Rb-Sr Isotopic Data from Three Suites of Igneous Rocks, Cape Breton Island. Nova Scotia

Authors

  • J. Duncan Keppie Department of Mines and Energy, P.O. Box 1087, Halifax, Nova Scotia B3J 2X1
  • A. N. Halliday Scottish Unlversltles Research and Reactor Centre East Kilbride. Glasgow, Scotland, G7S OQU

DOI:

https://doi.org/10.4138/1605

Abstract

Rb-Sr isotopic data is presented for three suites of Igneous rocks: the Fourchu Group; volcanic rocks from Ingonish Island; and the Gulch Brook pluton. Results from the Late Precambrlan metavolcanic rocks of the Fourchu Group show considerable scatter due to varying degrees of resetting of the Rb-Sr system. Seven of the sight samples yield an age of 407 ± 46 Ma which is believed to reflect the effects of the Acadian Orogeny in this area. A two-point whole-rock Isochron from the two freshest samples yield a c. 640 Ma age which could approximate the extrusive age. The redistribution of strontlus and rubidus Isotopes suggests that the Initial 87Sr/88Sr ratios in the range 0.703-0.705 should be viewed with caution. A whole-rock Rb-Sr isochron for volcanic rocks froe Ingonlsh Island yield an age of 412 ± 15 Ma, Interpreted to closely date the time of extrusion and crystallization. An Initial 87Sr/88Sr ratio of 0.706 suggests the lower crust as one possible source region. The 413 ± 10 Ma Rb-Sr whole-rock Isochron obtained from samples of the weakly foliated Gulch Brook pluton In northern Cape Breton Island probably represents the age of crystallization. Micas, which define the weak foliation In the pluton, have previously yielded a Rb-Sr mineral Isochron of 320 Ma or Individually calculated ages of 363 Ma for muscovlte and 350 Ma for blotite (Cormier, 1980; written con. 1984) and are believed to represent resetting by a later thermal event during the Late Devonian-Early Carboniferous. The initial 87Sr/88Sr ratio of 0.7045 ± 0.0004 suggests a lower crustal source for the magma, possibly with a mantle component as wall. The similarity of the ages and Initial 87Sr/88Sr ratios of the Ingonlsh Island volcanic rocks and the Gulch Brook Pluton suggest that they nay be volcanic/subvolcanic equivalents of one another. RÉSUMÉ On présents les signatures isotoplques Rb-Sr de trois séries de roches Ignées: le groupe de Fourchu, des volcanites de L’ile d'Ingonlsh et le pluton de Gulch Brook. L'analyse des métavolcanites tardiprécanbriennes du groupe de Fourchu montre une très grande dispersion des valours imputable au reajustement à divers degrés du système Rb-Sr. Sept des huit échantillons ont donné un âge de 407 ± 46 Ma qui semble traduire les effets de L’orogénie acadianne dans ce secteur. Un Isochrone de roche globale à deux points, provenant des deux échantiIIons les moins aItérés, a prodult un âge autour de 640 Ma qui pourrait approximer L’âge de L’épanchement. La redistribution des isotopes de strontium et rubidium rend discutables les rapports 87Sr/88Sr initiaux aux environs de 0.703-0.705. Un Isochrone Rb-Sr de roche globale pour les volcanites de L’ile d'Ingonlsh à rendu un âge de 412 ± 15 Ma qui correspondralt de très près au temps de L’extrusion et la cristalllzatlon. Le rapport 87Sr/88Sr initial de 0.706 évoque une génèse crustale profonde. Des échantiIIons provenant du pluton très peu folié de Gulch Brook, dans L’ile du Cap-Breton septentrionale, ont produit un Isochrone Rb-Sr de roche globale de 413 ± 10 Ma qui date probablement L’âge de sa cristalllzatlon. Une étude antérleure des micas (qui impriment au pluton sa faible foliation) a déjà donné un Isochrons Rb-Sr de minéral de 320 Ma ou des âges calculés de façon individualle de 363 Ma pour la muscovite et 350 Ha pour la blotIte (Confer, 1980; com. écrlte, 1984). On pense y deviner un réajustement lié à un épisode theraique ultérieur tardidévonlen ou éocarbonifère. Le rapport 87Sr/88Sr initial de 0.7045 ± 0.0004 suggère we participation de la create continentals inférieure avec une possible contribution aantellique. La similitude des âges et des rapports 87Sr/88Sr initiaux des volcanites de L’ile d'lngonlsh ainsi que du pluton da Gulch Brook nous invite à considérer leur consangulnlté. [Traduit par le journal]

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Published

1986-08-01

How to Cite

Keppie, J. D., & Halliday, A. N. (1986). Rb-Sr Isotopic Data from Three Suites of Igneous Rocks, Cape Breton Island. Nova Scotia. Atlantic Geoscience, 22(2). https://doi.org/10.4138/1605

Issue

Vol. 22 No. 2 (1986)

Section

Articles

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