440 Ma igneous activity in the Meguma Terrane, Nova Scotia, Canada; part of the Appalachian overstep sequence?

Abstract

Abraded zircons from the basal rhyolitic tuff member of the White Rock Formation, which disconformably overlies the Cambrian-Early Ordovician Meguma Group, have yielded a nearly concordant U-Pb age of 442+ or -4 Ma interpreted as the age of extrusion. This age straddles the approximately 443 Ma Ordovician-Silurian boundary. Abraded zircons from the Brenton granite lie on a chord with an upper intercept age of 439+4/-3 Ma, which is interpreted to be the age of intrusion, thus supporting its inferred subvolcanic nature. On the other hand, monazite from the Brenton pluton yielded a nearly concordant analysis with a ²⁰⁷ Pb/ ²⁰⁶ Pb age of 380+ or -3 Ma, which is interpreted to be the time of the low pressure, high temperature metamorphism. Using these new data, the following observations suggest that the Meguma and Avalon terranes were neighbors during the Silurian-Early Devonian: (1) both have latest Ordovician-earliest Silurian, bimodal, subaerial, alkalic-tholeiitic, rift-related, volcanic rocks with Nd signatures indicating a similar continental basement source; (2) both show a similar progression of depositional environments: subaerial in the earliest Silurian, progressively deeper-water marine strata in the Llandovery and Wenlock, switching to gradually shallowing marine environments in the Ludlow, and reverting to subaerial in the Pragian; 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 terranes form part of the overstep sequence that extends across most of the northern Appalachians. Published data indicate that Avalonia was adjacent to Gondwana in the Neoproterozoic, that it separated from Gondwana in the Early Ordovician, and was accreted to eastern Laurentia in the Late Ordovician-Early Silurian. On the basis of the data and correlations presented here, we suggest that the Meguma Terrane travelled with Avalonia. This is consistent with the absence of a phase of deformation between the Cambro-Ordovician Meguma Group and the Silurian White Rock Formation.