Biotite as a petrogenetic discriminator: Chemical insights from igneous, meta-igneous and meta-sedimentary rocks in Iran

Chemical compositions of Fe-Mg biotite have been used to understand the petrogenesis of metamorphic and ig-neous rocks. However, biotite is affected by sub-solidus hydrothermal alteration, metamorphism, and chemical exchange with other common coexisting phases such as garnet and muscovite. Therefore, the interpretation of igneous and metamorphic processes using biotite compositions is not always straightforward. Here we compare biotite compositions in igneous rocks, meta-igneous rocks, and meta-sedimentary rocks from localities in north -east (Dehnow, Khalaj, Khajeh Morad) and central (Jandaq and Airekan) Iran, with similar rock types in the global GEOROC database and from other localities, in order to constrain associated petrogenetic classification schemes. We find important compositional contrasts in biotite associated with muscovite and/or garnet (in both igneous and metamorphic rocks), suggesting careful use of common discrimination schemes. For example, magmatic bi-otite associated with garnet and/or muscovite (i.e., Bt + Ms, Bt + Ms + Grt, Bt + Grt) is often enriched in Al and depleted in Fe, Mg, and Ti, likely due to crystallization prior to muscovite but synchronous with or following gar-net crystallization. Metamorphic biotites in garnet-and/or muscovite-bearing rocks tend to be enriched in Ti, Fe, and Mg and depleted in Al. The contrasting compositional behavior of magmatic and metamorphic biotites also poses problems for garnet-biotite, biotite-muscovite, and Ti-in-biotite thermometers. Our analysis indicates that biotite rare earth and trace element concentrations are strongly influenced by co-existing garnet and muscovite. When magmatic biotite crystallization occurs with muscovite and garnet, HREE concentrations respectively de-crease and increase. ? 2021 Published by Elsevier B.V.

Automated summary

Chemical compositions of Fe-Mg biotite can provide valuable insights into the petrogenesis of metamorphic and igneous rocks, but they are often influenced by co-existing phases such as garnet and muscovite. Different compositional variations in biotite are observed in igneous and metamorphic rocks, leading to challenges in using biotite compositions for petrogenetic classification schemes.