6. Conclusions
Progressive deformation of hornblende-biotite tonalite of the Cerro de Costilla complex (Baja California, México) in a mylonitic shear zone involved cleavage slip in biotite to initiate discontinuous foliae, which became linked via fine-grained aggregates involving fragmental and recrystallized biotite, recrystallized quartz, fine-grained plagioclase and titanite after biotite, fragmental plagioclase, minor fragmental hornblende and recrystallized myrmekite, to form more continuous foliae anastomosing around resistant plagioclase framework grains. At an intermediate stage of fabric development, continuous foliae were localized around plagioclase grain boundaries forming a distinct S/C-type pattern. Although the C-surfaces are parallel to the shear-zone boundary, the S-surface are controlled by the orientations of plagioclase grain boundaries (e.g., Passchier and Simpson, 1986), and so do not represent a progressively rotating foliation that marks the direction of maximum finite elongation, as in the original S/C terminology (Berthé et al., 1979).
The amount of strain accumulation increases markedly close to the contact between the tonalite and the wall-rock orthogneiss, where hornblende disappears in favor of biotite, and continued deformation has resulted in elongation of recrystallized quartz aggregates to form lenticular ribbon-like foliae, as well as the development of biotite-quartz-plagioclase ’beards’ on resistant plagioclase clasts. Increasing separation of the clasts and consequent extension and recrystallization of the beards produced a fine-grained foliated matrix between the clasts. This was accompanied by collapse of biotite foliae into the inter-clast sites, resulting in partial loss of the S/C structure in favor of a more symmetrical anastomosing foliation. Extensive recrystallization of the matrix produced fine-grained decussate aggregates of biotite and aggregates of quartz and biotite with low-energy grain shapes.
The deformation occurred at approximately 475 ± 50°C, which was too low for extensive plagioclase recrystallization, and was accompanied and assisted by chemical reactions, such as: (1) replacement of biotite by symplectic plagioclase and titanite and by replacement of minor interstitial K-feldspar by myrmekite, these fine-grained aggregates recrystallizing readily to provide fine-grained granular aggregates capable of contributing to foliation initiation and development; (2) replacement of hornblende by biotite, which provided a higher proportion of weaker material; and (3) transport of biotite and quartz components in solution to sites of low compressive stress and shear strain in the lee of plagioclase porphyroclasts.
Coupling of deformation, diffusion, fluid advection and reaction in the wet core of the mylonite zone led to a strong fabric and strain gradient over a very short distance (approximately 50 cm) between samples D and C. The loss of hornblende across the transition, coupled with an increase in modal biotite and quartz, and enhanced dissolution-precipitation accommodated strain led to weakening and localization. The overall contributing factors in weakening were: (1) the progressive breakdown by fracturing, fragmentation and reaction of the load-bearing plagioclase framework; (2) the progressive development of continuous biotite-rich foliae; (3) flow of quartz into lenticular foliae, once the restricting plagioclase framework had sufficiently broken down; (4) elimination of strong hornblende accompanied by modal increase of weaker biotite; (5) dissolution, transport and re-deposition of biotite and quartz components in lower shear-strain sites between zones of continuous biotite-rich foliae; and (6) minor contributions from fine-grained aggregates of other minerals.