Two geologically distinct regions within the Meguma terrane have been studied in detail and are shown to have contrasting thermal histories. This conclusion is based on 31 new 40Ar/39Ar age spectra on micas, K-feldspars, and hornblendes from several of the plutons in southern Nova Scotia, which are satellites to the much larger South Mountain batholith (SMB). In addition, we report four K-feldspar age spectra from a locality in the northeastern part of the SMB and seven fission track ages from both the southern and northern areas.The complex age spectrum of hornblende from a mafic phase associated with the Barrington Passage pluton suggests a minimum intrusive age of 385 Ma, a result consistent with geologic evidence that this pluton represents an early, less evolved magmatic pulse. Later intrusive activity appears to have been coeval with the intrusion of the SMB at ca. 370 Ma ago. In the southern region, argon clocks in feldspars were completely reset and mica clocks variably reset by a later thermal event. The cumulative geochronologic and geologic evidence constrains this event to 300–320 Ma ago. In the SMB, this Alleghanian/Hercynian thermal disturbance is much less pronounced and appears to be localized to small areas that are often associated with economic mineralization.K-feldspars from the southern plutons record an episode of argon loss 220–230 Ma ago, which is less evident in the SMB to the north. Upper Triassic dike injection in the southern region, associated with the initial rifting phase of the Canadian Atlantic margin, accompanied this milder thermal pulse.Apatites in the two regions record a mean fission track age of ca. 180 Ma, which we attribute to the final cooling of the terrane below about 100 °C. The timing of this event coincides with regional uplift recorded in sedimentary sequences along much of the Maritime continental shelf.

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Fourteen granitoid plutons studied in the Baie d'Espoir area of southern Newfoundland are divisible into a northern group of eight and a southern group of six. The former tend to be undeformed, have thermal aureoles, and intrude greenschist facies Ordovician and Silurian rocks of the Dunnage zone following a period of folding and thrusting towards the SE. Four Rb-Sr whole-rock isochrons yield similar ages around 430 Ma for these plutons, fixing the deformation within early Silurian times. The latter plutons syntectonically intruded without aureole effects, and are all foliated and elongate parallel to the regional fabric of amphibolite facies gneisses and migmatites of the Gander zone. Whole-rock Rb-Sr isochrons around 350 Ma for both syntectonic and post-tectonic plutons indicate a rapid metamorphism, deformation, intrusion and uplift of the Gander zone in this area. There appear to be no systematic relationships between petrological and geochemical characteristics of the plutons and their tectono-stratigraphic setting. This study conclusively invalidates earlier hypotheses that deformed granitoid rocks of the Gander zone are older than the undeformed granitoids of the Dunnage zone; indeed, just the reverse is true.

The "Straddling Granite" was previously thought to straddle the Hermitage Bay -Dover fault system, which marks the boundary between the Avalon and Gander Zones of eastern Newfoundland, and hence to fix the earliest juxtaposition of these two zones before 504 2 12 Ma. More detailed investigation shows that these rocks are divisible into two distinct geochemical suites, the Indian Point Granite to the west and the Hardy's Cove intrusive suite to the east. Of the samples that provided the 504 Ma isochron, five were obtained from the Hardy's Cove suite and one from within the fault zone; hence the isochron must be taken as representing only a typical date for plutonic rocks within the Avalon Zone. The Indian Point Granite intrudes the Gaultois Granite, dated at 350 2 18 Ma, and is thus considerably younger than the Hardy's Cove suite. These observations now allow for juxtaposition of the Avalon and Gander Zones to have been as late as Early Carboniferous.Le "granite de Straddling" Btait antkrieurement consider6 comme chevauchant le systkme de failles de la baie dlHermitageDover qui dklimite la fronti&re entre les zones d'Avalon et de Gander dans I'est de Terre-Neuve, et fixe le moment du commencement de la juxtaposition de ces deux zones pdalablement B 504 2 12 Ma. Une ktude plus dCtaillke indique que ces roches appartiement A deux skquences g6ochimiques distinctes, le granite d'Indian Point a I'ouest et la sequence intrusive d'ffardy's Cove B I'est. Panni les kchantillons qui ont servi 5 Btablir I'isochrone B 504Ma, cinq provenaient de la sCquence d'Hardy's Coveet un de I 'intkrieur de la zone de failles, donc I'isochrone doit Ctre consid6rB comme reprksentant uniquement une date typique pour les roches plutoniques de l'interieur de la zone d' Avalon. Le granite d'Indian Point recoupe le granite Gaultois, datk ti 350 + 18 Ma et est donc considerablement plus dcent que la s6quence d'Hardy's Cove. Ces observations permettent maintenant de prkciser que la juxtaposition des zones d'Avalon et de Gander recule aussi loin que le Carbonifhe infkrieur.[Traduit par le journal]Can.