Zoning in Plagioclase (Normal and Oscillatory) in the Feldspar Porphyries of Karsog Area, Lesser Himalaya, Himachal Pradesh, India

Growth of plagioclase crystals accompanied by rapid cooling and contemporaneous crystallisation with falling temperature is considered to cause disequilibrium due to incomplete reaction between the growing crystal and the residual sodic melt. This causes the formation of normal zoning. Regular oscillatory zoning has been explained mainly due to the process of diffusion of An-molecules towards the crystal face of growing plagiolcase in a magma. Growth of zoned plagioclase was accompanied by fractional crystallisation and accumulation of magma of trondhjernitic composition. During this process, zoned plagioclases came in contact with one another and resulted in synneusis. Partial resorption resulted in truncated zones followed by crystallisation of a more calcic rim.