Sita Bora^1, Vikoleno Rino², Hansa Joshi³, Manohar Singh Rawat³ and Santosh Kumar³*

¹Department of Geology, Government Degree College, Kanda, Bageshwar-263631(UK), India ²Department of Geology, Nagaland University, Kohima Campus, Kohima- 797004(NL), India ³Department of Geology, Centre of Advanced Study, Kumaun University, Nainital-263001(UK), India

(*Correspondence Author, E-mail: skyadavan@yahoo.com)

Abstract

The Meghalaya Plateau including the Mikir Hills mainly comprises the Proterozoic granite gneiss, migmatites, metasediments, Cambrian granite plutons, and Mesozoic-Tertiary cover rocks. Field relationships and magnetic susceptibility (MS) mapping of granite gneisses, granites and related rocks of Meghalaya Plateau have been conducted in order to assess the redox series (magnetite, oxidized vs. ilmenite, reduced) nature of granites. The observed MS values of Proterozoic granite gneisses and Cambrian granites largely correspond to magnetite (>3.0×10-3 SI unit) series (oxidized type) and a few to ilmenite (<3.0×10-3 SI unit) series (reduced type) granites. The Cambrian granite plutons are dominated by magnetite series (>3.0×10-3 SI unit) series granites, which appear to have inherited primarily from the infracrustal (igneous) sources. However, the observed highly oxidizing (magnetite series) condition of granites from these plutons were most likely achieved during synchronous mixing and fractionation in open magma chamber, as evident from the presence of mafic to hybrid microgranular enclaves, and accumulation of ferromagnesian minerals. Fine to medium grained leucocratic granites measure low MS values (<3.0×10-3 SI unit) corresponding to ilmenite (reduced) series granite because of their evolved nature as being depleted in ferromagnesian (magnetic) minerals. At places, a few Cambrian granites hosting the pyrite and chalcopyrite measure a relatively higher MS values as compared to the unmineralized granites. Magnetite series granites are found to be reduced as ilmenite series close to its intrusive contacts with metasedimentary country-rocks.

Keywords: Magnetic Susceptibility, Redox Series Granites, Meghalaya Plateau, NE India