Jillian Kendrick
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1st Class Honours, University Medal In Earth Sciences
B.Sc. (Honours) Thesis
(PDF - 18.9 Mb)
The petrology, chemistry, and time of intrusion of alkaline mafic bodies in the southwest Grenville Province have been used to differentiate between autochthonous and allochthonous domains in this region. In the allochthons, the characteristic mafic suite is a group of coronitic metagabbros, the Algonquin metagabbros, which intruded at approximately 1152 Ma. Metamorphism at ca. 1050 Ma produced spectacular coronitic textures between igneous olivine and plagioclase and between igneous Fe-Ti oxides and plagioclase. This study examines in detail one metagabbro body exposed as a continuous 150 m wide outcrop in a roadcut near Emsdale, Ontario. Corona structures are used to estimate the P-T conditions experienced by this coherent body during metamorphism, test hypotheses of corona formation from previous work, and deduce its textural history.
Samples are divided into three types differentiated by their mineral assemblages and textures. In Type 1 samples, which have the best preserved igneous texture, opx + Fe-Ti oxide symplectite pseudomorphs after primary olivine are separated from plagioclase by coronas of opx, ± cpx, amph ± bt, grt + amph ± cpx ± opx ± plag symplectite. Coronas surrounding Fe-Ti oxides include amph + bt and grt + amph ± cpx ± opx ± plag symplectite. Type 2 samples represent the next developmental stage; relict igneous plagioclase and clinopyroxene are progressively recrystallized and a sodic plagioclase moat appears between amphibole and garnet. Type 3 samples, the latest stage, retain little to no igneous texture, with coronas largely obscured by late amph + plag + cpx + opx.
P-T estimates indicate that coronas in the metagabbro preserve the products of three stages of metamorphism. Type 1 preserves a prograde assemblage at ca. 800 ± 70°C and 11.0 ± 0.5 kb, Type 2 represents peak metamorphism at ca. 880 ± 70 °C and 13.0 ± 1.0 kb, and Type 3 phases overprinting coronas are retrograde at ca. 750 ± 70 °C and 9.5 ± 0.5 kb. The types define a “hairpin” shaped P-T path of metamorphism for the metagabbro body with a small temperature range. Corona formation was a prograde metamorphic process driven by diffusion until metamorphic fluids were intruded into the metagabbro around peak metamorphism, causing textural reworking of igneous phases and coronas in rock near veins. The opx + Fe-Ti oxide symplectite pseudomorphs after olivine are proposed to have formed by transfer of ilmenite from magmatic Fe-Ti oxides in response to an increase in fO2 of the system
Keywords: Grenville Province, Algonquin metagabbro, granulite facies metamorphism, coronitic metagabbro, multistage coronas, olivine corona, Fe-Ti oxide corona, olivine pseudomorph
Pages: 207
Supervisor: Rebecca Jamieson