Geochemical Characteristics of Metasomatised Diorites in and around Umsopri of Ri-bhoi District, Meghalaya, India

Abstract

The metasomatised dioritic rocks are well exposed in and around Umsopri area (N 25°49/ and E91°39/), Ri-Bhoi district of Meghalaya, along with granitoid rocks. The gneissic complex represents the basement of the Shillong plateau comprising a group of high-grade metamorphic rocks including basic granulite, amphibolite, quartzofeldspathic gneiss, migmatite, calc-silicate gneiss and garnet sillimanite-bearing metapelite. The basement rocks of the Shillong plateau is intruded by Neoproterozoic granitoids of multiple phases. The granitoids are younger in age from south-west to north-east. The Shillong Plateau along with Mikir Hills experienced four major phases of felsic magmatic episodes at ~1800 Ma, ~1600 Ma, ~1400 Ma, and ~500 Ma. Petrographicaly, the rock is composed of K-feldspar, plagioclase, hornblende, quartz along with accessory phases like sphene, apatite, zircon, rutile, ilmenite, etc. The metasomatic effect has been inferred by the formation of biotite and sphene at the expanse of Hornblende. The rocks have moderate SiO2 content (52.55 to 55.84 wt%). Trace elements Rb, Ba, Nb shows a negative trend with SiO2 concentrations. The high field strength elements like Nb, Zr, La, Th, U show a negative correlation and Y shows a positive correlation with an increase in abundance of SiO2. Large ion lithophile element Rb shows a negative correlation with SiO2 while Sr shows a positive correlation with SiO2. Geochemical features indicates that the rock is metaluminous in character and tectonically in the WPG (within plate granitoid) field.