<p> The Hawke River Terrane of the Grenville Province in
eastern Labrador is dominated by a coronitic gabbronorite to
anorthosite body plus associated amphibolitic to granodioritic
bodies known as the White Bear Arm Complex (WBAC). Petrographic
and geochemical evidence supports a constant volume
sub-solidus diffusion origin for double coronas of orthopyroxene
and amphibole around olivine. Geochemical evidence
suggests that corona formation was not completely isochemical,
requiring an influx of potassium, iron and magnesium, but on a
whole-rock scale was probably closed to the latter two
elements. Nd and Sr isotopic evidence indicates that the
coronas did not form as a response to a much later (eg.
Grenville aged) metamorphic event, and probably. developed
during post-crystallisational cooling or during a metamorphism
shortly thereafter. </p> <p> The WBAC is cut by a monzonitic to granodioritic body
mineralogically and chemically similar to the WBAC
'monzonite', known as the Paradise Arm Pluton (PAP).
Petrographic, geochemical, and field relationship evidence
indicates an igneous origin for the PAP and the WBAC
monzonite. Both of these units have been subjected to low
grade (greenschist facies) metamorphism. The WBAC and the PAP
are hosted by the Paradise Metasedimentary Gneiss Belt, an aluminous paragneiss which has been raised to middle to upper
amphibolite grade metamorphism, except immediately adjacent to
the contact with the WBAC, where granulite grade has been
obtained, forming high-temperature aluminosilicates such as
osumilite and sapphirine. The contact zone has also been
contaminated by fluids from the gabbronorites, reflected by
petrography, elemental and isotopic geochemistry. </p> <p> Field relationships and geochemistry are ambiguous
regarding whether the monzonitic units were derived by
fractional crystallisation of the gabbronorite or partial
melting of the paragneisses, but trace element variation
favours the latter model. The PAP is a product of physical,
geochemical and Sr isotopic mixing between a granodioritic
end-member and a paragneissic one. </p> <p> Sm-Nd isotope systematics on whole-rocks do not provide a
precise emplacement age for the WBAC gabbronorite, but
indicate a Labradorian age (ca. 1.65 Ga old). Rb-Sr wholerock
systematics show a Labradorian-aged trend and a steeply
sloping trend interpreted as a mixing line with an unspecified
crustal component, possibly corresponding to the potassium
metasomatism indicated earlier. Rb-Sr on minerals from two
rocks produces a precise age of 1675 ± 15 Ma and a similar,
imprecise age. </p> <p> The WBAC monzonite and the Paradise Arm Pluton give Rb-Sr
ages of 1621 ± 11 Ma CMSWD=24) and 1573 % 18 Ma (MSWD=9)
respectively, with low initial 87Sr/ 86Sr ratios around 0.7040.
These are interpreted as a minimum emplacement age and a
mixing line, respectively. The PMGB paragneisses give a Rb-Sr
age of 1630 ± 16 Ma CMSWD=35), with an initial 87Sr/ 86Sr
composition around 0.704. All ages are equal within expanded
2σ error of each other (ie. MSWD=1), and give Labradorian
ages. There is no evidence of Grenville-aged (ca. 1.0 Ga old)
metamorphism. </p> <p> Rb-Sr isotopic and E Nd -E Br variations suggest crustal
contamination of the gabbronorites, although the contaminant
cannot be identified. Two separate mechanisms are required to
generate the observed trends: a fluid metasomatism and a bulk-rock
crustal contamination, or else contamination by currently
unidentified low Sr crustal material. The WBAC gabbronorites
provide evidence for 'extra'-LREE-depleted mantle beneath
eastern Labrador at 1.65 Ga, with E Nd of at least +7.6. </p> / Thesis / Master of Science (MSc)
| Identifer | oai:union.ndltd.org:mcmaster.ca/oai:macsphere.mcmaster.ca:11375/19598 |
| Date | 12 1900 |
| Creators | Prevec, Stephen |
| Contributors | McNutt, R.H., Geology |
| Source Sets | McMaster University |
| Language | English |
| Detected Language | English |
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