Precambrian Evolution of the Avalon Terrane in the Northern Appalachians: A Review

摘要

The Avalon terrane of the Northern Appalachians forms a distinctive tectonostrati-graphic belt defined primarily by the presence of late Precambrian (circa 600 Ma) volcano-sedimentary and granitoid rocks overlain by Lower Paleozoic sequences containing Acado-Baltic fauna. Avalon terrane is exposed in eastern Newfoundland, Cape Breton Island and the northern Nova Scotian mainland, southern New Brunswick, coastal southeastern Maine, and southeastern New England. The Precambrian evolution of Avalon terrane includes two major tectonothermal events that Involved both continental and oceanic basement. Local oceanic mafic magmatism and regional metamorphism of a mid-Proterozoic (late Helikian?) carbonate-clastic platform and its gneissic continental basement may record an early Hadrynian (750-800 Ma) rifting or subduction event and local platform collapse. Late Hadrynian (580-630 Ma) calc-alkaline granitoid plutonlso and widespread volcanism of tboleiltlc, calc-alkaline and occasionally peralkaline affinities record a closure event that is Interpreted to Involve ensialic arc, oceanic interarc, and both eztensional and trans-tensional continental back-arc settings. Closure terminated, perhaps through transform interactions, in the late Hadrynian Avalonian orogeny and was locally heralded by the development of flysch containing evidence of Vendian glaciatlon and followed by molasse-like successor basins. Latest Hadrynian volcanogenic redbeds and bimodal volcanism, associated with widespread back-arc transtension, may herald Inception of the Iapetus ocean and platform re-establishment during the earliest Paleozoic. Onset of overlapping, Atlantic-realm platform conditions in the Cambro- Ordovician was accompanied by minor, witnin-plate rift volcanism and is in places succeeded by late Ordovian-early Devonian bimodal volcanic rocks, shallow marine sediments and rare peralkaline plutons. Effects of a mld-Ordovician ("Taconian") tectonothermal event occur locally but are generally limited to major unconformities. Widespread early to mid-Devonian Acadian metamorphism and deformation may record the tectonic juxtaposi-tioning of Avalon and Gander terranes and was accompanied by the emplacement into both terrenes of locally voluminous Acadian granitoid plutons. However, portions of the Avalon terrane may have accreted earlier. Deposition of widespread Devono-Carboniferous, molasse-like sequences associated with bimodal tholeiitic to alkaline volcanics form an overlap sequence with the remainder of the Appalachian orogen, although the present position of the Avalon terrane with respect to the orogen is locally controlled by major Alleghanian faulting. Undeformed Triassic redbeds and mafic volcanic rocks herald initial rifting of the present Atlantic Ocean. RÉSUMÉ Dans les Appalaches septentrionales. la lanière d' Avalon forme une zone tectonostratigraphique distincte definie prlncipalement par la présence de roches volcano-sédimentaires et granitoldes tardi-précambriennes (circa 600 Ma) recouvertes par des séries paléozoiques inférieures qui renferment une faune acado-baltique. La lanière d'Avalon affleure dans Vest de Terre-Neuve, dans l'ile du Cap-Breton, dans le nord de la penlnsule de Nouvelle-Écosse, au Nouveau-Brunswick méridional, sur la côte sud-est du Maine et dans le sud-est de la Nouvelle-Angleterre. Deux épisodes tectonothermiques d'envergure ont affecté les socles continental et océanique et ainsi conditionne l’évolution précambrienne de la lanière d'Avalon. Un magmatisme océanique local et le métamorphisme régional d'une plate-forme à terrigènes et à carbonates du Protérozoique moyen (tardl-Helikien?), ainsi que de son socle gneissique continental, pourraient traduire une phase de rifting ou de subduction et d'effondrement local de celle-cl à l'éo-Hadrynien (750-800 Ha). Un plutonisme granitoide calco-alcallne et un volcanisme largement repandu à caractéres tholèiitique. calco-alcalin et, occasionellement, peralcalin tard-hadryuiens (580-630 Ha) concrétisent un épisode de serrage qui semble englober des régimes d'arc ensialique. d'inter arc océanique et des conteites d,arrière arc continental tant en extension qu'en transtenslon. Le serrage se conclut, peut-être par des eux transformants, lors de l’orogènie avalonienne au tardi-Hadrynien et s'illustra localenent par le développement de flyschs qui portent la trace d'une glaclation vendienne; après quoi s'implantérent des basins successeurs de type molassaique. Au fini-Hadrynien, des grès rouz volcanogèn