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)

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 xXAl(NaR)-1 and AlO(R(OH))-1 exchange vectors, where xX represents X-site vacancy and R is (Mg + Fe2+ + Mn). Variations in xX/(xX + 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 δ11B values from - 15·6 to 6·8‰; the lowest corresponding to granitic tourmalines (- 15·6 to - 11·7‰) and the highest to veins (1·9 to 6·8‰). Tourmalines from tourmalinites have intermediate δ11B values of -8·0 to +2·0‰. The observed variations in δ11B 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. © The Author 2005. Published by Oxford University Press. All rights reserved.