Some chemical variations in biotite, phlogopite, and muscovite, considering their tectonic setting

Several studies have been conducted to investigate the composition of micas (especially biotite) with the magmatic suites of the host rock (especially granitoid), thus to understand the tectonic environment of the host rock. This study deals with the issue of what compositional trends or elemental correlations exist in igneous micas when a large dataset with a given tectonic environment is used. In this regard, variations in the chemical composition of biotite, phlogopite, and muscovite from three tectonic environments were investigated, regardless of the composition of their host rock. Enrichment of Fe or Mg in micas, such as Mg-rich biotite or Fe-rich phlogopite, can make us wrong in determining whether a mineral is primary or secondary based solely on chemical composition. The negative and good correlation between FeO and MgO caused biotites and phlogopites of all three tectonic environments to follow the trend of the calc-alkaline orogenic suites of Abdel-Rahman’s classification. Considering the excellent and negative correlation between Al and Mg in muscovite, the substitution of 2Mg2+ = 3Al3+ is significant in this mineral. The data distribution shows that biotites and phlogopites belonging to rift and convergent environments can be divided into Al-rich and Al-poor groups or Mg-rich and Mg-poor groups.