440 Ma igneous activity in the Meguma terrane, Nova Scotia, Canada: Part of the Appalachian overstep sequence?

Abraded zircons from the basal rhyolitic tuff member of the White Rock Form ation, which disconformably overlies the Cambrian-Early Ordovician Meguma G roup, have yielded a nearly concordant U-Pb of 442 +/- 4 Ma interpreted as the age of extrusion. This age straddles the similar to 443 Ma Ordovician-S ilurian boundary. Abraded zircons from the Brenton granite lie on a chord w ith an upper intercept age of 439 +4/-3 Ma, which is intepreted to be the a ge of intrusion, thus supporting its inferred subvolcanic nature. On the ot her hand, monazite from the Brenton pluton yielded a nearly concordant anal ysis with a Pb-207/Pb-206 age of 380 +/- 3 Ma,which is interpreted to be th e time of the low pressure, high temperature metamorphism. Using these new data, the following observations suggest that the Meguma and Avalon terrane s were neighbors during the Silurian-Early Devonian: (1) both have latest O rdovician-earliest Silurian, bimodal, subaerial, alkalic-tholeiitic, rift-r elated, volcanic rocks with Nd signatures indicating a similar continental basement source; (2) both show a similar progression of depositional enviro nments: subaerial in the earliest Silurian, progressively deeper-water mari ne strata in the Llandovery and Wenlock, switching to gradually shallowing marine environments in the Ludlow, and reverting to subaerial in the Pragia n; and (3) both contain Rhenish-Bohemian Early Devonian fauna. Furthermore, the southeast to northwest transition from an offshore sandbar to a beach sand in the Silurian White Rock Formation suggests the presence of land to the north, now recognized as Avalonia. These conclusions support published suggestions that the Siluro-Devonian successions in the Meguma and Avalon t erranes form part of the overstep sequence that extends across most of the northern Appalachians. Published data indicate that Avalonia was adjacent t o Gondwana in the Neoproterozoic, that it separated from Gondwana in the Ea rly Ordovician, and was accreted to eastern Laurentia in the Late Ordovicia n-Early Silurian. On the basis of the data and correlations presented here, we suggest that the Meguma Terrane travelled with Avalonia This is consist ent with the absence of a phase of deformation between the Cambro-Ordovicia n Meguma Group and the Silurian White Rock Formation.