EMPLACEMENT-RELATED MICROSTRUCTURES IN THE DEFORMED CARAPACE OF A TONALITE PLUTON: EVIDENCE FOR FAST CHAMBER CONSTRUCTION

The San José pluton in Baja California, México, is “post-kinematic” in that emplacement post-dated most or all of the regional ductile deformation in the surrounding country rocks. The pluton contains two texturally distinct, separate pulses referred to as the northern and central units. The northern unit was the first to be emplaced, and had partly crystallized before being intruded by an off-center pulse of the central unit tonalite. The contact between these two tonalite units indicates that they were juxtaposed as magmas, yet a sharp solid-state deformation gradient occurs over approximately 1 km within the margin of the northern unit. Rocks at the inner edge of this deformation gradient show an intrusive igneous microstructure, whereas rocks at the outer edge of the gradient show a mylonitic microstructure. Owing to the “post-kinematic” timing of the pluton, this solid-state deformation apparently occurred during emplacement of the central unit into the northern unit, as opposed to occurring during a later regional deformation overprint.

If the solid-state deformation in the northern unit was caused by intrusion of the central unit, microstructures in the deformed rocks are directly linked to growth of the central chamber, and so provide an unusual opportunity to evaluate how rapidly it was constructed. Field and theoretical studies indicate that strain rates associated with pluton emplacement can be much faster than rates associated with regional tectonic deformation. Rate estimates for emplacement-related deformation range from approximately 10^-7 s^-1 to 10^-12 s^-1, whereas regional tectonic strain rates are thought to lie in the range of 10^-13 s^-1 to 10^-15 s^-1. Evidence for melt-present deformation in these rocks includes fractured primary plagioclase grains that have been filled with K-feldspar, quartz and more albitic plagioclase, suggesting that deformation occurred at temperatures near the wet tonalite solidus (~680° C). At these temperatures, plagioclase should deform largely by dislocation creep at tectonic strain rates. Instead, plagioclase underwent extensive brittle deformation. This, and other microstructural evidence, suggests that these rocks were deformed at relatively fast rates, and provides some confirmation that the central unit chamber was constructed very rapidly, perhaps in a matter of months to thousands of years.