Geochemistry and formation mechanisms of the tourmaline nodules in the Mashhad granite (g2)

Tourmaline nodules in the Mashhad granites (g2) formed of two parts: dark core and white halo which are scattered in the leucocratic granites. Petrographic studies of these parts (core, white halo and host granite) show tourmaline accumulation in core, quartz, muscovite, microcline and orthoclase accumulation in white halo and quartz, plagioclase, microcline, muscovite and biotite accumulation in host granite. Based on the microprobe and geochemical analysis, the combination of tourmaline nodules is schorl and close to dravite. Features such as oscillatory zoning, Ca, Mg increase in rim, medium Fe/Fe+Mg (56-63) and the REE distribution pattern compatible to metapelite melting that show these tourmalines have double origin. In first stage, tourmaline have schorl composition, then with upward movement of magma and mixing to enriched liquid and boron originated of reaction to host metapelites, open system formed and tourmaline composition changed to dravite.