MICROSTRUCTURES IN THE POST-TECTONIC SAN JOSé PLUTON, BAJA CALIFORNIA, MEXICO: EVIDENCE FOR BOTH MAGMATIC AND SOLID-STATE DEFORMATION AND THEIR BEARING ON PLUTON EMPLACEMENT

The San José hornblende-biotite tonalite shows well-developed foliation throughout. The foliation is highly concentric in the northern two-thirds of the pluton where it is nearly parallel to both internal and external contacts. Microstructural analysis indicates a progressive change from melt-dominated, magmatic deformation in the pluton’s interior to solid-state deformation near its outer margins. Detailed structural mapping in the pluton and surrounding country rocks, combined with SHRIMP U/Pb zircon chronology, suggest that the pluton is "post-tectonic". Thus, the solid-state deformation in the pluton’s outer margins apparently formed during emplacement, rather than during later regional deformation.

Solid-state deformation in the pluton produced both brittle and crystal-plastic features. Plagioclase and amphibole were locally fractured at a high angle to the foliation. Fractures in plagioclase were typically healed by plagioclase, quartz or both. In some instances compositional zonation in plagioclase is truncated along plagioclase-plagioclase grain boundaries, which may provide evidence for "contact melting". In the northern two-thirds of the pluton, the solid-state foliation is overprinted by S-C structures within 200-300 meters of the outer margin. S-surfaces are defined by the pre-existing solid-state foliation, and the cross-cutting C-surfaces consistently dip outwards at ~45 degrees. Without exception, the S-C structures indicate country rock-over-pluton sense of displacement.

The concentrically developed S-C structures, and consistent country rock-over-pluton sense of displacement along the C-surfaces suggest that the solid-state deformation occurred during emplacement-related expansion to the north. Expansion presumably occurred while melt was still present in the pluton’s interior. Questions that arise from this study include: (1) do fracture orientations in plagioclase relative to magmatic foliation orientations provide a shear-sense indicator during submagmatic flow; (2) what is the timing and duration of the microstructural transition from melt-dominated to solid-state deformation; and (3) did deformation mechanisms and fabric-forming processes change through time during and continue after crystallization.