Tourmaline is a common and locally abundant mineral in all products of the granite magmatism and associated hydrothermal activity in southwest England, particularly in the county of Cornwall. Tourmaline of magmatic origin is homogeneous and is marked by Fe/Mg, high F, and high Al in the place of divalent cations (R2 site). The substitution of Al for divalent cations such as Mg and Fe2+ is charge-compensated by a deficit of alkalies and protons in in other structural sites. Tourmaline compositions from the granites clearly reflect the sequence of increased differentiation among the magma types, with biotite granites as the least clearly evolved, and topaz granites as the most evolved. In contrast, tourmaline of hydrothermal origin within granite displays fine-scale compositional zonation with a general tendency toward more magnesian compositions nearer the schorl-dravite solid solution (i.e., little or no Al in the R2 site). Metasomatic tourmaline precipitated in the surrounding pelitic and mafic rocks also possesses fine-scale zonation, and generally reflects the compositions of the host rocks. In addition, tourmaline formed in the country rocks has a higher proportion of Fe3+ to Fe2+, which presumably indicates a higher oxidation state of fluids in the metamorphic rocks than in the granites. This variation may correlate with the mobilization of tin from the granites and its consequent deposition as cassiterite in the host rocks. The abundance of magmatic tourmaline is limited by the initial Fe-Mg content of the magmas to not more than a few modal or weight percent. Although they contain tourmaline, the biotite granites are not the most likely sources of boron for voluminous, late-stage tourmalinization. Magmas that contain only tourmaline or no Fe-Mg minerals at all, such as the those that formed the topaz granites, may have been sources of large quantities of boron. The most intense areas of tourmalinization do surround the small stocks and sheets of topaz microgranite. High concentrations of tourmaline, for example in hydrothermal veins and breccias, appear to require mixing of two different chemical reservoirs: one a source of boron (the magmas or fluids derived from them), and the other a source of Fe-Mg components (e.g., mafic to metapelitic host rocks and fluids equilibrated with them). The fine-scale chemical zonation of hydrothermal tourmaline reflects the fluctuating conditions that would be expected from fluid mixing in open systems. The mode of origin of large bodies of massive quartz-tourmaline rock, in which the tourmaline possesses the compositional characteristics of the magmatic tourmaline is still unknown. These rocks generally lack evidence of pervasive fracture-enhanced permeability, as is prominently developed and preserved in the hydrothermal systems, yet the liquidus temperatures of the quartz-tourmaline rocks, even with the addition of magmatic fluxes, are unrealistically high. The general relations of tourmaline and boron in the magmatic-hydrothermal systems of southwest England are similar to those in felsic magmas elsewhere: due to low Fe-Mg contents, the magmas themselves were incapable of conserving much boron as tourmaline. Most tourmaline is hydrothermic or metasomatic in origin and lies in rocks that surround the magmatic sources of boron where we propose that mixing of fluids containing boron and Fe-Mg components locally dumped large quantities of tourmaline. Because of this mechanism, the initial concentration of boron in the granitic magmas is difficult if not impossible to assess. New experimental results presented here, however, indicate that tourmaline-saturated granites contained at least several wt percent B2O3. From these experiments and the abundance of tourmalines in the veins, breccias, and altered country rocks throughout the Cornubian peninsula, we can conclude that boron was an important constituent of these magmas.
