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Journal of Petrology Advance Access originally published online on February 9, 2005
Journal of Petrology 2005 46(5):1013-1044; doi:10.1093/petrology/egi009
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© The Author 2005. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oupjournals.org

Petrographic, Chemical and B-Isotopic Insights into the Origin of Tourmaline-Rich Rocks and Boron Recycling in the Martinamor Antiform (Central Iberian Zone, Salamanca, Spain)

A. PESQUERA1,*, J. TORRES-RUIZ2, P. P. GIL-CRESPO1 and S.-Y. JIANG3

1 DEPARTAMENTO DE MINERALOGÍA Y PETROLOGÍA, UNIVERSIDAD DEL PAIS VASCO, 48080 BILBAO, SPAIN
2 DEPARTAMENTO DE MINERALOGÍA Y PETROLOGÍA, UNIVERSIDAD DE GRANADA, 18002 GRANADA, SPAIN
3 STATE KEY LABORATORY FOR MINERAL DEPOSITS RESEARCH, DEPARTMENT OF EARTH SCIENCES, NANJING UNIVERSITY, NANJING 210093, CHINA

RECEIVED JANUARY 16, 2004; ACCEPTED DECEMBER 8, 2004

Tourmaline in the Martinamor antiform occurs in tourmalinites (rocks with >15–20% tourmaline by volume), clastic metasedimentary rocks of the Upper Proterozoic Monterrubio formation, quartz veins, pre-Variscan orthogneisses and Variscan granitic rocks. Petrographic observations, back-scattered electron (BSE) images, and microprobe data document a multistaged development of tourmaline. Overall, variations in the Mg/(Mg + Fe) ratios decrease from tourmalinites (0·36–0·75), through veins (0·38–0·66) to granitic rocks (0·23–0·46), whereas Al increases in the same order from 5·84–6·65 to 6·22–6·88 apfu. The incorporation of Al into tourmaline is consistent with combinations of x{chi}Al(NaR)–1 and AlO(R(OH))–1 exchange vectors, where x{chi} represents X-site vacancy and R is (Mg + Fe2+ + Mn). Variations in x{chi}/(x{chi} + Na) ratios are similar in all the types of tourmaline occurrences, from 0·10 to 0·53, with low Ca-contents (mostly <0·10 apfu). Based on field and textural criteria, two groups of tourmaline-rich rocks are distinguished: (1) pre-Variscan tourmalinites (probably Cadomian), affected by both deformation and regional metamorphism during the Variscan orogeny; (2) tourmalinites related to the synkinematic granitic complex of Martinamor. Textural and geochemical data are consistent with a psammopelitic parentage for the protolith of the tourmalinites. Boron isotope analyses of tourmaline have a total range of {delta}11B values from –15·6 to 6·8{per thousand}; the lowest corresponding to granitic tourmalines (–15·6 to –11·7{per thousand}) and the highest to veins (1·9 to 6·8{per thousand}). Tourmalines from tourmalinites have intermediate {delta}11B values of –8·0 to +2·0{per thousand}. The observed variations in {delta}11B support an important crustal recycling of boron in the Martinamor area, in which pre-Variscan tourmalinites were remobilized by a combination of mechanical and chemical processes during Variscan deformation, metamorphism and anatexis, leading to the formation of multiple tourmaline-bearing veins and a new stage of boron metasomatism.

KEY WORDS: tourmalinites; metamorphic and granitic rocks; mineral chemistry; whole-rock chemistry; boron isotopes

* Corresponding author. E-mail: npppepea{at}lg.ehu.es


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[Abstract] [Full Text] [PDF]


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