Amphibole Chemistry in Low–temperature I–type Granites from Kashmar Area, Northeastern Central Iran Plate (CIP)

The Kashmar granitoid (43.5–42.4 Ma) forms an extensive part of iron–oxide type magmatic belt in northern side of the Doruneh Fault. It includes plutons of tonalite, granodiorite, granite and alkali feldspar granite which are metaluminous (ASI ≤ 1) in nature. They contain dominantly felsic minerals, representing I–type granites with low–temperature and low–pressure characteristics. According to microscopic studies and electron microprobe analyses, all amphiboles in Kashmar are monoclinic calcic hornblende which is common mafic mineral in distinction of I–type granites. The calculated structural formula for Kashmar amphiboles show characteristics of magnesio-hornblende as follow: They are low in Al2O3 and TiO2, high in Mg* (0.6–0.75), extremely low in AlVI (<0.1 apfu) but significantly higher in Fe3+, features indicating low temperature, low pressure and oxidized conditions. Using appropriate thermo barometers for Kashmar amphiboles, average temperature of 650 oC, pressures of ≤ 3 kb and logfO2 = –16.59 to –19.40 were calculated. These results define a thermal boundary of ~700 oC between felsic and mafic low–temperature I–type granites.