Rare Earth Element Abundances in some A-Type Pan-African Granitoids of Karbi Hills, North East India

This is a preliminary report of the rare earth metal abundances in certain Pan-African granitoids of Karbi Hills, Assam, North East India. Higher abundance of rare earth metals is apparently related to the mineralogical abundance of certain key rare earth element (REE)-bearing accessory minerals like allanite, xenotime, bastnaesite, apatite, zircon and monazite. The bulk chemistry of the granitoids suggests their A-type, calcium-deficient but alkali-rich, anomalous, anorogenic, within-plate granitoid character. Textural variants considered for the present study are: (1) porphyritic to sub-porphyritic, medium to fine-grained, mylonitized grey and pink-coloured granitoids of the Panbari-Geleki area in northern Karbi Hills, and (2) the non-porphyritic, medium to fine-grained granitoids of Kathalguri area in northwestern Karbi Hills. Compositionally, the plutons are granitic to granodioritic in composition. The study reveals that the grey porphyritic granitoids are poor in REE, but the pink porphyry/sub-porphyry variety is enriched in REE. The pink granitoids show richness in La (average 1086 ppm) and Ce (average 2329 ppm) in particular. The average LREE content in the nonporphyritic Kathalguri granitoids is 607.46 ppm, but their HREE content is slightly higher (average 26.50 ppm) than normal granitoids. Chondrite normalized LREE-HREE profile and ratios of Ce_N/Yb_N, La_N/Yb_N, La_N/Sm_N and Tb_N/Yb_N of individual plutons indicate co-magmatic derivation of magmatic masses from the lower crust.