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Sr, Nd and Pb isotopes in Proterozoic intrusives astride the Grenville Front in Labrador: Implications for crustal contamination and basement mapping

Geochimica et Cosmochimica Acta

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Abstract

We report Sr, Nd and Pb isotopic compositions of mid-Proterozoic anorthosites and related rocks (1.45-1.65 Ga) and of younger olivine diabase dikes (1.4 Ga) from two complexes on either side of the Grenville Front in Labrador. Anorthositic or diabasic samples from the Mealy Mountains (Grenville Province) and Harp Lake (Nain-Churchill Provinces) complexes have very similar major, minor and trace element compositions, but distinctly different isotopic signatures. All Mealy Mountains samples have ISr = 0.7025-0.7033, ??{lunate}Nd = +0.6 to +5.6 and Pb isotopic compositions consistent with derivation from a mantle source depleted with respect to Nd/Sm and Rb/Sr. Pb isotopic compositions for the Mealy Mountains samples are slightly more radiogenic than model mantle compositions. All Harp Lake samples have ISr = 0.7032-0.7066, ??{lunate}Nd = -0.3 to -4.4 and variable, but generally unradiogenic 207Pb 204Pb and 206Pb 204Pb compared to model mantle, suggesting mixing between a mantle-derived component and a U-depleted crustal contaminant. Crustal contaminants are probably a variety of Archean high-grade quartzofeldspathic gneisses with low U/Pb ratios and include a component that must be isotopically similar to the early Archean (>3.6 Ga) Uivak gneisses of Labrador or the Amitsoq gneisses of west Greenland. This would imply that the ancient gneiss complex of coastal Labrador and Greenland is larger than indicated by present surface exposure and may extend in the subsurface as far west as the Labrador Trough. If Harp Lake and Mealy Mountains samples were subjected to the same degree of contamination, as suggested by their chemical similarities, then the Mealy contaminants must be much younger, probably early or middle Proterozoic in age. The Labrador segment of the Grenville Front, therefore, appears to coincide with the southern margin of the Archean North Atlantic craton and may represent a pre mid-Proterozoic suture. ?? 1986.

Additional Publication Details

Publication type:
Article
Publication Subtype:
Journal Article
Title:
Sr, Nd and Pb isotopes in Proterozoic intrusives astride the Grenville Front in Labrador: Implications for crustal contamination and basement mapping
Series title:
Geochimica et Cosmochimica Acta
Volume
50
Issue:
12
Year Published:
1986
Language:
English
Larger Work Type:
Article
Larger Work Subtype:
Journal Article
Larger Work Title:
Geochimica et Cosmochimica Acta
First page:
2571
Last page:
2585
Number of Pages:
15