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TECTO-124536; No of Pages 23 ARTICLE IN PRESS Tectonophysics xxx (2009) xxx–xxx Contents lists available at ScienceDirect Tectonophysics journal homepage: www.elsevier.com/locate/tecto Conflicting structural and geochronological data from the Ibituruna quartz-syenite (SE Brazil): Effect of protracted “hot” orogeny and slow cooling rate Sylvain Petitgirard a,1 , Alain Vauchez a, ⁎, Marcos Egydio-Silva b , Olivier Bruguier a , Pierre Camps a , Patrick Monié a , Marly Babinski b , Mathieu Mondou a,b a Geosciences-Montpellier, Université de Montpellier 2 and CNRS, Place E. Bataillon, 34095-Montpellier Cedex05, France b Instituto de Geociências—Universidade de São Paulo, Rua do Lago, 562—Cidade Universitária CEP 05508-080 São Paulo—SP, Brazil article info abstract Article history: Received 1 August 2008 Received in revised form 29 December 2008 Accepted 26 February 2009 Available online xxxx Keywords: Hot orogen Magmatic structure and deformation AMS U-Pb and 40Ar-39Ar geochronology Slow cooling rate SE-Brazil neoproterozoic orogeny The Ibituruna quartz-syenite was emplaced as a sill in the Ribeira-Araçuaí Neoproterozoic belt (Southeastern Brazil) during the last stages of the Gondwana supercontinent amalgamation. We have measured the Anisotropy of Magnetic Susceptibility (AMS) in samples from the Ibituruna sill to unravel its magnetic fabric that is regarded as a proxy for its magmatic fabric. A large magnetic anisotropy, dominantly due to magnetite, and a consistent magnetic fabric have been determined over the entire Ibituruna massif. The magmatic foliation and lineation are strikingly parallel to the solid-state mylonitic foliation and lineation measured in the country-rock. Altogether, these observations suggest that the Ibituruna sill was emplaced during the high temperature (~750 °C) regional deformation and was deformed before full solidification coherently with its country-rock. Unexpectedly, geochronological data suggest a rather different conclusion. LA-ICP-MS and SHRIMP ages of zircons from the Ibituruna quartz-syenite are in the range 530–535 Ma and LA-ICP-MS ages of zircons and monazites from synkinematic leucocratic veins in the country-rocks suggest a crystallization at ~570–580 Ma, i.e., an HT deformation N35My older than the emplacement of the Ibituruna quartz-syenite. Conclusions from the structural and the geochronological studies are therefore conflicting. A possible explanation arises from 40 Ar– 39 Ar thermochronology. We have dated amphiboles from the quartz-syenite, and amphiboles and biotites from the country-rock. Together with the ages of monazites and zircons in the country-rock, 40 Ar– 39 Ar mineral ages suggest a very low cooling rate: b3 °C/My between 570 and ~500 Ma and ~5 °C/My between 500 and 460 Ma. Assuming a protracted regional deformation consistent over tens of My, under such stable thermal conditions the fabric and microstructure of deformed rocks may remain almost unchanged even if they underwent and recorded strain pulses separated by long periods of time. This may be a characteristic of slow cooling “hot orogens” that rocks deformed at significantly different periods during the orogeny, but under roughly unchanged temperature conditions, may display almost indiscernible microstructure and fabric. © 2009 Elsevier B.V. All rights reserved. 1. Introduction A major issue in understanding orogenic processes is to decipher the relationships between large-scale deformation and magmatism because they are tightly linked and frequently interdependent (e.g., Hutton, 1988; Tommasi et al., 1994; Bouchez and Gleizes, 1995; Brown, 2007; Le Roux et al., 2008). The anisotropy of magnetic susceptibility (AMS) has been extensively used to map the internal structure (magmatic foliation and lineation) of plutons and determine whether the emplacement of a magmatic body is early, coeval or late compared to the main deformation in an orogenic domain (e.g., Guillet et al., 1983; Bouchez and Gleizes, 1995; Borradaile and Henry, 1997; ⁎ Corresponding author. E-mail address: vauchez@dstu.univ-montp2.fr (A. Vauchez). 1 Now at: Laboratoire de Sciences de la Terre-Ecole normale Supérieure de Lyon, 46 Allée d'Italie 69364—Lyon Cedex07, France. Archanjo et al., 2002). A complementary approach consists in dating the main deformation that affected the country-rock and the crystallization of the magmatic rock. When magma intrusion is coeval with the deformation, it is expected that the fabric in the magmatic intrusive and in the country-rock is similar, as are the ages of both crystallization of the pluton and deformation of the country-rock. This paper documents a complex case study where mapping of the magmatic foliation and lineation in a magmatic body (i.e. the Ibituruna quartz-syenite, SE Brazil) indicates a common deformation history with the country-rocks. However, U–Pb geochronology carried out on high temperature minerals (zircon and monazite) substantiate a ca. 40 My delay between the main metamorphic event affecting country-rocks and emplacement of the magmatic body. We propose that this apparent inconsistency between structural mapping and isotopic dating may result from the thermal evolution of the Ribeira- Araçuaí “hot-orogen”, which is characterized by a low (b5 °C/My) regional cooling from synkinematic HT–LP (~750 °C–600 MPa) 0040-1951/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2009.02.039 Please cite this article as: Petitgirard, S., et al., Conflicting structural and geochronological data from the Ibituruna quartz-syenite (SE Brazil): Effect of protracted “hot” orogeny and slow cooling rate Tectonophysics (2009), doi:10.1016/j.tecto.2009.02.039
ÔØ ÅÒÙ×ÖÔØ Repeated granitoid intrusions during the Neoproterozoic along the western boundary of the Saharan metacraton, eastern Hoggar, Tuareg shield, Algeria: An AMS and U-Pb zircon age study B. Henry, J.P. Liégeois, O. Nouar, M.E.M. Derder, B. Bayou, O. Bruguier, A. Ouabadi, D. Belhai, M. Amenna, A. Hemmi, M. Ayache PII: S0040-1951(09)00233-9 DOI: doi: 10.1016/j.tecto.2009.04.022 Reference: TECTO 124584 To appear in: Tectonophysics Received date: 20 February 2009 Revised date: 7 April 2009 Accepted date: 19 April 2009 Please cite this article as: Henry, B., Liégeois, J.P., Nouar, O., Derder, M.E.M., Bayou, B., Bruguier, O., Ouabadi, A., Belhai, D., Amenna, M., Hemmi, A., Ayache, M., Repeated granitoid intrusions during the Neoproterozoic along the western boundary of the Saharan metacraton, eastern Hoggar, Tuareg shield, Algeria: An AMS and U-Pb zircon age study, Tectonophysics (2009), doi: 10.1016/j.tecto.2009.04.022 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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