Jared Butler

The petrography and mineral chemistry of granulite-facies metacarbonates from the Paleoproterozoic Lake Harbour Group (LHG), southwestern Baffin Island, reflect the combined effects of regional metamorphism and subsequent fluid infiltration. In the study area the LHG comprises polydeformed metacarbonates and quartzofeldspathic metasediments, intruded by syntectonic foliated monzogranites and post-deformation pegmatitic syenogranites. Impure marbles from the study area comprise two assemblages corresponding to variations in the proportion of carbonate versus silicate minerals: forsterite + spinel + calcite + dolomite, and forsterite + diopside + phlogopite + calcite; both consistent with relatively high XCO₂, and a minimum temperature of ca. 750^oC at 8.0 kbar. In the silicate-rich impure marbles, the earliest phase of metamorphism (M1a) involved the sequential formation of calcite, phlogopite, diopside, and forsterite by the following reactions: 3 Dol + Kfs + H₂O = Phl + 3 Cal + 3 CO₂, Dol + Qtz = Di + 2 CO₂, and 3 Dol + Di = 4 Cal + 2 Fo + 2 CO₂. The subsequent formation of vermicular K-feldspar and nepheline intergrowths is consistent with the reaction: 3 Ab + 3 Cal + Phl = Kfs + 3 Ne + 3 Di +3 CO₂, and implies decreasing XCO2, possibly reflecting to late- to post-M1a fluid infiltration. Scapolite, varying in composition from Me31 - Me77, is locally present in calc-silicates, clinopyroxenites, and monzogranites associated with the LHG, and suggests the infiltration of Cl-rich fluids possibly related to post-deformation pegmatitic syenogranite emplacement. In Kimmirut, southern Baffin Island, sapphire formation in calc-silicate lenses within stratigraphically correlative marbles is linked to the intense alteration of nepheline and scapolite. The scapolite + nepheline association described here may represent a precursor assemblage, implying that LHG metacarbonates from southwestern Baffin Island may be good targets for gemstone

Keywords: Metamorphism, metasomatism, calc-silicates, Trans-Hudson orogen, sapphires
Pages: 136
Supervisor: Rebecca A. Jamieson / M. St.-Onge