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D. Majumdar ¹, P. Dutta ¹

Affiliations
1 Department of Applied Geology, Dibrugarh University, Dibrugarh 786 004, IN

Abstract

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.

Keywords

Granitoids, Mineralogical Abundance, Plutons, Rare Earth Elements.

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T. K. Goswami ¹, D. Ray ², R. K. Sarmah ¹, U. Goswami ¹, P. Bhattacharyya ¹, D. Majumdar ¹, D. Bezbaruah ¹, P. Borgohain ¹

Affiliations
1 Department of Applied Geology, Dibrugarh University, Dibrugarh 786 004, IN
2 Physical Research Laboratory, Ahmedabad 380 009, IN

Abstract

A single piece of meteorite weighing approximately 12 kg and dimension 10"× 9"× 8" fell at Komargaon town (lat. 26°39'N; long. 93°46'E), Golaghat district, Assam, India on 13 November 2015 (12:00 h, IST). We report here eye-witness accounts and a preliminary description of the sample which includes macro- and microstructures and a tentative petrologic-chemical classification. According to local villagers, it was a bright sunny day with a clear sky.

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D. Majumdar , P. Dutta

Abstract

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.

Keywords

Pan-African, Mikir Hills, LREE, HREE, Differentiation, Fractionation

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P. Bhattacharyya , B. K. Tamuli , D. Majumdar

Abstract

Sulphide mineralization in a thin shear of about 25 mts wide locating in the Potin - Yazali road section of Subansiri district, Arunachal Pradesh has been reappraised. The mineralization is structurally controlled. Structural elements include shear fractures and shear foliations; lithological variant is confined to garnetiferous quartz-biotite-schist. A comprehensive EPMA analysis of host minerals viz., garnet and biotite has been evaluated and P&T condition for the host rock has also been determined by garnet-biotite geothermometry. Ore mineralogical study revealed dominance of chalcopyrite and pyrrhotite with minor quantities of sphalerite, occasional patches of arsenopyrite and isolated finely disseminated bodies of pyrite. From the EPMA analysis of sphalerite, sphalerite geobarometry has been evaluated. The equilibrium temperature estimated for the peak metamorphic assemblages are 480°c ± 24°c while the pressure value might be higher than estimated value of 2.5 Kbar.

Keywords

Sulphide Mineralization, Potin-Yazali Area, Shear Zone, Structural

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