Jd. Keppie et Te. Krogh, 440 Ma igneous activity in the Meguma terrane, Nova Scotia, Canada: Part of the Appalachian overstep sequence?, AM J SCI, 300(6), 2000, pp. 528-538
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.