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https://doi.org/10.5194/se-2020-38
© Author(s) 2020. This work is distributed under
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https://doi.org/10.5194/se-2020-38
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  20 Apr 2020

20 Apr 2020

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A revised version of this preprint is currently under review for the journal SE.

Unravelling the origins and P-T-t evolution of the allochthonous Sobrado unit (Órdenes Complex, NW Iberia) using combined U-Pb titanite, monazite and zircon geochronology and REE geochemistry

José Manuel Benítez-Pérez1,3, Pedro Castiñeiras2, Juan Gómez-Barreiro3, José Ramón Martínez Catalán3, Andrew Kylander-Clark4, and Robert Holdsworth5 José Manuel Benítez-Pérez et al.
  • 1Centro de Ciências e Tecnologias Nucleares. Instituto Superior Técnico. Universidad de Lisboa. Estrada Nacional 10 (km 139,7), 2695-066, Bobadela LRS, Portugal
  • 2Departamento de Mineralogía y Petrología. Facultad de Ciencias Geológicas, Universidad Complutens de Madrid. C/José Antonio Novais, 12. Ciudad Universitaria, 28040, Madrid, Spain
  • 3Departamento de Geología, Universidad de Salamanca, Pza. de los Caídos s/n, 37008, Salamanca, Spain
  • 4Department of Earth Science, University of California, Santa Barbara, CA 93106, United States
  • 5Earth Science Department, Durham University, Science Labs, Durham DH1 3LE, United Kingdom

Abstract. The Sobrado unit, within the upper part of the Órdenes complex (NW Iberia) represents an allochthonous tectonic slice of exhumed high grade metamorphic rocks formed during a complex sequence of orogenic processes in the middle to lower crust. In order to constrain those processes, U-Pb geochronology and REE analyses of accessory minerals in migmatitic paragneisses (monazite, zircon), and mylonitic amphibolites (titanite) were conducted using LASS-ICP-MS. The youngest metamorphic zircon age obtained co-incides with a Middle Devonian concordia monazite age (~ 385 Ma) and is interpreted to represent the minimum age of the Sobrado high-P granulite-facies metamorphism that occurred during the early stages of the Variscan Orogeny. Metamorphic titanites from the mylonitic amphibolites yield a Late Devonian age (~ 365 Ma), and track the progressive exhumation of the Sobrado unit. In zircon, cathodoluminescence images and REE analyses allow two aliquots with different origins in the paragneiss to be distinguished. An Early Ordovician age (~ 490 Ma) was obtained for metamorphic zircons, employing the TuffZirc algorithm, although with a large analytical dispersion. This age is considered to mark the onset of granulite-facies metamorphism in the Sobrado unit under intermediate-P conditions, and related to intrusive magmatism and coeval burial in a magmatic arc setting. A maximum depositional age for the Sobrado unit is established in the late Cambrian (~ 503 Ma). The zircon dataset also record several inherited populations. The youngest cogenetic set of zircons yield a crystallization age from TuffZirc algorithm of ~ 530 Ma and are thought to be related to the peri-Gondwana magmatic arc. The additional presence of inherited zircons older than ~ 530 Ma is interpreted as suggesting a West African Craton provenance.

José Manuel Benítez-Pérez et al.

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José Manuel Benítez-Pérez et al.

José Manuel Benítez-Pérez et al.

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Short summary
The Sobrado unit represents an allochthonous tectonic slice of exhumed high grade metamorphic rocks formed during a complex sequence of orogenic processes in the middle to lower crust. We have combined U-Pb geochronology and REE analyses (LASS-ICP-MS) of accessory minerals in migmatitic paragneisses (monazite, zircon), and mylonitic amphibolites (titanite) to constraint the evolution. A Middle-Devonian minimum age for HP metamorphism has been obtained.
The Sobrado unit represents an allochthonous tectonic slice of exhumed high grade metamorphic...
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