FLUID-FLUXED MELTING OCCURS ON A REGIONAL SCALE IN THE GRENVILLE PROVINCE

Fluid-fluxed melting is typically considered to generate insignificant amounts of melt during orogenesis¹. Instead, fluid absent hydrate breakdown is the primary mechanism suggested to explain large-scale melting in the crust during orogenesis. In recent years, various studies have presented field evidence and models to suggest that fluid-fluxed melting plays a more important role in melt-generation than we previously thought². One region where fluid-flux melting has been proposed as the dominant melting mechanism is the Muskoka domain in the Grenville province³. The Muskoka domain and its neighbouring equivalents make up a contiguous region over 200 km along the strike of the Grenville orogen. The rocks of this region are highly deformed, resulting in cryptic evidence for fluid-fluxed melting. These deformed rocks typically consist of stromatic and patch migmatites, exhibiting peritectic amphibole and volumes of leucosome far greater than what should be present from dehydration melting alone. In addition to this cryptic evidence, we have found rare occurrences of relatively undeformed rock that contain migmatites with a high abundance of amphibole localized around cross-cutting pegmatites. Textures and composition of these migmatite reaction zones are best explained by fluid-flux melting. Several of these migmatite reaction zones have been observed in rocks sharing chemical similarity to the stromatic mesosomes common throughout the region. In addition, zircon ages from the pegmatites and the stromatic leucosomes are coeval. This suggests that these are less-deformed equivalents of the stromatic migmatites observed extensively throughout this region. If that is the case, this would be a rare example of terrane-scale fluid-flux melting.

[1] Brown, M. Granite: From genesis to emplacement. Geol. Soc. Am. Bull. 125, 1079–1113 (2013).

[2] Weinberg, R. F. & Hasalová, P. Water-fluxed melting of the continental crust: A review. Lithos 212–215, 158–188 (2015).

[3] Slagstad, T., Jamieson, R. A. & Culshaw, N. G. Formation, Crystallization, and Migration of Melt in the Mid-orogenic Crust: Muskoka Domain Migmatites, Grenville Province, Ontario. J. Petrol. 46, 893–919 (2005).