Roberta Jean Hicks
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B. Sc. Honours Thesis
(PDF - 8.8 Mb)
A garnet amphibolite body in the Parry Island shear zone, Central Gneiss Belt, Grenville Province, shows textural and mineralogical evidence of retrograde metamorphism to amphibolite facies. Garnets with plagioclase coronas, and clinopyroxene replaced by amphibole, illustrate the reaction Gnt + Cpx + H2O = Plag + Hb, with the degree of reaction dependent upon deformation and fluid access. Petrological, compositional and thermobarometric studies support the proposal that these rocks are retrograded form granulite facies. The least deformed sample best preserves relict textures and phases (relatively abundant clinopyroxene-plagioclase intergrowths associated with highly embayed garnets, and minor plagioclase corona development around less embayed garnets), and gives compositional evidence (An content in plagioclase, Al2O3/SiO2, and AlIV in amphiboles) of former higher temperatures. The same sample yields the highest pressure and temperature estimates. Another sample has amphiboles with pleochroism indicative of growth over a range of P-T conditions, and shows evidence that the most embayed garnets formed under higher pressure conditions. Textural similarities between the amphibolite and a meta-eclogite in the Britt domain suggest an earlier high-grade history for the amphibolite, however, evidence for an eclogite facies history is inconclusive. Temperature and pressure estimates average 650oC and 6-7 kbars.
Keywords: Amphibolite, eclogite facies, geothermobarometry, granulite facies, Grenville Province, metamorphic
history, retrogression
Pages: 102
Supervisor: Rebecca Jamieson