40Ar/39Ar and U-Pb mineral ages from the Brookville Gneiss: implications for terrane analysis and evolution of Avalonian "basement" in southern New Brunswick

Authors

  • R. D. Dallmeyer Department of Geology, University of Georgia, Athens, Georgia 30602, U.SA.
  • R. Doig Department of Geological Sciences, McGill University, Montreal, Quebec H3A 2A7, Canada
  • R. D. Nance Department of Geological Sciences, Ohio University, Athens, Ohio 45701, USA.
  • J. B. Murphy Department of Geology, St. Francis Xavier University, Antigonish, Nova Scotia B2G ICO, Canada

DOI:

https://doi.org/10.4138/1707

Abstract

New 40Ar/39Ar hornblende and U-Pb zircon data from units previously thought to represent basement to the Avalon composite terrane in southern New Brunswick yield latest Precambrian or early Cambrian metamorphic cooling ages and a late Precambrian protolith age. Hornblendes from the Brookville Gneiss and from an intrusive amphibolite body within the gneiss yield 40Ar/39Ar isotope correlation ages of 542 ± 4 and 338 ± 2 Ma, respectively. These ages are interpreted to date post-amphibolite facies metamorphic cooling. Euhedral zircons from the Point Pleasant orthogneiss, a quartz dioritic gneiss previously interpreted to be the oldest component of the Brookville Gneiss, show slight inheritance and yield 247Pb/206Pb ages ranging from 603 to 631 Ma. The youngest of these is interpreted to be a maximum age for the orthogneiss protolith. These results, together with other recent U-Pb age data, conflict with previous interpretations of the Brookville Gneiss and the metasedimentary Green Head Group with which it is associated, as a mobilized Proterozoic basement-cover succession upon which an Avalonian ensialic arc developed at c. 600-633 Ma. Instead, the age of the orthogneiss matches that of the arc, and the orthogneiss protolith is likely to have been originally intrusive into the Green Head Group. Although the arc succession is unaffected by high-grade metamorphism, the metamorphic cooling age recorded in the "basement" closely follows evidence of within-arc extension at c. 330 Ma. Hence, the arc/"basement" contrasts in tectonothermal regime could be those of varying structural level within the Avalonian arc rather than requiring the proposed existence of entirely separate terranes. RÉSUMÉ De nouvelles données 40Ar/39Ar sur hornblendes et U-Pb sur zircons, issues d'unités considérées jadis comme représentant le socle de la Lanière composite d'Avalon au Nouveau-Brunswick meridional, ont livré des âges de refroidissement métamorphique finiprécambriens ou éocambriens ainsi qu'un âge de protolithe tardipréambrien. Des hornblendes extraites du Gneiss de Brookville ont livré un âge de corrélation isotopique par 40Ar/39Ar de 542 ± 4 Ma; d'autres, provenant d'un bâti amphibolitique intrusif au sein du gneiss, ont livré un âge de 538 ± 2 Ma. On interprète ces âges comme datant le refroidissement métamorphique au-delâ du faciès à amphibolites. Des zircons idiomorphes provenant de l'orthogneiss de Point Pleasant, un gneiss quartzodioritique interprèt auparavant comme le plus ancient constituant du Gneiss de Brookville, montrent un 1éger remaniement et livrent des âges 247Pb/206Pb s'étalant de 603 à 631 Ma. On interprèté le plus jeune de ces âges comme l'âge maximal du protolithe de l'orthogneiss. Ces résultats, tout comme d'autres données U-Pbrecentes, contredisent les interprètations antérieures considéranl le Gneiss de Brookville et le Groupe métasédimentaire de Green Head, auquel il s'associe, comme une succession socle-couverture mobilisée protérozoique sur laquelle un arc sialique avalonien s'est développé il y a en v. 600 à 635 Ma. L'âge de l'orthogneiss correspond plutôt â celui de l'arc et il est probable que le protolithe de l'orthogneiss faisait à l'origine intrusion au sein du Groupe de Green Head. Bien que la succession d'arc n'ait subi aucun métamorphisme de degré é1evé l'âge de refroidissement métamorphique enregistré dans le "socle" Concorde étroitement avec la manifestation d 'une extension intra-arc vers 550 Ma. Par consequent, les contrastes de régime tectonothermique arc/"socle" pourraient reflèter les variations du niveau structural au sein de l'arc avalonien plutôt que necessiter l'existence proposée de lanières tout a fait distinctes. [Traduit par le journal]

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Published

1990-11-01

How to Cite

Dallmeyer, R. D., Doig, R., Nance, R. D., & Murphy, J. B. (1990). 40Ar/39Ar and U-Pb mineral ages from the Brookville Gneiss: implications for terrane analysis and evolution of Avalonian "basement" in southern New Brunswick. Atlantic Geoscience, 26(3). https://doi.org/10.4138/1707

Issue

Vol. 26 No. 3 (1990)

Section

Articles

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