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The petrogenesis of the Ta-bearing Fir carbonatite system, east-central British Columbia, Canada Chudy, Thomas Christof


This dissertation investigates the petrogenesis of the Fir carbonatite system (Monashee Mountains of British Columbia), which is particularly interesting because of its high degree of deformation, the relatively minor presence of associated silicate rocks and its comparably high content of Ta. A detailed examination of the rock textures and microstructures shows that the two main fabrics, a primary gneissic and a secondary fine-grained, foliated fabric, are the results of plastic flow and dynamic recrystallization during deformation. The primary fabric developed under peakmetamorphic conditions and was overprinted by retrograde mylonitic shear zones. The microtextural record and the equilibration temperatures are compared to regional marbles that share the same tectonometamorphic history. Both lithologies reveal a very similar petrological record indicative of metamorphic equilibration, however, some calculated temperatures in the carbonatites (∼700 °C) exceed the peak-metamorphic conditions (620 - 650 °C), which indicates that the magmatic crystallization temperatures are preserved despite amphibolite-facies metamorphism. Apart from minor calcium and sodium amphiboles the Fir system contains predominantly the sodium-calcium amphiboles winchite and katophorite which define two major mineralogical facies. The amphiboles have high F contents (<1.6 a.p.f.u.) and show a systematic increase in the Na₂O content from the margin to center of the carbonatite. This distribution is paralleled by the Nb-Ta mineralization with ferrocolumbite in the winchite facies and pyrochlore in the katophorite facies. The two Nb-Ta-oxide minerals are stable under acidic and alkaline conditions, respectively, which are strongly influenced by the proximity to the country rocks. The geochemical composition is characterized by low REE and LILE concentrations, elevated Ta₂O₅ contents and subchondritic Nb/Ta ratios. The observed compositional differences can be attributed to the simple mineralogy that developed by in-situ differentiation of a primary dolomitic weakly fractionated mantle melt. The key factors for the enrichment of Ta in the Fir system are the relatively high F content of the melt and the high emplacement temperatures which increased the solubility of Ta in the melt and favoured the relative late precipitation of pyrochlore leading to the accumulation of Ta₂O₅.

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