THE TRANSITION FROM MAGMATIC TO HIGH-TEMPERATURE SOLID-STATE DEFORMATION - IMPLICATIONS FROM THE MOUNT STUART BATHOLITH, WASHINGTON

Criteria for syntectonic emplacement of plutons are commonly ambiguous. The strongest single criterion is probably the preservation of a continuous transition from submagmatic to high-temperature solid-state deformation. This transition is not commonly documented. An exception is the Mount Stuart batholith, which displays a variety of syntectonic structures within its sheared northeastern margin, including the following: (1) S-C fabrics that range from submagmatic to high-temperature solid-state; (2) solid-state foliation and lineation defined by amphibolite-facies assemblages that overprint and are parallel to magmatic foliation and lineation; (3) folds of magmatic and solid-state foliation that have hinge lines parallel to magmatic and solid-state lineation and to equivalent structures in wall rocks; (4) dikes that were folded and boudinaged over a wide range of rheological states with boudin necks locally filled by tonalite; (5) pegmatite dikes and mineralized joints that are sub-perpendicular to magmatic and solid-state lineation; and (6) submagmatic and high-temperature solid-state ductile shear zones. We suggest that many plutons emplaced during regional deformation do not preserve evidence for syntectonic deformation because of the transitory nature of the submagmatic state and the obscuring effects of post-emplacement deformation. Syntectonic features are most likely preserved in plutons cooled at slow to moderate rates, or in plutons deformed at high strain rates during emplacement. The optimum conditions for preservation may occur in plutons emplaced along fault zones in the mid-crust, such as the Mount Stuart batholith, and in intrusions at deeper levels that were rapidly exhumed and/or intruded during the waning stages of regional deformation.