Intergrowth of Biotite and Chlorite in an Amphibolitic Schist: Prograde or Retrograde Reaction\ulcorner

각섬암에서 관찰된 흑운모와 녹니석의 협재 조직: 전진 또는 후퇴변성작용에 의한 것인가\ulcorner

Intergrowth texture of biotite and chlorite crystals within an amphibolitic schist of the northwestern Okchon metamorphic belt was investigated using back-scattered electron (BSE) imaging and high-resolution transmission electron microscopy (HRTEM). BSE images show that thin chlorite and biotite packets are mixed along (001) plane to result in intergrowth texture. In addition, rutile particles of submicron size occur exclusively at the boundaries between biotite and chlorite stacks. HRTEM investigation and remnant biotite layers are closely associated with such boundaries, suggestinga possibility that chlorite layers were formed from biotite during retrograde metamorphic reaction. Such intepretation of the origin of intergrowth texture can be further supported biotite is approximately 2 w%, and that of chlorite usually lower than 0.2 wt%. Ti was apparently leached out during the alteratin of biotite to precipitate rutile particles at the such rutile particles could be an important indicator showing that the intergrowth texture of chlorite and biotite is originated by a retrograde metamorphism rather than by incomplete chlorite-to-biotite reaction during prograde metamorphism. Biotite crystals contain intercalated chlorite layers will result in somewhat high Mg and Al, and the use of such inhomogeneous biotite will result in impreciese geothermobarometric calculations.