USING ELECTRON BACKSCATTER DIFFRACTION TO UNDERSTAND TEXTURES IN SECTOR-ZONED GARNETS

Interpretation of textures has been a key component of understanding rock history ever since the inception of microscopes and rock thin sections. In recent years, the development of the electron backscatter diffraction (EBSD) technique has revitalised texture studies because it enables systematic mapping of crystallographic orientations in polished rock surfaces, providing an additional strand of information previously not available. To illustrate the utility of EBSD and its potential future impact on texture-based studies, a case study is presented of sector-zoned garnets from Sulitjelma (Norway). Sector-zoned garnets have been recognised for a long time (e.g. Harker 1939) and have been investigated sporadically since (e.g. Ferguson et al. 1980; Andersen, 1984; Burton 1986; Rice & Mitchell 1991; Vance et al 1998). They form in graphite-bearing pelites and involve the preferential inclusion of graphite and equant quartz grains (type 1 inclusions) at dodecahedron face edges coupled with the growth of rod-shaped quartz grains (type 2 intergrowths) in garnet [110] directions. The textures are considered to form under hydrostatic conditions (Rice & Mitchell 1991). Mapping of garnet and quartz crystallographic orientations reveal a number of important relationships: A. matrix quartz grains define a weak c-axis crystallographic preferred orientation (CPO) parallel to the rock schistosity; B. type 1 quartz inclusions commonly have crystallographic orientations similar to matrix quartz grains; C. type 2 quartz intergrowths typically have crystallographic orientations different to the matrix quartz grains and independent of the host garnet orientation; D. garnets commonly have crystallographic sectors defined by low angle misorientations that are independent of the textural sector zones; E. some garnet porphyroblasts are really three or four garnets that nucleated close together (i.e. they have high angle misorientations). These results suggest that while the type 1 quartz inclusions can represent trapped matrix grains (as suggested by previous studies), the type 2 quartz intergrowths are more likely to be newly nucleated grains rather than modifications of existing grains. A model will be presented to explain garnet and quartz crystallographic texture formation under non-hydrostatic conditions.