Rb-Sr isotopic data from three suites of igneous rocks, Cape Breton Island, Nova Scotia

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Authors:Keppie, J. Duncan; Halliday, A. N.
Author Affiliations:Primary:
N.S. Dep. Mines and Energy, Halifax, NS, Canada
Scott. Univ. Res. and React. Cent., United Kingdom
Volume Title:Maritime Sediments and Atlantic Geology
Source:Maritime Sediments and Atlantic Geology, 22(2), p.162-171. Publisher: Atlantic Geoscience Society, Fredericton, NB, Canada. ISSN: 0711-1150
Publication Date:1986
Note:In English with French summary. 21 refs.; illus. incl. 3 tables, sketch maps
Summary:Rb/Sr isotopic data are presented for three suites of igneous rocks: the Fourchu group, volcanic rocks from Ingonish Island, and the Gulch Brook pluton. Results from the late Precambrian metavolcanic rocks of the Fourchu group show considerable scatter due to varying degrees of resetting of the Rb-Sr system. Seven of the eight samples yield an age of 407 + or - 46 m.y. 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 yields an approx 640 m.y. age which could approximate the extrusive age. The redistribution of Sr and Rb isotopes suggests that the initial 87Sr/86Sr ratios in the range 0.703-0.705 should be viewed with caution. A whole-rock Rb-Sr isochron for volcanic rocks from Ingonish Island yields an age of 412 + or - 15 m.y., interpreted to closely date the time of extrusion and crystallization. An initial 87Sr/86Sr ratio of 0.706 suggests the lower crust as one possible source region. The 413 + or - 10 m.y. Rb-Sr whole-rock isochron obtained from samples of the weakly foliated Gulch Brook pluton in N 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 m.y. or individually calculated ages of 363 m.y. for muscovite and 350 m.y. for biotite (Cormier, 1980; written comm. 1984) and are believed to represent resetting by a later thermal event during the late Devonian-early Carboniferous. The initial 87Sr/86Sr ratio of 0.7045 + or - 0.0004 suggests a lower crustal source for the magma, possibly with a mantle component as well. The similarity of the ages and initial 87Sr/86Sr ratios of the Ingonish Island volcanic rocks and the Gulch Brook Pluton suggest that they may be volcanic/subvolcanic equivalents of one another. (Authors' abstract) [A.W.H.]
Subjects:Absolute age; Alkaline earth metals; Dates; Fourchu Group; Geochemistry; Geochronology; Hadrynian; Igneous rocks; Intrusions; Isochrons; Isotopes; Metals; Neoproterozoic; Paleozoic; Petrology; Plutons; Precambrian; Proterozoic; Rb/Sr; Silurian; Sr-87/Sr-86; Stable isotopes; Strontium; Upper Precambrian; Volcanic rocks; Whole rock; Canada; Cape Breton Island; Eastern Canada; Maritime Provinces; Nova Scotia; Gulch Brook Pluton; Ingonish Island; Ratios
Abstract Numbers:87M/5394
Record ID:1987031428
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from Mineralogical Abstracts, United Kingdom, Twickenham, United Kingdom
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