Abstract
Late Pan-African A-type, within plate, anorogenic granitoids dominate the lithology of the magmatic complex of northwest Karbi Hills, NE India. Petrographic analysis of these granitoids indicate enrichment of rare earth bearing accessory minerals like allanite, titanite, zircon, xenotime and monazite, constituting about 5% of total mineralogy, with concomitant rise in REE content (up to 2202.50 ppm; av.614.64 ppm), especially LREE. There are at least two episodes of textural modifications; the former are coarser, show crenulated crystal boundary with occasional interpenetrative grain structure; feldspars suffer extensive saussuritization and sericitization; biotite and plagioclases are studded with many volatile phases, viz., apatite, zircon etc. Quartz of latter generation are mostly fine grained, spherical, grown in a process of reducing surface or interfacial free energy during post magmatic recrystallization episode and production of microcline perthite. In our present study, mineralogical identifications have been done by optical microscopy and X-ray diffraction; physico-chemical status of study granitoids have been evaluated based on comprehensive EPMA analysis of constituent biotite, plagioclase and potash feldspar. The crystallization temperature extracted from EPMA of biotites suggests low emplacement temperature <700OC and pressure 1.65 -2.56 kb, corresponding to a depth of crystallization from 6.36-9.86 km. The plot of ∑FeO/∑(FeO+MgO) vs. % MgO, oscillating zoning in constituent zircons and Hf (t) of zircon (-12.20 to-18.90), refer to the derivation of magma from an older magmatic crust.

Keywords
Karbi Hills, Pan-African magmatism, EPMA, rare earth elements

Cite this paper
Dilip Majumdar, Pankhi Dutta, Abhijit Gogoi, Contribution to the physico-chemical condition of granitoid emplacement in a part of Karbi Hills, NE India , SCIREA Journal of Geosciences. Volume 1, Issue 2, December 2016 | PP. 50-77.

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