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|Title:||Petrology, Geochronology and Geochemistry of the White Bear Arm Complex and Associated Units, Grenville Province, Eastern Labrador|
|Keywords:||Petrology;Geochronology;Geochemistry;White Bear Arm;Grenville;Eastern Labrador|
|Abstract:||<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>|
|Appears in Collections:||Open Access Dissertations and Theses|
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