Fluid-present melting of the deep crust: implications for the tectonics of large convergent orogens

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The interaction of fluids with rocks in the deep continental crust plays a critical role in the redistribution of mass and energy during the buildup and collapse of mountain belts and the genesis of ore deposits. During the construction of large mountain belts, crustal thickening generates elevated temperatures and pressures in the deep crust. Widespread fluid influx into the deep hot crust beneath mountain belts promotes partial melting, which drastically weakens large regions of continental crust. These melt-weakened regions may act as large-scale shear zones that facilitate mountain buildup or may flow laterally towards orogenic fronts resulting in the formation of high-elevation plateaux. For example, geophysical imaging suggests that large regions of partially molten crust are present beneath the Himalayan mountain belt and the lateral movement of this melt-weakened material is proposed to play a first-order role in the development of the Tibetan Plateau. While widespread fluid-fluxed partial melting may have important tectonic consequences, the sources, compositions and transport pathways of these fluids are poorly constrained. This is due, in part, to the lack of access to the deep crust beneath modern mountain belts. The Grenville Province of southwestern Ontario represents the deeply eroded core of an ancient Himalayan-scale mountain belt. This region represents a natural laboratory to investigate the processes that are occurring beneath modern mountain belts — such as the Himalaya in Asia and the Andes in South America. This research program will investigate the role of fluid infiltration and partial melting on the tectonic evolution of an ancient mountain belt using both geological field mapping and geochemical laboratory work. Fieldwork will constrain the transport pathways for fluids in the deep crust exposed in Ontario. Laboratory work will evaluate the source and compositions of the fluids as well as the timing of fluid infiltration using modern geochemical techniques at the University of Waterloo. The results of this research program will provide insights into the role of fluid infiltration in the deep crust on the tectonic evolution of modern and ancient mountain belts. Fluids play an important role in scavenging elements of economic interest (e.g. gold, uranium, and tin) from the deep crust and transporting them to the shallower crust where they generate mineral deposits. However, our understanding of fluid–rock interaction in the deep crust is still rudimentary. Therefore, a broader implication of this research program is an improved understanding of fluid–rock interaction beneath modern and ancient mountain belts, which can be used to improve genetic models of metalliferous ore deposits and to prioritize exploration for these deposits in Canada and around the world.