Characterizing Tourmaline in Metapelitic Schists from The Gasht Area, North Iran and Boron Source in Metamorphic Fluids

Abstract

Metapelitic rocks in the Gasht area include micaschist, kyanite schist, andalusite schist, garnet schist, staurolite schist, cordierite schist and sillimanite schist. Tourmaline occurs as accessory mineral in all of these rock types. These schists are metamorphosed regionally and are affected by contact metamorpism subsequently. Based on the textural relations and the fact that CaO and TiO2 contents in the studied tourmalines are low, they are formed during regional metamorphism. They appear in the rock matrix and as inclusions in other minerals, especially biotite and albite. The studied tourmalines are of alkali type and are rich in dravite end-member. Cl and F contents are below the detection limit testifying for hydro-tourmaline nature of the studied minerals. Na and K are higher in the X-site in comparison with Ca. Low Ca shows neglegible amounts of Ca end-member or uvite. Mg content is much higher than Fe in the structure of the studied tourmalines, which are not zoned or show weak zoning and are grown at nearly constant P-T conditions. These tourmalines are crystallized from pelitic to psammitic protoliths in equlibrium with a fluid phase, rich in Al. Boron in fluid more likely was from the boron adsorbed on clay minerals surface in the protolith, released during metamorphism and boron from B-bearing mica breakdown during high tempertaure metamorphism, while boron released from the subducting oceanic crust is not a likely source for tourmaline crystallization in the studied rocks since there is no evidence for tourmaline in the associated oceanic crust mafic rocks.