Geochronology of the Twillingate Granite and Herring Neck Group, Notre Dame Bay, Newfoundland

• Published in: Canadian Journal of Earth Sciences
• Main Authors: Williams, Harold, Dallmeyer, R. D., Wanless, R. K.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Canadian Science Publishing 1976
• Subjects: Newfoundland; Single Island
• Online Access: http://dx.doi.org/10.1139/e76-166; http://www.nrcresearchpress.com/doi/pdf/10.1139/e76-166

Zircons from the Twillingate Granite form a linear array with a concordia intercept of 510 + 17 – 16 m.y. Mafic dikes that cut deformed granite along the southern contact of the pluton at New World Island record 40 Ar/ 39 Ar hornblende ages of 440 ± 10 and 473 ± 9 m.y. These ages indicate that the mafic dikes are Ordovician, and confirm their correlation with mafic volcanic rocks of the nearby Herring Neck Group. The dates also suggest that Herring Neck volcanism and mafic dike intrusion extended from Early to Late Ordovician. Together with the zircon age, they define a narrow chronologic bracket for intrusion, deformation, and metamorphism of the Twillingate Granite (`~510–475 m.y.).Within the central portion of the Twillingate pluton. a metamorphosed mafic dike cutting massive granite and an amphibolite inclusion within Foliated granite yield similar, 40 Ar/ 39 Ar hornblende ages of 443 ± 11 and 438 ± 9 m.y. These ages are anomalously young compared with the 473 m.y. age of a mafic dike cutting deformed granite at New World Island. They are interpreted to indicate prolonged metamorphism and/or slower post-meta mo rphic cooling for central portions of the pluton compared to its southern margin.The isotopic ages support the view that the Twillingate Granite and nearby mafic volcanic rocks are collectively part of a single island-arc complex. The granite may have been generated during a period of subduction as sociated with plate convergence and closing of the proto-Atlantic ocean. A 510 m.y. (Late Cambrian) age for the granite suggests that convergence began rather early in the evolution of the Northern Appalachians. In addition, where dated Newfoundland ophiolite suites appear to be younger than, or contemporaneous with some granitic plutons (such as Twillingate), it is likely that they formed in a marginal ocean basin environment behind an older island-arc terrane.Les zircons du granite de Twillingate forment un réseau linéaire avec intercept à 510 + 17 – 16 Ma. Les dykes mafiques qui recoupent le granite ...