TECTONIC CONDITIONS FOR PEGMATITE-INDUCED HYDRATION AND KM-SCALE WEAKENING IN DEEP OROGENIC CRUST

The Parry Sound domain, a granulite nappe within the Central Gneiss Belt of the Grenville orogen exhibits a newly recognized carapace of retrogression and transposition at its southwestern margin. The base of the carapace and the high-strain rocks of subjacent domains form the kms-wide Twelve Mile Bay shear zone. In the base of the carapace, retrogression to upper amphibolite facies assemblages due to hydration is associated with the intrusion of pegmatites along spaced fractures. The consequent rheological softening, allowed the Parry Sound domain segment of the Twelve Mile Bay shear zone to develop. Identification of the general tectonic setting in which this process can occur allows better prediction and recognition in other orogenic systems. We establish the following constraints. (1) A hot, dry, presumably granulite block must override a hydrous, fertile footwall. (2) Due to downward heat conduction or other mechanisms accompanying emplacement, the footwall must melt sufficiently to produce granitic magma but not become incompetent relative to the overriding block. The footwall can retain its strength due to melt expulsion or restricted fertility, or the nappe can move from over an incompetent to over a competent footwall. (3) Magma pressure induces fractures within the margin of the dry hanging wall that are then filled with the magma. (4) The hydrous magma yields water upon crystallization sufficient to infiltrate the host and cause retrogression. Thermal modeling demonstrates the feasibility of melting of fertile footwall rocks in response to a 10 km thick lower crustal block wedging into the middle crust. Tectonic conditions amenable for such nappe-like emplacement of a granulite block over fertile material are likely only in large, hot orogens where the middle crust is sufficiently weak to accommodate nappe expulsion.