Microstructure of serpentinite mylonites from the Josephine ophiolite and serpentinization in retrogressive shear zones, California

The Josephine Peridotite has experienced deformational episodes over a wide range of temperatures. Deformation within the stability field of a serpentine group mineral appears invariably accompanied by retrogressive metamorphism (serpentinization). Two general types of deformed serpentinites have been identified: (1) incohesive serpentinites (most common), which appear to have experienced low-temperature, predominantly brittle deformation and (2) serpentinite mylonites, which have experienced plastic flow near the upper limit of antigorite stability. The distinction between these two is based on textural and mineralogical criteria.

Serpentinite mylonites are cohesive, homogeneously foliated, and typically strongly lineated rocks composed primarily of antigorite and magnetite. They occur in planar-bounded zones enclosed within the Josephine Peridotite that were the loci of both noncoaxial deformation and serpentinization. We discuss the development of two important shear-sense indicators, porphyroclast systems and oblique shear-band foliations as observed in these rocks.

Porphyroclast systems in serpentinite mylonites are often comprised of cores and mantles of different mineralogy. Cores are usually composed of primary pyrogenetic minerals, whereas mantles, including tails, are composed of products of the serpentinization reaction. These systems are therefore termed "porphyroctasts with neocrystallization tails.".

Oblique shear bands are common micro-structures in serpentinite mylonites. Shear bands which offset the mylonitic foliation along narrow, discrete fractures are termed "f-type," whereas those which offset the mylonitic foliation along narrow, ductile "micro-shear zones" are termed "d-type". The latter are considered more reliable for shear-sense determinations. Strains accomplished by movement on shear bands are probably small. Strain resulting from a volume increase dur-ing serpentinization while holding the shear-zone width constant is sufficient to produce the shear bands. Therefore, we suggest that oblique shear bands may develop to accom-modate volume increases associated with deformation.