Assessing silicate melt behaviour with granite microstructure

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The upward flow and crystallization of magma (molten rock) are two of the most important processes in the formation and evolution of Earth's crust. These processes are particularly important within the continental crust, where the majority of magmas are granitic in composition. Understanding the evolution of granitic magma is critical because (a) granite is a principal carrier of many economic minerals, including gold, copper, tin, tungsten and uranium, and (b) the genesis and emplacement of granitic magma has had a significant bearing on continental evolution from the Precambrian to the present day.*** Little is known about the details of granitic magma migration and crystallization because we have not yet developed a set of rigorous methods to measure and evaluate these processes. As such, we are currently unable to describe, in any given body of granite, how the crystal and liquid parts of the magma behaved, and how the rate of cooling affected crystal growth and grain dimensions. This gap in knowledge severely limits our understanding of the most fundamental physical, chemical, spatial and temporal conditions of granite formation and, in turn, the evolution of the continental crust. In the proposed program of research, I will investigate the internal behaviour of granitic magma by applying recent advances in the analysis and interpretation of rock microstructures. These investigations will be made on samples collected during new field work planned for Canada and South Africa. An immediate result of my research program will be a new set of methods to (1) identify the mobility of crystals and liquid during granite crystallization, and (2) quantify the crystallization rate of granitic intrusions. These methods will be used to parameterize the evolution of granitic magma and ultimately improve our understanding of large continental magmatic systems.*** The proposed research will consist of two interrelated themes: I. the internal behaviour of high-silica granites (>70 wt.% SiO2); and II. the thermal evolution of granitic intrusions. We will use crystallographic orientation relationships of the mineral quartz to determine if crystals were mobile during crystallization and if crystalmelt segregation occurred. The results will allow us to evaluate the influence of intrusion composition and dimensions on the physical evolution of granitic bodies. To address gaps in our understanding of cooling behaviour of granitic magma, we will develop and calibrate a new crystallization rate speedometer, based on the morphology of the mineral plagioclase and the chemical composition of the granite. Given the nearly ubiquitous occurrence of plagioclase in granitic rocks, this method will be applicable to the study of magma dynamics and thermal histories of magmatic complexes worldwide. The results of my research will also provide a new platform for the understanding of granite-related mineral deposits.**