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Microstructural, chemical and textural records during growth of snowball garnet
Author(s) -
ROBYR M.,
CARLSON W. D.,
PASSCHIER C.,
VONLANTHEN P.
Publication year - 2009
Publication title -
journal of metamorphic geology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.639
H-Index - 114
eISSN - 1525-1314
pISSN - 0263-4929
DOI - 10.1111/j.1525-1314.2009.00824.x
Subject(s) - geology , electron backscatter diffraction , chevron (anatomy) , orientation (vector space) , population , crystallography , geometry , lineation , rotation (mathematics) , spiral (railway) , mineralogy , microstructure , seismology , chemistry , tectonics , paleontology , mathematics , demography , sociology , mathematical analysis
The growth history of two populations of snowball garnet from the Lukmanier Pass area (central Swiss Alps) was examined through a detailed analysis of three‐dimensional geometry, chemical zoning and crystallographic orientation. The first population, collected in the hinge of a chevron‐type fold, shows an apparent rotation of 360°. The first 270° are characterized by spiral‐shaped inclusion trails, gradual and concentric Mn zoning and a single crystallographic orientation, whereas in the last 90°, crenulated inclusion trails and secondary Mn maxima centred on distinct crystallographic garnet domains are observed. Microstructural, geochemical and textural data indicate a radical change in growth regime between the two growth sequences. In the first 270°, growth occurred under rotational non‐coaxial flow, whereas in the last 90°, garnet grew under a non‐rotational shortening regime. The second population, collected in the limb of the same chevron‐type fold structure, is characterized by a spiral geometry that does not exceed 270° of apparent rotation. These garnet microstructures do not record any evidence for a modification of the stress field during garnet growth. Concentric Mn zoning as well as a single crystallographic orientation are observed for the entire spiral. Electron backscatter diffraction data indicate that nearly all central domains in the snowball garnet are characterized by one [001] axis oriented (sub‐)parallel to the symmetry axis and by another [001] axis oriented (sub‐)parallel to the orientation of the internal foliation. These features suggest that the crystallographic orientation across the garnet spiral is not random and that a relation exists among the symmetry axis, the internal foliation and the crystallographic orientation.