Nature of Biotites from the Granitoids of Guwahati and Mayong Areas of Shillong Plateau, Northeastern India and their Petrogenetic Significance.

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]