Characteristics and consequences of H2O undersaturation and low melt fertility in mountain belt interiors

Nicholas A Lucas , Owen Weller, Marc R St-Onge

Continental deformation and crustal magmatism in orogenic settings are fundamentally linked to fluid availability and melting processes. Critical rheological transitions occur at approximately 7% melt volume, whereby melt-film connectivity reduces rock strength by as much as c. 90%, and at 20%, marking an onset of solid-to-liquid behaviour. The Mesoproterozoic Grenville orogen in Canada provides an ideal orogenic locality to study such processes owing to its popular view as a 'large hot orogen' characterised by mid-crustal weakening; yet, large regions of the exposed orogenic mid-crust remain poorly understood in terms of melting history. Here we present new constraints on peak metamorphic P-T conditions, H2O saturation and melt productivity across a 130-km across-strike transect perpendicular to the Sole Thrust in the western Grenville Province. While phase equilibrium modelling reveals relatively constant peak P-T conditions of ~10 kbar and ~750 C across the transect, associated melt development was heterogeneous due to variable bulk rock chemistry and hydrous mineral content. Localised occurrences of aluminous pelite yielded greater than 20 vol.% melt, but most regions record subsolidus conditions or generated low melt volumes, below the melt connectivity threshold, due to being fluid-undersaturated. Pervasive ductile flow and large-scale crustal weakening were therefore unlikely for this segment of the orogen. Even under conditions of H2O saturation, temperatures exceeding 800 C would have been required at 10 kbar to generate sufficient melt for bulk viscous flow (greater than 20% melt).