UHP-metamorphic rocks from Dora Maira/Western Alps and Kokchetav/Kazakhstan; new insights using cathodoluminescence petrography

Thin sections of ultrahigh pressure (UHP) metamorphic rocks from the Dora Maira Massif (Italy) and the Kokchetav Massif (Kazakhstan) were investigated using the hot cathode cathodoluminescence (CL) technique. Coloured images of important, but otherwise invisible growth features could be easily identified with this tool within seconds. These features are in excellent correlation with chemical variations of minerals revealed by electron microprobe (EMP). Generally, CL is induced by activator-elements (e.g. Mn and REE) and lattice defects whereas so-called quencher-elements like Fe may reduce or even extinct luminescence. Since X-ray-intensity mapping images (MAPS) of minerals can take up to 50 hours, the CL-method represents an ideal and rapid approach prior to chemical characterization. In addition to typical carbonates such as calcite, Mg-bearing calcite and dolomite, a number of rock forming and accessory minerals including Mg- and Mg-Ca-garnets, diopsidic and jadeitic pyroxenes, kyanite, K-feldspar, quartz, coesite, diamond, zircon, apatite, and bearthite were examined. Features observed in garnets include small-scale oscillatory zoning patterns, changes in morphology during growth as well as different crack generations which were partly annealed. SiO (sub 2) phases (coesite, quartz, chalcedony) as well as exsolution textures of dolomite and Mg-bearing calcite are easy to distinguish due to their different CL-colours. Pyroxene displays complex zonation patterns and -to some extent- exsolution-textures of K-feldspar. Kyanite reveals distinct growth zones; in combination with mineral inclusion studies it is possible to discriminate between different kyanite-forming reactions. The different crystallographical orientation of twinned kyanite crystals leads to various luminescence colours, thus, the suture of the twin plane is well defined. Prior to SHRIMP analyses, knowledge of the internal structures of zircon is indispensable. Even very tiny coesite crystals are easy to distinguish from quartz or chalcedony by their disparate luminescence colours. Accessory luminescent minerals like diamond, apatite, bearthite are easy to identify in thin section even if they occur in very small abundance within the matrix or as inclusions. The CL method presented here for UHP-metamorphic rocks is recommended as a pathfinder for the discovery of internal structures of minerals prior to their chemical characterization using EMP.