TEXTURE DEVELOPMENT IN DEFORMED GRANODIORITES FROM THE SANTA-ROSA MYLONITE ZONE, SOUTHERN CALIFORNIA

We report on texture development of granodiorite which has been progressively deformed to mylonite and phyllonite in the Santa Rosa mylonite zone in southern California. Since it is extremely difficult to measure pole figures of materials with complex diffraction patterns, it was necessary to use sophisticated neutron diffraction techniques with high resolution and position-sensitive detectors which enable the mathematical deconvolution of at least part of the spectrum. We could construct separate pole figures for biotite, quartz and plagioclase which are the major components. In the granodiorite protolith the preferred orientations of all phases are weak. In the mylonite biotite and quartz develop strong preferred orientations, whereas feldspars remain randomly oriented. Quartz has c-axes in the intermediate strain axis as is common in other mylonites. Interestingly, biotite a-axes show preferred orientation, suggesting that slip may be involved rather than just rigid body rotation. In phyllonite with a grain size of about 50 μm, the biotite fabric is very strong. However the quartz fabric has largely vanished, perhaps by superplastic processes which do not produce texture. A weak plagioclase fabric has developed. © 1990.