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

Research Article| May 01, 1989 Microstructure of serpentinite mylonites from the Josephine ophiolite and serpentinization in retrogressive shear zones, California GREGORY T. NORRELL; GREGORY T. NORRELL 1Department of Geological Sciences, State University of New York at Albany, Albany, New York 12222 Search for other works by this author on: GSW Google Scholar ANTONIO TEIXELL; ANTONIO TEIXELL 2Instituto Geológico y Minero de Espana, Mayor 20, 22700 Jaca, Spain Search for other works by this author on: GSW Google Scholar GREGORY D. HARPER GREGORY D. HARPER 1Department of Geological Sciences, State University of New York at Albany, Albany, New York 12222 Search for other works by this author on: GSW Google Scholar Author and Article Information GREGORY T. NORRELL 1Department of Geological Sciences, State University of New York at Albany, Albany, New York 12222 ANTONIO TEIXELL 2Instituto Geológico y Minero de Espana, Mayor 20, 22700 Jaca, Spain GREGORY D. HARPER 1Department of Geological Sciences, State University of New York at Albany, Albany, New York 12222 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1989) 101 (5): 673–682. https://doi.org/10.1130/0016-7606(1989)101<0673:MOSMFT>2.3.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Permissions Search Site Citation GREGORY T. NORRELL, ANTONIO TEIXELL, GREGORY D. HARPER; Microstructure of serpentinite mylonites from the Josephine ophiolite and serpentinization in retrogressive shear zones, California. GSA Bulletin 1989;; 101 (5): 673–682. doi: https://doi.org/10.1130/0016-7606(1989)101<0673:MOSMFT>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract 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. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.