On the Genesis of Lapsa Buru Kyanite, Singhbhum District, Bihar

A number of massive pegmatoid kyanite veins occur along shear planes at Lapsa Buru, in the western part of the Singhbhum shear zone, Bihar. The host rocks are Precambrian metatuff and metachert, now represented by muscovite-biotite-chlorite schist and quartzite, which have been affected by four phases of deformation and metamorphism. During the first two phases granitic rocks were emplaced within the shear zone at temperature between 650°-700°C and pressure around 2 Kbar. During the 3rd phase albite metasomatism took place within the shear zone under pressure of about 5 Kbar. At the same time, kyanite veins were emplaced along the northern boundary of the shear zone under slightly higher pressure ranging between 5-6 Kbar. A review of experimental data indicates that temperature around 450°-500°C prevailed during kyanite segregation. It has been suggested that under these P-T conditions alumina and silica were mobilised from the alumina rich volcanogenic rocks of the shear zone. Presence of tourmaline, topaz, and dumortierite in the kyanite veins indicates that boron and flourine may have aided the mobilisation process. The resulting kyanite was affected by minor shearing and local retrogressive metamorphism during the fourth phase of deformation in the region.