Feldspar Pb isotopic compositions from Paleozoic tonalitic to leucogranitic Caledonian plutons from the northern Appalachian Mountains of the U.S.A. and Canada range from nonradiogenic to radiogenic relative to the evolution curve of Stacey and Kramers (1975). Pb isotopic values in the feldspars are generally independent of modal and major and trace element contents of the plutons. Plutons with the least radiogenic Pb isotopic signatures (Northern group) have intruded allochthonous rocks closest to the North American craton, and plutons with the most radiogenic Pb (Southern group) have intruded Avalonian basement. Feldspars from plutons in the Central group have intermediate Pb isotopic compositions between the Northern and Southern groups and do not define an isotopically unique group. The predominant source of the felsic magmas was the continental lithosphere as indicated by the general Pb isotopic similarity of the plutonic feldspars with either Grenville or Chain Lakes basement in the case of the Northern group and with Avalonian basement rocks in the case of the Southern group. The source region for plutons in the Central group may have contained a mixture of Grenvillian and Avalonian components. Plutons in the northern Appalachians are as a group more evolved in their Pb isotopic compositions relative to Caledonian plutons in the British Isles.
New U–Pb ages for late Precambrian volcanic and associated plutonic units in the Mira (Avalon) terrane of southeastern Cape Breton Island indicate that volcanic suites were erupted over a span of at least 100 Ma. The oldest dated rock is a quartz–feldspar rhyodacitic porphyry from the unit that hosts the Mindamar Zn–Pb–Cu–Ag–Au deposit in the Stirling belt, which has an age of [Formula: see text]. The most widespread volcanism and plutonism occurred at ca. 620 Ma in the East Bay Hills and Coxheath Hills belts, and probably the Sporting Mountain belt, as indicated by U–Pb ages and U–Pb maximum ages for rhyolite flows and U–Pb and Ar–Ar ages of crosscutting plutons, as well as stratigraphic constraints. Younger volcanic rocks occur in the Coastal belt, from which a rhyodacitic crystal tuff is dated at [Formula: see text] and a pluton is dated at 574 ± 3 Ma. A rhyolite flow from the contiguous Main-à-Dieu sequence yields a maximum age of ca. 563 Ma, and a minimum age for this sequence is indicated by overlying latest Precambrian to Cambrian fossiliferous sedimentary rocks. Middle Devonian plutonism in the Mira terrane is confirmed by an age of [Formula: see text] from the Lower St. Esprit granodiorite in the Coastal belt. The range of ages of volcanic and plutonic rocks in Mira terrane is similar to that in other parts of Avalon terrane in eastern Newfoundland and southern New Brunswick. Many of the dated rocks contain xenocrystic zircons of Middle Proterozoic ages which suggest a South American source.
N ew U-Pb (zircon) data from the northern Boisdale Hills show that rhyolite on Long Island and syenogranite o f the nearby Mount Cameron pluton have similar ages o f 505 ± 3 Ma and 509 ± 2 Ma, respectively. These ages are similar to those known or inferred from some felsic plutonic units elsewhere in central Cape Breton Island and suggest that latest Cambrian to earliest Ordovician igneous activity was widespread in the Bras d'Or terrane. The new dates also suggest that stratigraphic and structural relationships in the Boisdale Hills are more complex than previously interpreted. The dated rhyolite and other volcanic and sedimentary rocks in the northern Bourinot belt in the Boisdale Hills were previously included in undivided Middle Cambrian Bourinot Group. In the southern Bourinot belt, the Bourinot Group was subdivided into the Eskasoni, Dugald, and Gregwa formations. The Eskasoni Formation is dominantly a bimodal volcanic suite with petrological characteristics indicative o f origin in a continental within-plate tectonic setting. Its present contacts, both with adjacent older metamorphic and plutonic rocks and with the apparently overlying fossiliferous Dugald and volcanogenic Gregwa formations, are faulted. Our new mapping demonstrates that the Eskasoni, Dugald and Gregwa forma tions can be extended into the central Bourinot belt where they were previously undivided. However, continuity cannot be demonstrated between these Middle Cambrian units and the dated volcanic and associated sedimentary units in the northern Bourinot belt. Hence, assuming that both U-Pb and fossil ages are correct, our interpretation is that the Upper Cambrian-Lower Ordovician Northern Boisdale Hills volcanic unit is younger than the Bourinot Group, although petrochemical data suggest that it formed in a similar tectonic regime. The presence in the Bourinot belt o f fauna characteristic o f the Acado-Baltic faunal province appears to tie the Bras d'Or terrane to other Avalonian (peri-Gondwanan) terranes. However, the Bras d'Or terrane differs from the adjacent Mira terrane which includes Lower as well as Middle and Upper Cambrian units and lacks volcanic and plutonic rocks o f this age. D e nouvelles donndes U-Pb (zircon) provenant de la partie nord des collines Boisdale montrent que la rhyolite sur l'lle Long et le syenogranite du pluton du mont Cameron k proximite ont des Ages similaires, de 505 ± 3 Ma et 509 ± 2 Ma respectivement. Ces ages sont similaires a ceux connus ou supposes pour quelques unites plutoniques felsiques ailleurs dans le centre de l'ile-du-Cap-Breton, et suggerent que l'activite ignee de la fin du Cambrien au debut de l'Ordovicien etait tepandue dans le terrain de Bras d'Or. Ces nouvelles datations sugg&rent aussi que les relations stratigraphiques et structurales dans les collines Boisdale sont plus complexes que ne le montraient les interpretations anterieures. La rhyolite datee et d'autres roches volcaniques et sedimentaires de la partie nord de la ceinture de Bourinot dans les monts Boisdale etaient auparavant...
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