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The Alamo Complex consists of structural-metamorphic domes surrounded by low-grade metasedimentary rocks of Upper Proterozoic to Lower Cambrian age that form part of the Schist Graywacke Complex (Central Iberian Zone, Spain). Tourmaline is ubiquitous throughout the domes, in which it occurs in tourmalinites, psammo-pelitic schists, quartzites, gneisses, migmatites, leucogranites, aplo-pegmatites, and quartz veins. Overall, tourmaline compositions can be described within the four component system schorl-dravite-foitite-magnesiofoitite, with [K.sub.2]O < 0.23%, low Ca contents, Mg/(Mg+Fe) = 0.25-0.71 and X/(X+Na) = 0.18-0.43, where X = vacancies in the X site. Field relations and petrographic observations, combined with tourmaline [sup.40]Ar/[sup.39]Ar data, provide evidence of intense boron metasomatism affecting this region. Tourmaline [sup.40]Ar/[sup.39]Ar step-heating spectra for Cambrian-Ordovician orthogneisses are complex, yielding a pseudo-plateau age of ~370 Ma that is interpreted to reflect Variscan rejuvenation of older tourmaline. Tourmaline [sup.40]Ar/[sup.39]Ar data for mylonitized and folded tourmalinites yield disturbed spectra with pseudo-plateau ages of ~355-342 Ma that are unsuitable for precise age determination. These ages, however, are consistent with published ages (340-350 Ma) for the second Variscan deformation ([D.sub.2]). Tourmaline from fine-layered tourmalinite and metasedimentary rocks yield well-defined plateau ages of 317 and 315 Ma, respectively, recording an additional metasomatic event concomitant with anatexis and evolution of B-bearing granites, pegmatites, and hydrothermal fluids. The different tourmaline-forming stages reflect significant boron cycling within the continental crust of the Central Iberian Zone, driven by deformation, metamorphism, and magmatism during the Variscan orogeny. Boron-rich aqueous fluids related to Cambro-Ordovician magmatism are considered to be the primary source of boron. Keywords: Tourmaline, petrography, chemical composition, [sup.39]Ar/[sup.40]Ar dating, Alamo Complex, Central Iberian Zone, Spain

The Li-F-Be-B-P bearing Pinilla de Fermoselle (PF) pegmatite occurs in the apical part of a leucogranite body. It shows a clear non-symmetrical vertical zoning from the contact with the leucogranite to a contact with the metamorphic country rocks. The pegmatitic facies evolve upward from (1) the undifferentiated Lower Border Zone (LBZ), with quartz, feldspars, muscovite, biotite, and black tourmaline, through (2) the Intermediate Zone (IZ), with quartz, muscovite, zinnwaldite, black tourmaline, and Fe-Mn phosphates, to (3) the highly evolved Upper Border Zone (UBZ), with quartz, albite, lepidolite, zinnwaldite, elbaite, and beryl. The composition of the pegmatite-forming minerals suggests that a residual melt become progressively enriched in F and Li until the crystallization of the apical UBZ, whereas P partitions in the melt only until the intermediate levels of differentiation attained in the IZ. Chemical variations in the mica and tourmaline as well as in the feldspar and Fe-Mn phosphate minerals are consistent with an internal evolution by crystal fractionation processes. A plausible model for the crystallization of the PF pegmatite involves a rapid, in situ, bottom-up crystallization from significantly undercooled liquids. The lack of metasomatic effects in the metamorphic host-rock and the estimated P content of the initial leucogranite melt suggest that the PF pegmatite mainly crystallized under closed system conditions.

The Berry-Havey pegmatite (Oxford pegmatite field, Androscoggin County, Maine, U.S.A.), enriched in Li, F, B, Be, and P, is intruded in hornblende-rich amphibolite, with minor biotite or diopside. The pegmatite has a complex internal structure, with four texturally and compositionally different zones that show an increasing degree of evolution inward: wall zone, intermediate zone, core margin, and core zone. The main minerals are quartz, feldspars, Al-micas, tourmaline, with minor Fe-micas, garnet, beryl, amblygonite-montebrasite, Fe-Mn phosphates, and apatite. Tourmaline is present in all zones of the pegmatite, showing different textures: black anhedral crystals in the wall and intermediate zones; black prisms of up to 40 cm in length in the intermediate zone; black tapered prisms, surrounded by a pseudo-graphic intergrowth of quartz or albite with black ± green/bluish tourmaline, and constituting a continuous layer under the core zone; multicolored and "watermelon" zoned crystals in the core zone; and gemmy deep green and color-zoned "watermelon" tourmaline prisms, up to 15 cm length, inside the pockets. A complete chemical evolution from Mg-rich schorl in the wall zone to elbaite with an important deprotonation in the pockets inside the core zone is observed. The most plausible exchange vectors for this chemical evolution are FeMg-1, YAlWO[YR2+W(OH)]-1 and Al[X]X(R2+Na)-1 (where R2+ = Fe2++Mg2++Mn2++Zn2+), for the tourmalines from the wall and intermediate zones. In the core margin, tourmaline composition evolves from schorl toward Li-rich species through the substitution (YAlYLiYR-22+). Later, during the crystallization of the core zone, this exchange vector combined with the substitution ([X] YAl0.5XNa-1YLi-0.5). Finally, the gemmy tourmalines from the pockets show a deprotonation related to the exchange vector YAlWO2YLi-1W(OH)-2 and may be classified as darrellhenryite. These substitutions may reflect an increase in oxygen fugacity and a decrease in Li and F related to the crystallization of lepidolite and amblygonite-montebrasite in the core zone adjacent to or within the pockets. The crystallization of these minerals would reduce the availability of Li and F for the very latest tourmaline crystals, growing inside the pockets, where the deprotonation becomes important. Chemical and textural variation in tourmaline is consistent with a fractional crystallization process for the internal evolution of the Berry-Havey pegmatite. Crystallization of the tourmaline layer under the core zone may be related to the exsolution of the fluid phase implied in the formation of pockets. [ABSTRACT FROM AUTHOR]

The Alamo Complex consists of structural-metamorphic domes surrounded by low-grade metasedimentary rocks of Upper Proterozoic to Lower Cambrian age that form part of the Schist Graywacke Complex (Central Iberian Zone, Spain). Tourmaline is ubiquitous throughout the domes, in which it occurs in tourmalinites, psammo-pelitic schists, quartzites, gneisses, migmatites, leucogranites, aplo-pegmatites, and quartz veins. Overall, tourmaline compositions can be described within the four component system schorl-dravite-foitite-magnesiofoitite, with K2O < 0.23%, low Ca contents, Mg/(Mg+Fe) = 0.25-0.71 and X/(X+Na) = 0.18-0.43, where X = vacancies in the X site. Field relations and petrographic observations, combined with tourmaline 40Ar/39Ar data, provide evidence of intense boron metasomatism affecting this region. Tourmaline 40Ar/39Ar step-heating spectra for Cambrian-Ordovician orthogneisses are complex, yielding a pseudo-plateau age of ~370 Ma that is interpreted to reflect Variscan rejuvenation of older tourmaline. Tourmaline 40Ar/39Ar data for mylonitized and folded tourmalinites yield disturbed spectra with pseudo-plateau ages of ~355-342 Ma that are unsuitable for precise age determination. These ages, however, are consistent with published ages (340-350 Ma) for the second Variscan deformation (D2). Tourmaline from fine-layered tourmalinite and metasedimentary rocks yield well-defined plateau ages of 317 and 315 Ma, respectively, recording an additional metasomatic event concomitant with anatexis and evolution of B-bearing granites, pegmatites, and hydrothermal fluids. The different tourmaline-forming stages reflect significant boron cycling within the continental crust of the Central Iberian Zone, driven by deformation, metamorphism, and magmatism during the Variscan orogeny. Boron-rich aqueous fluids related to Cambro-Ordovician magmatism are considered to be the primary source of boron. [ABSTRACT FROM AUTHOR]