Géochronologie U-Pb par ablation laser et ICP-MS (LA-ICP-MS ...

TECTO-124536; No of Pages 23
ARTICLE IN PRESS
Tectonophysics xxx (2009) xxx–xxx
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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