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Please use this identifier to cite or link to this item: http://hdl.handle.net/10481/32077

Title: Tourmaline 40Ar/39Ar chronology of tourmaline-rich rocks from Central Iberia dates the main Variscan deformation phases
Authors: Bea Barredo, Fernando
Pesquera Pérez, Alfonso
González Montero, Pilar
Torres-Ruiz, José
Gil-Crespo, Pedro Pablo
Issue Date: 2009
Abstract: During crustal thickening, metapelites taken to depth release boron-bearing hydrothermal fluids because of progressive heating and dehydration. These fluids swiftly percolate upwards, especially if the crust is being actively deformed, to form tourmaline where the PT conditions and the chemical composition of the host-rock are favorable. The age of the so-formed tourmaline would record the age of the upward admittance of B-bearing fluids and, presumably, the age of the deformation. This process has been documented in the Martinamor Antiform of Central Iberia, a region where tourmaline-bearing rocks are particularly abundant. Metasomatic tourmaline from the Late Cambrian San Pelayo orthogneisses (zircon U-Pb age of 496 ± 5 Ma) yielded 40Ar/39Ar plateau ages at 370 ± 5 Ma and 342 ± 5 Ma. The first value represents the crystallization age of the tourmaline and is so far the most precise estimation of the age of crustal thickening in Central Iberia (D1). The second value reflects a partial loss of Ar caused by the second deformation phase (D2). Tourmaline from mylonitized and folded tourmalinites developed above D2 shear zones yield perturbed spectra with mean “plateau” ages of 347 ± 9 Ma and 342 ± 9 Ma which may represent either the resetting of older tourmaline or the formation of new tourmaline by focused boron metasomatism. After the metamorphic peak and simultaneously with the emplacement of the main granitoids of the Avila Batholith (310-315 Ma), another episode of boron metasomatism precipitated a new generation of tourmaline, which appears either concentrated in fine-layered tourmalinites (318 ± 2 Ma) or disseminated within Ediacaran-Cambrian metasediments (316 ± 2 Ma). The source of boron was the breakdown of previously formed tourmaline during melting reactions. Lastly, tourmaline from a leucogranitic body yielded a saddle-shaped age spectrum with a minimum age of ca. 296 Ma, roughly coeval with the youngest leucogranites. Although further work is required, our results suggest that tourmaline can be a useful chronological marker for dating deformation and magmatism.
Sponsorship: This work has been supported by the Spanish grants CGL2006-07938 and CLG2005-05863, and the Andalucian grant RNM1595.
Publisher: Universidad de Barcelona; Instituto de Ciencias de la Tierra Jaume Almera
Keywords: Tourmaline
40Ar/39Ar dating
Deformation
Iberia
Boron
Metasomatism
URI: http://hdl.handle.net/10481/32077
ISSN: 1695-6133
1696-5728
Rights : Creative Commons Attribution-NonCommercial-NoDerivs 3.0 License
Citation: Bea Barredo, F.; et al. Tourmaline 40Ar/39Ar chronology of tourmaline-rich rocks from Central Iberia dates the main Variscan deformation phases. Geologica Acta, 7(4): 399-412 (2009). [http://hdl.handle.net/10481/32077]
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