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Journal of Petrology Advance Access published online on February 9, 2005
Journal of Petrology, 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
Received January 16, 2004
Accepted December 8, 2004

Article

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. PESQUERA 1*, J. TORRES-RUIZ 2, P. P. GIL-CRESPO 1, and S.-Y. JIANG 3

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

* To whom correspondence should be addressed.
A. PESQUERA, E-mail: npppepea{at}lg.ehu.es


  Abstract

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.

Keywords: tourmalinites; metamorphic and granitic rocks; mineral chemistry; whole-rock chemistry; boron isotopes.
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