Abstracts
Abstract
Rb-Sr i sot op I c data is presented for three suites of Ip^eous rocks: the Fourchu Group; volcanic rocks from Ingonlsh Island; and the Gulch Brook pluton. Results froi 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 elp^t sables yield an age of 407 ± 46 Ma which is be I leved 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 strontlui and rubldlui Isotopes suggests that the Initial ^Sr/^r ratios In the range 0.703-0.705 should be viewed with caution.
A Hhole-rock Rb-Sr isochron for volcanic rocks froe Ingonlsh Island yield an age of 412 ± 15 Ma, Interpreted to closely date the tlie of extrusion and crystallization. An Initial ^Sr/^Sr ratio of 0.706 suggests the lower crust as one possible source region.
The 413 ± 10 Ma Rb-Sr whole-rock Isochron obtained froi samples of the weakly foliated Gulch Brook pluton In northern Cape Breton Island probably represents the age of crystallization. Micas, vtilch 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 therial event during the Late Devonian-Early Carboniferous. The Initial ^Sr/^r 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 ^Sr/B^Sr ratios of the Ingonlsh Island volcanic rocks and the Gulch Brook Pluton suggest that they nay be volcanlc/subvolcanlc equivalents of one another.
RESUME
On presents les signatures Isotoplques Rb-Sr de trols series de roches Igiees: le groupe de Fourchu, des volcanltes de I'fle d'Ingonlsh et le pluton de Gulch Brook. L'analyse des metavolcanltes tardlprecanbrlemes du groupe de Fourchu montre une tres grande dispersion des valours imputable au reajustement a divers degres du system Rb-Sr. Sept des hult echanti I Ions ont dome un age de 407 ± 46 Ma qui sewble tradulre les effets de I'orogenie acadleme dans ce secteur. Un Isochrone de roche globale a deux points, provenant des deux echanti I Ions les wo Ins a I teres, a prodult un age autour de 640 Ma qui pourrait approxlmer I'age de I'epanchement. La redistribution des isotopes de strontium et rubidium rend dlscutables les rapports ^Sr/^r Inltlaux aux environs de 0.703-0.705.
Un Isochrone Rb-Sr de roche globale pour les volcanltes de I'tie d'Ingonlsh a rendu un age de 412 ± 15 Ma qui correspondralt de tres pres au temps de I'extrusion et la cristalllzatlon. Le rapport wSr/*%r Initial de 0.706 evoque une genese crustale profonde.
Des echantiI Ions provenant du pluton tres peu folie de Gulch Brook, dans I'fle du Cap-Breton septentrionale, ont prodult un Isochrone Rb-Sr de roche globale de 413 ± 10 Ma qui date probablement I'age de sa cristalllzatlon. Une etude anterleure des micas (qui imprlment au pluton sa faible foliation) a deja dome un Isochrons Rb-Sr de ilneral de 320 Ma ou des Ages calcules de facer Individual re de 363 Ma pour la nuscovlte et 350 Ha pour la blotIte (Confer, 1980; com. ecrlte, 1984). On pense y deviner un reajustewnt lie a un episode theraique ulterleur tardldevonlen ou eocarbonifere. Le rapport ^Sr/^Sr Initial de 0.7045 ± 0.0004 suggere we participation de la create continentals inferleure avec une possible contribution aantellique. La slillltude des ages et des rapports ^Sr/^r Inltlaux des volcanltes de I'tie d'lngonlsh a Ins I que du pluton da Gulch Brook nous Invite a conslderer leur consangulnlte.
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