Replacement of cordierite by basic plagioclase

Abstract

The process is described of the replacement of cordierite by basic plagioclase associated with subordinate chlorite and sporadical biotite. It took place during regressive metamorphism of the metamorphic Krynki series (Precambrian substrate of NE Poland) developed in the hornfels facies, with calcium and alkalies supplied probably under a low pressure and temperatures ranging from 330 to 500° C. The recognized plagioclase has been analysed on a universal stage for ills composition, facial development and orientation in reIation to the cordierite being replaced.