Sm-Nd and Rb-Sr isotope systematics of an Archean anorthosite and related rocks from the Superior Province of the Canadian Shield

Saved in:
Authors:Ashwal, L. D.; Wooden, J. L.; Phinney, W. C.; Morrison, D. A.
Author Affiliations:Primary:
Lunar and Planet. Inst., Houston, TX, United States
Other:
NASA/Johnson Space Cent., Exp. Planet. Branch, United States
Volume Title:Earth and Planetary Science Letters
Source:Earth and Planetary Science Letters, 74(4), p.338-346. Publisher: Elsevier, Amsterdam, Netherlands. ISSN: 0012-821X
Publication Date:1985
Note:In English. 34 refs.; illus. incl. 2 tables, geol. sketch map
Summary:The Sm/Nd and Rb/Sr isotopic data for the Bad Vermilion Lake anorthosite complex (BVL) in the Rainy Lake area of NW Ontario, Canada, show that direct ages of Archaean anorthosites can be obtained with these isotopic systems, despite the effects of low-grade metamorphism. There is sufficient spread of Sm/Nd between plagioclase megacrysts and coexisting mafic groundmass to allow the determination of reasonably precise internal Sm/Nd isochrons. Anorthosite samples from BVL show an unusually large range in LIL concentrations, such that there is sufficient spread in Rb/Sr for a whole-rock isochron (2690 + or - 100 m.y., ISr = 0.70097 + or - 8). This variability may have been caused by Rb introduction during hydrothermal alteration and/or low-grade metamorphism. The Sm/Nd isochron for BVL (2747 + or - 58 m.y., epsilonNd = 2.0 + or - 1.4) includes data for anorthosite, gabbro and metabasalt, and is consistent with the consanguineity of these units in the Rainy Lake area. The age is interpreted as the age of crystallization of the anorthosite complex and related plutons and mafic rocks. Visibly altered samples show evidence for disturbance of the Sm/Nd and Rb/Sr systems. In one altered porphyritic dyke plagioclase appears to have exchanged LREE with the relatively REE-rich basaltic matrix. The sample yields an internal Sm/Nd age of 2160 + or - 50 m.y. which may correspond with the time of local heating or represent a partial resetting from a still younger event. The Nd and Sr results add to the growing body of data indicating that the Superior Province is underlain by depleted mantle. Rb/Sr and Sm/Nd data for BVL anorthosite and related rocks are presented. [P.Br.]
Subjects:Absolute age; Anorthosite; Archean; Dates; Geochronology; Hydrothermal alteration; Igneous rocks; Isotopes; Low-grade metamorphism; Metamorphism; Metasomatism; Plutonic rocks; Precambrian; Rb/Sr; Sm/Nd; Canada; Canadian Shield; Eastern Canada; Minnesota; North America; Ontario; Rainy Lake; Superior Province; United States; Abitibi; Bad Vermilion Lake; Keewatin Group
Abstract Numbers:86M/1246
Record ID:1986014011
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from Mineralogical Abstracts, United Kingdom, Twickenham, United Kingdom
Tags: Add Tag
No Tags, Be the first to tag this record!
Be the first to leave a comment!
You must be logged in first