Geothermobarometry of granitic pegmatites of Nagamalai - Pudukottai area, Madurai Block, South India

Abstract

The pegmatites in the Neoproterozoic A-type granites of NagamalaiPudukottai area are of two typesgray and pink. These pegmatites contain hornblende. In this work an attempt is made to understand the nature of magma, the process of crystallisation and the Pressure-Temperature (P-T) conditions of formation of the rock with the help of the Electron microprobe (EPMA) study of hornblende from both varieties of pegmatites. The EPMA of the hornblende from the granitic pegmatites indicate their calcic nature. The structural formula of these calcic amphiboles corresponds to hastingsite. The Al pressure calibration suggests the formation of the gray and pink pegmatites to be 5 kilobar (average). Based on the amphibole-plagioclase thermometer the magmatic temperature deduced for gray and pink pegmatites is 760C and 745C respectively. These temperature values suggest the later formation of pink pegmatites. The study reveals the formation of the granites and associated pegmatites from a less evolved, hydrous, and less viscous calcic melt in the upper crust.