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

Chemical Geology 261 (2009) 171–183
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Chemical Geology
journal homepage: www.elsevier.com/locate/chemgeo
Miocene incorporation of peridotite into the Hercynian basement of the Maghrebides
(Edough massif, NE Algeria): Implications for the geodynamic evolution of the
Western Mediterranean
O. Bruguier a, ⁎, D. Hammor b , D. Bosch a , R. Caby a
a Equipe Manteau-Noyau, Géosciences Montpellier, Université de Montpellier II, Place E. Bataillon, 34 095 Montpellier, France
b Université Badji-Mokhtar, BP12, El-Hadjar, Annaba 23 000, Algeria
article
info
abstract
Article history:
Accepted 13 November 2008
Keywords:
Peridotite
Western Mediterranean
Monazite
U–Pb geochronology
Laser ablation
A laser ablation ICP-MS U–Pb age of 17.84±0.12 Ma (late Burdigalian) was obtained from monazites separated
from a leucocratic diatexite collected in close proximity to a small peridotite massif incorporated into the
lower crustal sequence of the Edough Massif (north-eastern Algeria), a southern segment of the peri-
Mediterranean Alpine Belt. Monazites extracted from a neighbouring deformed leucogranite intruding early
Paleozoic phyllites yield a consistent age of 17.4±1.3 Ma. Zircons occurring in the same leucogranite, with
magmatic characteristics, have an age of 308±7 Ma interpreted as dating magmatic crystallisation of the
leucogranite and reflecting partial melting during the Hercynian orogeny. Low Th/U domains (Th/Ub0.10)
from the same grains substantiate recrystallisation during a younger metamorphic event whose upper age
limit is 286±11 Ma. These results emphasize the polycyclic evolution of basement rocks preserved in the
crystalline units of the western Mediterranean and indicate that part of their metamorphic features were
inherited from older, Hercynian, events.
Taken together with published Ar–Ar dates, the late Burdigalian age of monazites indicates a rapid cooling rate
of c. 370 °C/Ma and is regarded as closely approximating the emplacement of the peridotites into the
Hercynian basement. The monazite ages are significantly younger than those recorded for orogenic
peridotites from the Betic-Rif orocline and for the timing of lithospheric extension forming the Alboran sea.
It is also younger than rifting and back-arc extension opening the Liguro–Provençal basin. The late Burdigalian
age is interpreted as dating the incipient rifting event that opened the Algerian basin, which is consequently
not a continuation of the Liguro–Provençal basin. At the scale of the western Mediterranean, these
observations concur with current models supporting slab roll-back and an eastwards migration of extension
in the western Mediterranean, but suggest that the Algerian basin opened as a result of torsion and stretching
of the Thethyan slab due to its steepening under the Alboran microplate.
© 2008 Elsevier B.V. All rights reserved.
1. Introduction
⁎ Corresponding author. Fax: +33 4 67 14 47 85.
E-mail address: bruguier@gm.univ-montp2.fr (O. Bruguier).
Sizeable pieces of deep mantle material crop-out worldwide
within orogenic belts involving major continental collisions (Brueckner
and Medaris, 2000) or within regions with rift-thinned continental
margins (e.g., Nicolas et al., 1987; Schärer et al., 1995).
Although minor components of most metamorphic belts, the understanding
of how and when orogenic peridotites were emplaced within
the continental crust and their subsequent exhumation is paramount
for the knowledge of the processes operating at the Earth crustmantle
interface, in subduction as well as rift environments. The
question of determining the age of the peridotites, and in particular
their emplacement within their host rocks, is a challenging geochronological
problem since many of the minerals commonly used as
robust chronometers (e.g., zircon) do not typically occur in peridotite.
Others, such as the Sm/Nd system, commonly give rise to ambiguous
interpretations as they define trends which are variously regarded
either as reliable dates or on the contrary as mixing lines between
different mantle components (see discussion in Brueckner et al.,1996).
Some chronometers, more adequate for peridotites (e.g., Re/Os), are
often subject to disturbances and may yield minimum model ages
(Snow and Schmidt, 1999), which in addition relate to phases of
mantle differentiation, rather than to the crustal emplacement of the
peridotites (Reisberg and Lorand, 1995). The Lu/Hf system has proved
to be a valuable chronometer (Blichert-Toft et al., 1999), but requires
fractionating phases or large inter-sample variations in order to get a
spread of data to allow for a precise age determination. This restricts
the studies aimed at tackling the timing of emplacement of orogenic
peridotites and thus hampers any chronology and comparison
between crustal processes and mantle dynamics and in particular
0009-2541/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.chemgeo.2008.11.016