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

Citation
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
Citations number
38
Categorie Soggetti
Earth Sciences
Journal title
AMERICAN JOURNAL OF SCIENCE
ISSN journal
00029599 → ACNP
Volume
300
Issue
6
Year of publication
2000
Pages
528 - 538
Database
ISI
SICI code
0002-9599(200006)300:6<528:4MIAIT>2.0.ZU;2-1
Abstract
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.