The investigation of O-H-B isotopes of tourmaline in biotite- muscovite granite of Mashhad

Several distinct morphologies of tourmaline have been identified in Mashhad biotite-muscovite granite: nodule, pegmatite, aplite, quartz-tourmaline veins, tourmaline-rich veins and radial tourmalines. The δ18O and δD values in solar tourmaline (12.4 and -69 ‰), nodule tourmaline (11.8 and -63), quartz-tourmaline vein (11.7 and -57), pegmatite tourmaline (11.62-11.67 and -59 to -73), aplite tourmaline (11.39 and -57), tourmaline-rich vein (11.82 and -62) and mica schist tourmaline (11.06 and -77) with low changes are similar together. These values show same origin for tourmalines fluid source in biotite-muscovite granite. The difference in δ18O values between quartz and tourmaline (Δqtz-tur) are positive (between+2.0 and+2.2‰), and this show quartz and tourmaline are in equilibrium. The δ18O thermometry of tourmaline - quartz minerals show a range between 492°C and 579 °C for tourmaline crystallization. The calculated δ18O, and δD values of the initial fluid in equilibrium with tourmaline suggest a magmatic and primitive magmatic water of peraluminous granite source. All tourmalines show similar δ11B values (with a narrow range between −7.4 and −10.8‰). This indicates a same boron source for them that resulting during highly evolved magmatic differentiation. Based on the 11B values of tourmalines in Mashhad fall within the range reported for granite-related tourmaline and similar to the S-type source granites that derived by Continental crust. The δ11B values of tourmalines show the δ11B values of the magma of the biotite-muscovite granite.