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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]

Intrusive granitoid bodies and quartzofeldspathic gneiss outcrops in the Guwahati and Mayong areas are considered part of the northern extension of the basement gneissic complex of the Shillong Plateau. They are medium to coarse-grained porphyritic granitoids essentially composed of quartz, K-feldspar, plagioclase, biotite, hornblende, sphene, magnetite, apatite, zircon, allanite, and bastnaesite in different proportions. Mineralogical characteristics of these granitoids reveal that they are monzogranite. The biotites are re-equilibrated in Guwahati granitoids whereas they are primary in Mayong granitoids. Ti-in biotite, Al-in hornblende and mineral assemblage biotite-K-feldspar-magnetite were used to determine temperature, pressure and oxygen fugacity during the crystallization of parent magmas. The Guwahati granitoids crystallized and re-equilibrated at temperatures ranging between 602°C and 752°C (avg. 685°C) with pressure (5.2–5.6 kbar, avg 5.4 kbar) from low oxidized magmas (log fO2 −16.33 to −14.36 bar), buffered below FMQ to within NNO. The Mayong granitoids crystallized at temperatures range 694°C to 715°C (avg. 705 °C) with pressure (4.7 to 5.1 kbar, avg. 5 kbar) under reduced oxidized condition (log fO2 −17.88 to −17.62 bar) buffered below FMQ. The calc-alkaline nature, low to moderate oxygen fugacity and high H2O (4.5–7.4 wt%) content suggests that Guwahati granitoids emplaced in subduction-related tectonic setting whereas alkaline nature, reduced oxygen fugacity and low content of H2O (2.15 wt%) suggest anorogenic tectonic setting for Mayong granitoids. Also, from the estimated pressure and depth of emplacement conditions it suggests that both the granitoids are mid-level plutons. [ABSTRACT FROM AUTHOR]