田上花こう岩中のアルカリ長石の微細組織とその形成史

Abstract

Alkali feldspars in the Tanakami granite, southwest Japan, contain a complicated fine-scale mixture of various micro-textures; lamellar crypto- and micro-perthites and micropore-rich patch perthites. Mineralogical examination mainly by TEM and SEM disclosed that there are two formational stages of these micro-textures, the subsolidus cooling stage and hydrothermal alteration stage. In the subsolidus cooling stage, lamellar crypto- and micro-perthite were formed by exsolution. Then nearly regularly arranged dislocations were formed at interfaces of Ab-rich lamellae and Or-rich feldspars. Hydrothermal alteration caused the dissolution of feldspar grains and re-precipitation of new feldspar grains mainly around the dislocations. These processes result in neo-formation of patch perthites that comprise irregular microcline, tweed orthoclase, numerous micropores and albite grains. Bulk compositions of cryptoperthite have excessive Or content (around Or90) in comparison to those expected for primary alkali feldspars. It is estimated that retrograde inter-crystalline Na-K exchange between alkali feldspars and plagioclases occurs in the subsolidus stage. On the contrary, bulk compositions of patch perthite have poor Or content (Or64-71) from those of the cryptoperthite. This feature indicates that the chemical compositions of alkali feldspars changed drastically as well as the micro-textures in the hydrothermal alteration stage. Coexisting plagioclases also underwent chemical change from the subsolidus cooling stage through the hydrothermal alteration stage. Especially in the hydrothermal alteration stage, alteration by percolation of hydrothermal water occurred in the internal areas of the plagioclase grains. These processes result in lower Or content (max Or2.7) and higher Ab content (>Ab90) of the plagioclases, and neo-formation of patchy zoning including micropores and fluorite subgrains. Summarizing all of these features, the relation between micro-textural development and chemical compositional changes in feldspars was discussed taking into consideration their formation processes such as inter-crystalline Na-K exchange and hydrothermal reaction.