LAUSR

Nodular tourmalines composed of dark cores and white haloes occur in the Mashhad leucogranite. The cores are made up of tourmaline and quartz, and the haloes of quartz, muscovite, microcline and orthoclase. The host granite consists of quartz, plagioclase, microcline, muscovite and biotite. Geochemical analyses show that the studied tourmalines are schorl. They are characterized by oscillatory zoning, increasing concentrations of Ca and Mg from core to rim, moderate Fe/Fe + Mg ratios (56–63), and REE distribution patterns similar to those of the host granite. The tourmalines exhibit high Ti content due to breakdown of biotite. Petrographic and geochemical results show that the formation of tourmaline can be attributed to two stages. In the first stage, bubbles containing vapor-rich liquid, Fe, and B were generated by magma differentiation. During rise and cooling of the magma, core tourmaline was produced with Fe rich schorl composition. In the second stage, the tourmaline composition changed to that of Mg rich schorl because of the decomposition of biotite (biotite is considered as a source of Mg), and the mixing with vapor-rich liquid in bubbles