MICROSTRUCTURE OF PERTHITES IN SYENITES FROM THE YAMATO MOUNTAINS, EAST ANTARCTICA

Cryptoperthites in intrusive syenite from the Yamato Mountains were studied by transmission electron microscopy. They consist of two domains, one being narrow Albite-twinned plagioclase, the other a triclinic K-feldspar. The crystal symmetry of the majority of K-feldspars from the Yamato Mountains is triclinic; while monoclinic symmetry is rare. The triclinicity of K-feldspar ranges from 0.34 to 0.59. There is no difference in triclinicity between K-feldspar from the northern and southern Yamato Mountains. The exsolution lamelae do not develop a zig-zag pattern in either area. This fact suggests that the cooling rate of intrusive syenites from the Yamato Mountains is not very slow. Plagioclase in myrmekite, perthite and antiperthite in syenite from the southern Yamato Mountains is oligoclase (An_Ab_Or_∿An_Ab_Or_) with low Or content, whereas plagioclase of composition An_Ab_Or_∿An_Ab_Or_ occurs as patches and wavy strings in perthite from the northern area. Although plagioclase is more anorthitic in syenite from the south than from the north, the chemical composition of K-feldspar is similar in both areas