Tourmaline 40Ar/39Ar chronology of tourmaline-rich rocks from Central Iberia dates the main Variscan deformation phases


  • F. BEA



Tourmaline, 40Ar/39Ar dating, Deformation, Iberia, Boron, Metasomatism


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.


Allen, P., 1991. Provenance research; Torridonian and Wealden. In: Morton, A.C., Todd, S.P., Haughton, P.D.W. (ed.). Developments in sedimentary provenance studies. Geological Society Special Publications, 57, 13-21.

Andriessen, P.A.M., Hebeda, E.H., Simon, O.J., Verschure, R.H., 1991. Tourmaline K-Ar ages compared to other radiometric dating systems in Alpine anatectic leucosomes and metamorphic rocks (Cyclades and Southern Spain). Chemical Geology, 91, 33-48.

Anglin, C.D., Jonasson, I.R., Franklin, J.M., 1996. Sm-Nd dating of scheelite and tourmaline: Implications for the genesis of Archean gold deposits, Val d’Or, Canada. Economic Geology, 91, 1372-1382.

Arnaud, N.O. and Kelley, S.P., 1995. Evidence for excess argon during high pressure metamorphism in the Dora Maira Massif (western Alps, Italy), using a ultra-violet laser ablation microprobe 40Ar-39Ar technique. Contributions to Mineralogy and Petrology, 121, 1-11.

Azor, A., Expósito, I., González Lodeiro, F., Simancas, J.F., Martínez Poyatos, D., 2004. Propuesta de un modelo evolutivo para la Zona de Ossa Morena. In: Vera, J.A. (ed.). Geología de España, Madrid, SGE-IGME, 188-189.

Baksi, A.J., 2006. Guidelines for assesing the reliability of 40Ar/39Ar plateau ages: Application to ages relevant to hotspot tracks.

Bea, F., Montero, P., Molina, J.F., 1999. Mafic precursors, peraluminous granitoids, and late lamprophyres in the Avila batholith: A model for the generation of Variscan batholiths in Iberia. Journal of Geology, 107, 399-419.

Bea, F., Montero, P., Talavera, C., Zinger, T., 2006. A revised Ordovician age for the oldest magmatism of Central Iberia: U-Pb ion microprobe and LA-ICPMS dating of the Miranda do Douro orthogneiss. Geologica Acta, 4, 395-401.

Bea, F., Montero, P., Zinger, T., 2003. The Nature and Origin of the Granite Source Layer of Central Iberia: Evidence from Trace Element, Sr and Nd Isotopes, and Zircon Age Patterns. Journal of Geology, 111, 579-595.

Bea, F., Pereira, M.D., Corretgé, L.G., Fershtater, G.B., 1994. Differentiation of strongly peraluminous, perphoshorous granites. The Pedrobernardo pluton, central Spain. Geochimica et Cosmochimica Acta, 58, 2609-2628.

Chen, C.H., Depaolo, D.J., Lan, C.Y., 1996. Rb-Sr microchrons in the Manaslu granite: Implications for Himalayan thermochronology. Earth and Planetary Science Letters, 143, 125-135.

Dallmeyer, R.D., 1979. 40Ar/39Ar Dating: Principles, Techniques, and Applications in Orogenic Terranes. In: Jaeger, E., Hunziker, J.E. (eds.). Springer-Verlag, Berlin, Lectures in Isotope Geology, 77-105.

Dallmeyer, R.D., Catalan, J.R.M., Arenas, R., Ibarguchi, J.I.G., Alonso, G.G., Farias, P., Bastida, F., Aller, J., 1997. Diachronous Variscan tectonothermal activity in the NW Iberian Massif: Evidence from 40Ar/39Ar dating of regional fabrics. Tectonophysics, 277, 307-337.

Díaz Montes, A., Navidad, M., González Lodeiro, F., and Martínez Catalán, J.R., 2004. El Ollo de Sapo. In: Vera, J.A. (ed.). Geología de España, Madrid, SGE-IGME, 69-72.

Díez Balda, M.A., 1986. El Complejo Esquisto-Grauwáckico, las series Paleozoicas, y la estructura Hercínica al sur de Salamanca. Salamanca, Ediciones Universidad de Salamanca, 162 pp.

Díez Balda, M.A., Martínez Catalán, J.R., Ayarza, P., 1995. Syncollisional extensional collapse parallel to the orogenic trend in a domain of steep tectonics: The Salamanca Detachment Zone (Central Iberian Zone, Spain). Journal of Structural Geology, 17, 163-182.

Fernández-Suárez, J., Arenas, R., Jeffries, T.E., Whitehouse, M., Villaseca, C., 2006. A U-Pb Study of Zircons from a Lower Crustal Granulite Xenolith of the Spanish Central System: A Record of Iberian Lithospheric Evolution from the Neoproterozoic to the Triassic. Journal of Geology, 114, 471-484.

Fitch, F.J., Miller, J.A., 1972. 40Ar/39Ar dating of detrital tourmaline and tourmalinized rocks; schists and micas. Journal of the Geological Society of London, 128, 291-294.

Frei, R., Pettke, T., 1996. Mono-sample Pb-Pb dating of pyrrhotite and tourmaline: Proterozoic vs Archean intracratonic gold mineralization in Zimbabwe. Geology, 24, 823-826.

Frei, R., Pettke, T., 1997. Mono-sample Pb-Pb dating of pyrrhotite and tourmaline: Proterozoic vs. Archean intracratonic gold mineralization: Reply. Geology, 25, 670-671.

Kawakami, T., 2001. Tourmaline breakdown in the migmatite zone of the Ryoke metamorphic belt, SW Japan. Journal of metamorphic Geology, 19, 61-75.

Koppers, A.A.P., 2002. ArArCALC - software for 40Ar/39Ar age calculations. Computers and Geoscience, 28, 605-619.

Kudryashov, N.M., Gavrilenko, B.V., Apanasevich, E.A., 2004. U-Pb, Pb-Pb tantalite and tourmaline dating of rare metal pegmatites from the Kolmozero-Voron’ya greenstone belt (NE Baltic Shield). Geochimica et Cosmochimica Acta, 68, A678-A678.

Lanphere, M.A., Dalrymple, G.B., 1976. Identification of excess 40Ar by the 40Ar/39Ar age spectrum technique. Earth and Planetary Science Letters, 32, 141-148.

Leeman, W.P., Sisson, V.B., 1996. Geochemistry of boron and its implications for crustal and mantle proceses. In: Grew, E.S., Anovitz, L.M. (eds.). Boron: Mineralogy, petrology and geochemistry. Reviews in Mineralogy, 33, 645-707.

Leeman, W.P., Sisson, V.B., Reid, M.R., 1992. Boron geochemistry of the lower crust: Evidence from granulite terranes and deep crustal xenoliths. Geochimica et Cosmochimica Acta, 56, 775-788.

Martínez Catalán, J.R., Poyatos, D., Bea, F. (coordinators), 2004. La Zona Centroibérica. In: Vera, J.A. (ed.). Geología de España, Madrid, SGE-IGME, 68-133.

Matte, P., 2001. The Variscan collage and orogeny (480-290 Ma) and the tectonic definition of the Armorica microplate: a review. Terra Nova, 13, 122-128.

Montero, P., Bea, F., González-Lodeiro, F., Talavera, C., Whitehouse, M., 2007. Zircon crystallization age and protolith history of the metavolcanites and metagranites of the Ollo de Sapo Domain in central Spain. Implications for the Neoproterozoic to Early-Paleozoic evolution of Iberia. Geological Magazine, 144, 963-976.

Montero, P., Bea, F., Zinger, T.F., Scarrow, J.H., Molina, J.F., Whitehouse, M.J., 2004. 55 Million Years of Continuous Anatexis in Central Iberia: Single Zircon Dating of the Peña Negra Complex. Journal of the Geological Society, 161, 255-264.

Moran, A.E., Sisson, V.B., Leeman, W.P., 1992. Boron depletion during progressive metamorphism: Implications for subduction processes. Earth and Planetary Science Letters, 111, 331-349.

Pereira, M.D., 1998. P-T conditions of generation of the Pena Negra anatectic complex, central Spain. Petrology, 6, 555-563.

Pereira, M.D., Shaw, D.M., 1997. Behaviour of boron in the generation of an anatectic complex: The Peña Negra complex, central Spain. Lithos, 40, 179-188.

Pesquera, A., Torres-Ruiz, J., Gil-Crespo, P.P., Jiang, S.Y., 2005. 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). Journal of Petrology, 46, 1013-1044.

Pesquera, A., Velasco, F., 1997. Mineralogy, Geochemistry and geological significance of tourmaline-rich rocks from the Paleozoic Cinco Villas massif (western Pyrenees, Spain). Contributions to Mineralogy and Petrology, 129, 53-74.

Qiu, H.N., Jiang, Y.D., 2007. Sphalerite 40Ar/39Ar progressive crushing and stepwise heating techniques. Earth and Planetary Science Letters, 256, 224-232.

Quesada, C., Dallmeyer, R.D., 1994. Tectonothermal evolution of the Badajoz-Córdoba shear zone (SW Iberia): characteristics and 40Ar/39Ar mineral age constraints. Tectonophysics, 231, 195-213.

Richards, J.P., Wagner, P.A., 1997. Mono-sample Pb-Pb dating of pyrrhotite and tourmaline: Proterozoic vs. Archean intracratonic gold mineralization: Comment. Geology, 25, 669-670.

Rodríguez, J., Cosca, M.A., Gil Ibarguchi, J.L., Dallmeyer, R.D., 2003. Strain partitioning and preservation of 40Ar/39Ar ages during Variscan exhumation of a subducted crust (Malpica - Tui complex, NW Spain). Lithos, 70, 111-139.

Scarrow, J., Bea, F., Montero, P., Molina, J.F., Vaugham, A.P.M., 2006. A precise 40Ar/39Ar age for Central Iberian camptonitic

lamprophyres. Geologica Acta, 4, 451-460.

Schatz, O.J., Dolejs, D., Stix, J., William-Jones, A.E., Layne, G.D., 2004. Partitioning of boron among melt, brine and vapor in the system haplogranite-H2O-NaCl at 800 °C and 100 MPa. Chemical Geology, 210, 135-147.

Shaw, D.M., Smith, P. L., 1991. Concentration of B, Sm, Gd, and H in 24 reference materials. Geostandards Newsletters, 15, 59-66.

Simancas, J.F., Carbonell, R., González Lodeiro, F., Pérez Estaún, A., Juhlin, C., Ayarza, P., Kashubin, A., Azor, A., Martínez Poyatos, D., Almodóvar, G.R., Pascual, E., Sáez, R., Expósito, I., 2003. The Crustal Structure of the Transpressional Variscan Orogen of SW Iberia: The IBERSEIS Deep Seismic Reflection Profile. Tectonics, 22(6), 1-16. DOI: 10.1029/2002TC001479.

Villa, I.M., 1990. Geochronology and excess Ar geochemistry of the Lhotse Nup leucogranite, Nepal-Himalaya. Journal of Volcanology and Geothermal Research, 44, 89-103.

Villaseca, C., Downes, H., Pin, C., Barbero, L., 1999. Nature and composition of the lower continental crust in central Spain and the granulite-granite linkage: Inferences from granulitic xenoliths. Journal of Petrology, 40, 1465-1496.

Von Goerne, G., Franz, G., Robert, J.L., 1999. Upper thermal stability of tourmaline plus quartz in the system MgO-Al2O3-

SiO2-B2O3-H2O and Na2O-MgO-Al2O3-SiO2-B2O3-H2OHCl in hydrothermal solutions and siliceous melts. Canadian Mineralogist, 37, 1025-1039.

Wijbrans J.R., Pringle M.S., Koppers, A.A.P., Scheveers R., 1995. Argon Geochronology of Small Samples Using the Vulkaan Argon Laserprobe. Proceedings Koninklije Nederlandse Akademie van Wetenschappen, Biological, Chemical, Geological, Physical and Medical Sciences, 98, 185-218.







Most read articles by the same author(s)