Cryptoperthites and cooling rate in a layered syenite pluton: a chemical and TEM study

Alkali feldspars in the Klokken layered syenite (South Greenland) are optically either coarse, turbid, deuteric patch perthites or clear, unaltered, fine braid microor cryptoperthites. Irregular, clear volumes can be found in most turbid grains. Electron microscopy shows that all clear crystals or volumes are cryptoperthites in which the periodicity of the exsolution lamellae increases systematically with distance from the roof of the intrusion, from 40 nm to 300nm. They are composed of low albite and diagonally associated maximum microcline. A secondary coarsening is responsible for optically visible braid microperthites.The layered series consists of interleaved, granular syenites and coarser grained, laminated syenites with mineral layering. Microprobe analyses show a downward decrease in Ca and Ba in the granular syenite feldspars consistent with chilling from the roof inwards. Most laminated syenite feldspars contain 1% An increasing to 5% in mafic layers.The periodicity of the exsolution lamellae in the laminated syenite feldspars is consistently up to twice that in adjacent granular syenites. Periodicity is not influenced by Ab:Or or An content, but high-An feldspars have continuous zig-zag intergrowths, while others are more lozengeshaped. Lamellar periodicities largely reflect cooling rate in the pluton. Cooling times in the exsolution interval were probably about 10³–10⁴ years at the top of the series, whereas existing experimental data suggest that the periodicities would be reached in a few days or years. The periodicities at the top of the granular series are only slightly greater than experimentally determined initial wavelengths for spinodal decomposition, suggesting that little or no coarsening occurred despite the long cooling times. Coarsening did occur lower in the pile. Framework ordering and twinning must slow or stop coarsening.The difference in periodicity between the granular and laminated syenite series may be caused by differences in amount of water incorporated in the feldspar structure from the magma. The exsolution of this water at low temperatures could be the cause of the deuteric unmixed patch perthites. Small volumes of cryptoperthite which have escaped deuteric unmixing may be common in igneous rocks in general and may be useful relative indicators of cooling rate.