Mineral Chemistry and Geothermometry of Biotite in the Granitoids, Located in and around Jirang-Patharkhamah Area, Ri-Bhoi District, Meghalaya, India.

Granitoids are well exposed in Jirang-Patharkhamah area (latitude N 25°57′− 25°41′ and longitude E 91°30′–91°38′). The area lies mostly in Ri-Bhoidistrict and is also extended to West Khasi Hills district of Meghalaya. The granitoids are medium grained and nonporphyritic. The colour is dominantly grey although some granitoids are pink in colour. The common minerals in order of abundance are quartz, plagioclase, K-feldspar, biotite and hornblende. Secondary minerals are sericite, chlorite, muscovite, secondary hornblende and secondary biotite. Among accessory minerals, sphene, zircon, apatite, calcite, zoisite, magnetite and monazite are significant. The intergrowth textures such as perthite, myrmekite in these rocks indicate that the rocks were affected by low temperature alteration. The representative samples ploy in the field of granite in various geochemical discrimination diagrams. The aim of the present paper is geochemical characterisation of the granitoids using major and trace elements (including REE). The whole rock geochemistry reveals that the granitoid is I-type, calc-alkaline, metaluminous to weakly peraluminous and an attempt is made to examine the nature of the magma based on biotite mineral chemistry. The biotite chemistry indicates that the granitoid is I-type and calc-alkaline. The paper also aims to estimate temperature of emplacement of granitoids based on two feldspar geothermometer and Ti-in biotite geothermometer. The temperature calculated from two feldspar thermometry is in the range 512–602°C and the average is 550°C. The temperature calculated by Ti-in biotite thermometer is in the range 623 to 656°C and the average is 645°C. Geothermometric results plays an important role in definition of petrogenetic history and regional tectonism. [ABSTRACT FROM AUTHOR]