ASSESSMENT OF CONDITIONS OF THE MUONG LAT MAGMA GRANITE: FROM THE ANALYSIS OF THE BIOTITE COMPOSITION

Abstract

    Studying characteristics of the chemical composition of rock-forming minerals is of particular importance, as a basis for assessing formation conditions as well as the initial magma source. Biotite is one of the rock-forming minerals of the Muong Lat granite, therefore in this study we analyzed the composition of biotite using the electron probe microanalysis (EPMA) method. The biotite mineral extracted from the Muong Lat granite is primary biotite and has a composition corresponding to iron-rich biotite (Fe-biotite), with high aluminium contents (3.34 - 3.66), its A/CNK values range from 1.75 to 1.93 (average of 1.84), and Mg# ones are from 28.08 to 35.27 which are typical of S-type granite. Calculated crystallization temperatures ranging from 610°C to 685°C indicate a "cold" granite characteristic. Crystallization pressure is estimated from 4.6 kbar to 5.7 kbar, corresponding to a mid-crustal depth of 14 km to 17 km.