Metamorphism of Nepheline Syenite Gneisses from Chhotanagpur Granite Gneiss Complex, Northeastern Puruliya District, Eastern India

Two contiguous, but spatially separated occurrences of nepheline syenites in the northeastern part of Puruliya district, one having biotite as the principal mafic component and the other having amphibole with subordinate biotite as mafic minerals are studied. Although modal mineralogy of nepheline syenites of these two bodies contrasts considerably, bulk chemical compositions of them remain almost same. Petrographic features give evidences of metamorphism. It is Inferred that regional metamorphism of biotite-bearing nepheline syenite under granulite facies condition caused the appearance of amphibole-Bearing nepheline syenite. During this metamorphic transition, possible reactions between minerals are also drawn.