Petrography and Rare Earth Element Geochemistry of Pan-African Granites of a Part of Mikir Hills, NE India

Petrography and rare earth element (REE) geochemistry of representative anorogenic granite samples of Pan-African origin from the Kathalguri-Lungsung-Lalpahar area in western Mikir Hills of NE India were analyzed in an attempt to determine their origin, evolutionary status and tectonic setting. Petrographic study indicates appearance of REE enriched accessory minerals like zircon, monazite, apatite, sphene, allanite, xenotime etc., thereby, forcing reasonably high absolute REE concentrations. Chondrite normalized REE patterns suggest richness in LREE (194-2l38 ppm; av. 688.05 ppm) and low HREE content (27-65 ppm; av. 44 ppm). The LREE and HREE distribution patterns maintain consistent parallelism in the studied samples suggesting comagmatic origin, although, two samples, however, show minor deviation in cases of La and Eu values only. The ratios ofCe_N/Yb_N (7.66-35.35); La_N/Yb_N (3.18-16.67); La_N/Sm_N (2.72-5.27) and Th_N/Yb_N (0.9l-l.87) suggest differentiation and REE fractionation during magma production and derivation from mantle sources under variable degree of contamination with crustal material. Evaluated Eu/Eu^∗ (0.26-0.91 av. =0.47) ratios over a consistent level indicate moderate but less fluctuating f₀₂ level of magma at the source.