| Summary: | 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 ...
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