Dana Thomson

B.Sc. (Honours) Thesis

Investigating fluid exchange between granitic hosts, dykes and enclaves in the South Mountain Batholith

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This study performed detailed geochemical analyses of primary and secondary minerals from the Sambro Head dyke in order to further investigate the hypothesis that it was formed via hybridization. Recent studies have suggested that mafic enclaves within the (Late Devonian) South Mountain Batholith are mela-granitic restites formed in part from the melting of metasedimentary rocks. Detailed textural studies conducted in the Sambro Head area have revealed clear evidence of magma mixing between the host granite and a mafic dyke intrusion. Textural and micro-analytical studies also show resorption textures and Ba zoning in K-Feldspar megacrysts, indicating significant temperature changes, supporting the hypothesis that hybridization has occurred. However, hybridization and fluid-alteration of the dyke makes determining the original composition extremely difficult. This study investigated the fluid exchange reactions between the host granitic rock and the Sambro Head dyke using whole rock relationships as well as the analysis of fluid-bearing phases. Whole rock samples from the dyke, the mixing zone, and the adjacent peraluminous granites were collected and analyzed using the Electron Probe Micro-Analyzer (EPMA) and Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS). The dyke’s mineral assemblage consists of plagioclase + quartz + biotite + oxides, along with secondary chlorite + muscovite and a significant amount of apatite, much of which appears to be primary. The extensive chlorite + muscovite replacement of biotite indicates continued fluid exchange reactions prior to final crystallization of the dyke. EPMA maps and thin section data show that this replacement is fairly consistent in all samples from within the dyke, with slightly higher replacement of biotite occurring near the edges of the dyke. The biotite selvage adjacent to the mixing zone also suggests evidence of disequilibrium and fluid exchange between the dyke and the granitic host. Small groups of cordierite can be found in similar places, along the edge of the dyke. Three geothermometry methods: Titanium-in-biotite, biotite-apatite and chlorite geothermometry were applied to calculate mineral formation temperatures of the dyke. Peak formation temperatures using the Titanium-in-biotite and biotite apatite methods were ~648°C and ~542°C respectively. The resulting chlorite peak formation temperature was ~325°C. These were used to create a temperature profile and crystallization history of the Sambro Head dyke supporting the hypothesis that hybridization occurred.

Keywords: dyke, hybridization, fluid reactions, magma mixing, geothermometry
Pages: 66
Supervisor: Richard Cox