Chemical characteristics of biotites in post-kinematic granites and associated pegmatites of Nagamalai-Pudukottai area, Madurai district, Tamil Nadu, South India

Biotite is a ubiquitous mineral in granitoid rocks. Biotite composition in granitoids rocks is a useful tool to investigate and characterise the magma types and tectonic setting. The present study relates to electron microprobe analysis of biotites from Atype granites and the associated pegmatites of Nagamalai-Pudukottai area, Madurai block in the Southern Granulite Terrain. The biotites in both granites and pegmatites are Fe-rich. The Fe/(Fe+Mg) content in granites is higher than that in pegmatites. Higher Mg concentration is noticed in biotites of pegmatites. The Al^VI values of both the rocks are < 1, indicating their magmatic nature. A low Ti content is also found in these rocks. The X_Mg value of biotites in granites and pegmatites is low. The low Ti and X_Mg values of biotites in both granites and pegmatites indicate prevalence of low oxygen fugacity and the origin of these biotites as products derived from a reduced magma. The high MgO content in the biotites of pegmatite may be due to the close association of hornblende. The petrogenetic diagram MgO- FeO^t-Al₂O₃, indicates an anorogenic tectonic setting for the emplacement of granites and pegmatites in the study area.