin&, des tufs a lapilli felsiques, des poudingucs volcaniclastiques, ainsi que des plutons a composition principalcmcnt gabbro'i'que et sy^nogranitique. Ces unites sont typiqucment beaucoup moins deTormdes et me"tamorphisees que 1'est le groupe d'unites plus ancien. Les volcanites el les plutonites semblent appartenir a des lignees cogen^tiques bimodales; leurs caracteres pe"trochimiques suggerent une gtSncsc au sein d'un arc volcanique en contexte de rift.
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