CHEMICAL VARIATION AND SIGNIFICANCE OF TOURMALINE FROM SOUTHWEST ENGLAND

Tourmaline is a common and locally abundant mineral in all products of the granite magmatism and associated hydrothermal activity in southwe st England, particularly in the county of Cornwall. Tourmaline of magm atic 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 fo r divalent cations such as Mg and Fe2+ is charge-compensated by a defi cit of alkalies and protons in in other structural sites. Tourmaline c ompositions from the granites clearly reflect the sequence of increase d differentiation among the magma types, with biotite granites as the least clearly evolved, and topaz granites as the most evolved. In cont rast, tourmaline of hydrothermal origin within granite displays fine-s cale compositional zonation with a general tendency toward more magnes ian compositions nearer the schorl-dravite solid solution (i.e., littl e or no Al in the R2 site). Metasomatic tourmaline precipitated in the surrounding pelitic and mafic rocks also possesses fine-scale zonatio n, and generally reflects the compositions of the host rocks. In addit ion, 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 m ay correlate with the mobilization of tin from the granites and its co nsequent deposition as cassiterite in the host rocks. The abundance of magmatic tourmaline is limited by the initial Fe-Mg content of the ma gmas to not more than a few modal or weight percent. Although they con tain tourmaline, the biotite granites are not the most likely sources of boron for voluminous, late-stage tourmalinization. Magmas that cont ain only tourmaline or no Fe-Mg minerals at all, such as the those tha t 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 b oron (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 hydrother mal tourmaline reflects the fluctuating conditions that would be expec ted from fluid mixing in open systems. The mode of origin of large bod ies of massive quartz-tourmaline rock, in which the tourmaline possess es the compositional characteristics of the magmatic tourmaline is sti ll unknown. These rocks generally lack evidence of pervasive fracture- enhanced permeability, as is prominently developed and preserved in th e hydrothermal systems, yet the liquidus temperatures of the quartz-to urmaline rocks, even with the addition of magmatic fluxes, are unreali stically high. The general relations of tourmaline and boron in the ma gmatic-hydrothermal systems of southwest England are similar to those in felsic magmas elsewhere: due to low Fe-Mg contents, the magmas them selves were incapable of conserving much boron as tourmaline. Most tou rmaline is hydrothermic or metasomatic in origin and lies in rocks tha t surround the magmatic sources of boron where we propose that mixing of fluids containing boron and Fe-Mg components locally dumped large q uantities of tourmaline. Because of this mechanism, the initial concen tration 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 perc ent B2O3. From these experiments and the abundance of tourmalines in t he veins, breccias, and altered country rocks throughout the Cornubian peninsula, we can conclude that boron was an important constituent of these magmas.