An Investigation on the Composition of Biotite from Mashhad Granitoids, NE Iran

Compositions of biotite from three different rock types of Mashhad granitoids, i.e., granodiorite, monzogranite and leucogranite in NE of Iran have been documented by electron microprobe and wet chemistry for Fe3+ and Fe2+. Mashhad granitoids have been geochronologically and petrologically grouped into G1 and G2 phases. Microprobe data show that the total Fe contents in biotite from G2 leucogranite are higher than those in biotite from G1 granites. In addition, the oxidation state of iron determined by wet chemistry shows that Fe3+/(Fe2+ + Fe3+) ratio in biotite from G2 leucogranite is 0.10 indicating relatively reducing whereas, in G1 ones is 0.18 and 0.23 suggesting more oxidizing conditions. The most outstanding compositional characteristics of Mashhad biotite are differences in total Al contents and Fe/(Fe+Mg) ratios. In the annite-siderophylite-phlogopite-eastonite (ASPE) quadrilateral, represented based on the above parameters, biotite samples from G1 and G2 granites define two distinct and non-overlapping trends. Each trend is characterized by a pronounced trend of increasing total Al at relatively narrow Fe/(Fe+Mg) values. The total Al contents of G1 biotite are in the range of 2.8 to 3.1, whereas, in G2, 3.3 to 3.6 (apfu). Fe/(Fe+Mg) values of G1 biotite are in the range of 0.52 to 0.59 which is considerably lower than those from G2 biotite, 0.67 to 0.72. The trend of increasing Al contents at constant Fe/(Fe+Mg) is relatively common and observed in biotite from several locations worldwide and attributed to considerable contributions from aluminous supracrustal material, either by assimilation or anatexis.