SESolid EarthSESolid Earth1869-9529Copernicus GmbHGöttingen, Germany10.5194/se-6-643-2015High-temperature metamorphism during extreme thinning of the continental crust: a reappraisal of the North Pyrenean passive paleomarginClercC.camille.clerc@cnrs-orleans.frLahfidA.MoniéP.LagabrielleY.ChopinC.PoujolM.BoulvaisP.RingenbachJ.-C.MasiniE.de St BlanquatM.Laboratoire de Géologie, CNRS-UMR8538, Ecole normale supérieure, 24 rue Lhomond, 75231 Paris,
FranceGéosciences Montpellier, CNRS-UMR 5243, Université de Montpellier 2, Place Eugène Bataillon, 34095 Montpellier,
FranceBRGM/ISTO, 3 avenue Claude Guillemin, 45000 Orléans, FranceGéosciences Rennes, UMR 6118, Université Rennes 1, Campus de Beaulieu, 35042 Rennes,
FranceTOTAL, CSTJF, avenue Larribau, 64000 Pau, FranceGET, 14 avenue Edouard Belin, 31400 Toulouse, Francenow at: ISTO, 1A rue de la Férollerie, 45100 Orléans, France
Invited contribution by C. Clerc, recipient of the EGU Outstanding Student Poster (OSP) Award 2012.
C. Clerc (camille.clerc@cnrs-orleans.fr)8June20156264366812January201520February201513April201516May2015This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://se.copernicus.org/articles/6/643/2015/se-6-643-2015.htmlThe full text article is available as a PDF file from https://se.copernicus.org/articles/6/643/2015/se-6-643-2015.pdf
An increasing number of field examples in mountain belts show that the
formation of passive margins during extreme continent thinning may occur
under conditions of high to very high thermal gradient beneath a thin cover
of syn-rift sediments. Orogenic belts resulting from the tectonic inversion
of distal margins and regions of exhumed continental mantle may exhibit
high-temperature, low-pressure (HT-LP) metamorphism and coeval
syn-extensional, ductile deformation. Recent studies have shown that the
northern flank of the Pyrenean belt, especially the North Pyrenean Zone, is
one of the best examples of such inverted hot, passive margin. In this study,
we provide a map of HT-LP metamorphism based on a data set of more than
100 peak-temperature estimates obtained using Raman spectroscopy of the
carbonaceous material (RSCM). This data set is completed by previous
PT (pressure and temperature) estimates based on mineral assemblages, and new 40Ar–39Ar
(amphibole, micas) and U–Pb (titanite) ages from metamorphic and magmatic
rocks of the North Pyrenean Zone. The implications on the geological
evolution of the Cretaceous Pyrenean paleomargins are discussed. Ages range
mainly from 110 to 90 Ma, and no westward or eastward propagation of the
metamorphism and magmatism can be clearly identified. In contrast, the new
data reveal a progressive propagation of the thermal anomaly from the base to
the surface of the continental crust. Focusing on the key localities of the
Mauléon basin, Arguenos–Moncaup, Lherz, Boucheville and the Bas-Agly, we
analyze the thermal conditions prevailing during the Cretaceous crustal
thinning. The results are synthetized into a series of three regional
thematic maps and into two detailed maps of the Arguenos–Moncaup and Lherz
areas. The results indicate a first-order control of the thermal gradient by
the intensity of crustal thinning. The highest grades of metamorphism are
intimately associated with the areas where subcontinental mantle rocks have
been unroofed or exhumed.
Introduction
Distal domains of present-day passive margins are inaccessible
environments from where very little in situ observations have been
obtained so far (Sibuet et al., 1979; Shipboard Scientific Party 1987; Boillot
et al., 1987; Sawyer et al., 1994; Whitmarsh et al., 1996, 1998).
The understanding of the processes controlling their structural and
thermal evolution hence requires the comparison with exhumed analogs of
fossil passive margins. The first and most-studied analogs have been the
paleomargins exposed along the Alpine arc (Lemoine et al., 1987; Froitzheim
and Eberli 1990; Manatschal and Nievergelt 1997; Manatschal 2004). Other
fossil margins have recently been identified within the Zagros (Wrobel-Daveau
et al., 2010) and Appalachian–Caledonian orogenic belts (Andersen et al., 2012;
Chew et Van Staal, 2014). At present, the Pyrenean domain is also considered
as hosting relevant analogs of distal passive margins, and its pre-orogenic
evolution is being intensely revisited (Lagabrielle and Bodinier, 2008;
Jammes et al., 2009; Lagabrielle et al., 2010; Masini 2011; Clerc et al., 2012;
Clerc et al., 2013; Masini et al., 2014; Tugend et al., 2014).
Simplified structural map of the northern flank of the Pyrenean belt.
The area between the North Pyrenean Fault (NPF) and the North Pyrenean Frontal Thrust (NPFT)
is known as the North Pyrenean Zone (NPZ).
Unlike the Alpine analog, the North Pyrenean domain did not undergo
subduction, and the thermal pattern recorded in the pre- and syn-rift
material has not been overprinted by major crustal overthrusts or
subductions. The pre-Pyrenean inverted margins hence offer suitable direct
access to the thermal imprint of crustal thinning and subsequent continental
breakup. Regional high-temperature, low-pressure (HT–LP) metamorphism is
known along the northern rim of the Pyrenean belt where it developed coevally
with a major Cretaceous crustal thinning event leading (Ravier, 1959;
Bernus-Maury, 1984; Azambre and Rossy, 1976; Golberg and Leyreloup, 1990;
Dauteuil and Ricou, 1989; Clerc and Lagabrielle, 2014). The HT metamorphic
rocks, deriving mainly from pre-rift to syn-rift sediments, are distributed
in a narrow WNW–ESE belt defined as the North Pyrenean Zone (NPZ), bounded by
two major post-metamorphic thrusts, the North Pyrenean Fault (NPF) to the
south and the North Pyrenean Frontal Thrust (NPFT) to the north (Fig. 1). The
NPZ hosts about 40 outcrops of sub-continental peridotite – among which
the Lherz body, lithotype of the lherzolite – whose exhumation is directly
attributed to the extreme thinning of the lithosphere (Kornprobst
and Vielzeuf, 1984; Lagabrielle and Bodinier, 2008).
At a regional scale, the NPZ is thought to represent the portion of the
lithosphere that accommodated most of the deformation during the
counterclockwise rotation of the Iberian plate with respect to the Europa plate
in the mid-Cretaceous (Choukroune and Mattauer, 1978). The zone
affected by the HT-LP metamorphism is characterized by an intense ductile
deformation – S1 of Choukroune (1972); Choukroune (1976) – first
attributed to compression and later reinterpreted as resulting from
syn-extensional or transtensional deformation (Golberg 1987; Golberg and
Leyreloup, 1990; Lagabrielle et al., 2010; Clerc and Lagabrielle, 2014).
Several estimates of peak temperatures have been published (Golberg and
Leyreloup 1990, 1990; Vauchez et al., 2013), but no comprehensive metamorphic
map is available so far at the scale of the whole NPZ.
(a) Simplified geological map of the North Pyrenean Zone,
compiled from Choukroune and Séguret (1973), Golberg and Leyreloup (1990), Debroas
(2003) and Jammes et al. (2009), and from the 1 / 50 000 geological map of the BRGM. (b) Map of peak metamorphic
temperatures estimated using RSCM geothermometry (black dots). Only some values are reported on
the map. See Table 1 for an exhaustive list and location of all the RSCM data. Temperatures of
Golberg and Leyreloup (1990) are also reported for the eastern NPZ (white dots). (c) Map of
isometamorphic zones; data compiled are from this study and the literature (Choukroune and
Séguret, 1973; Bernus-Maury, 1984; Golberg, 1987; Golberg and Leyreloup, 1990).
In this study, we provide a map of the HT Pyrenean metamorphism based on a
data set of more than 100 peak-temperature estimates obtained using
Raman spectroscopy of the carbonaceous material (RSCM) on samples collected
during the past 5 years along the whole NPZ (Fig. 2). From east to west
the sampled regions are the Bas-Agly basin, the Tarascon and Aulus basins
(eastern NPZ), the Ballongue basin (central NPZ) and the Mauléon basin
(western NPZ). This data set is completed by previous PT estimates based on
analysis of mineral assemblages from the Bas-Agly, Boucheville and Pays de
Sault areas by Golberg and Leyreloup (1990). We provide 18 new
40Ar–39Ar (amphibole, micas) ages and 1 U–Pb (titanite) age from
metamorphic and magmatic rocks of the North Pyrenean Zone. We further report
some characteristics of the deformation associated with the HT metamorphic
imprint, and we discuss the significance of the thermal event in the frame of
the extension of the continental crust and local mantle exhumation. Triassic
and Jurassic aborted rifting events predated the development of a major
Cretaceous crustal thinning event, which culminated in the crustal separation
between the Iberia and European plates (Puigdefabregas and Souquet, 1986;
Vergés and Garcia-Senz, 2001). Continental rifting in the Pyrenean domain
occurred in response to the counterclockwise rotation of Iberia relative to
Europe, coeval with the onset of oceanic spreading in the Bay of Biscay
between Chron M0 and A33o (approximately 125–83 Ma) (Le Pichon et al., 1970;
Choukroune and Mattauer, 1978; Olivet, 1996; Gong et al., 2008; Jammes et al., 2009). After an early rifting episode during the late Aptian, narrow,
non-connected Albian basins opened north of the basement of the Pyrenean
Axial Zone, along a wide domain opened between Iberia and Europe (Choukroune
and Mattauer, 1978; Olivet, 1996; Jammes et al., 2009). They connected
together during the Cenomanian when the rift zone became wider and deeper.
The main infills of the basins are dark-colored pelites, sandstone, and
breccias deposits, referred to as “Flysch noir” or “Flysch ardoisier” in
the literature (Debroas, 1976, 1978, 1990; Souquet et al., 1985). The black
flysch is organized into three megasequences (I, II, and III), which are the
records of three successive steps in the opening of the basins. Megasequence
I corresponds to the opening of narrow half grabens, megasequence II
registers the opening of en echelon 10 km wide basins, and megasequence III
records the coalescence of the basins into a large trough with internal and
external parts separated by central highs (Debroas, 1990).
The kinematics of the Iberian plate during Aptian–Albian and younger
Cretaceous times is still strongly debated (Olivet 1996; Sibuet, Srivastava,
et Spakman 2004; Jammes et al., 2009; Vissers et Meijer 2012a, b). Three main types are generally opposed: (i) the
transtensional rift model (Choukroune et Mattauer 1978; Olivet 1996), which
involves a dominant left-lateral strike-slip along the NPF; (ii) a model
implying most of the left-lateral movement during the Jurassic to Aptian
times followed by orthogonal extension during Albian to Cenomanian times
(Schettino et Turco 2010; Jammes et al., 2009); (iii) and a scissor-opening
model, which implies the existence of an important subduction beneath the
Pyrenean belt (Srivastava et al., 1990; Sibuet et al., 2004; Vissers and Meijer 2012a, b). For these authors, the extension
observed in the pre-Pyrenean domain at that time (Dinarès-Turell and
Garcia-Senz, 2000, 2002) would result from back-arc extension
or gravitational instability of the lithosphere above the subducted oceanic
lithosphere. This scenario implies the northward subduction of a large
portion of the Neotethyan oceanic domain, which is hardly compatible with
current reconstructions of the Alpine Tethys (Manatschal and Bernouilli,
1999; Handy et al., 2010; Schettino and Turco, 2010). Furthermore, recent
seismic tomography (Souriau et al., 2008; Chevrot et al., 2014a, b)
precluded the existence of a subducted hundredth-of-a-kilometer-long oceanic
lithosphere. Instead, they report evidence for the subduction of a thinned
Iberian crust to ca. ∼ 70 km depth.
Compilation of the age and localization of the Cretaceous metamorphism and magmatism,
modified and updated from Debroas and Azambre (2012). (1): Montigny et al., 1986; 40K–40Ar
on amphiboles from magmatic and metamorphic rocks. (1'): Montigny et al., 1986; 40K–40Ar
on micas. (1”): Montigny et al., 1986; 40K–40Ar on feldspath(oids). (1”'): Montigny et
al., 1986; 40K–40Ar on bulk rocks. (2): Golberg and Maluski, 1988. (3): Albarède and
Michard-Vitrac, 1978; 40Ar–39Ar on phlogopite from metasediments. (4): Albarède and
Michard-Vitrac, 1978; 40Ar–39Ar on orthoclase from metasediments. (4'): Albarède and
Michard-Vitrac, 1978; 87Rb–87Sr on phlogopite from metasediments. (5): Thiébaut et al.,
1988; 40K–40Ar (on bulk rock?) from Triassic meta-evaporites. (6): Hervouët et al.,
1987; 40K–40Ar on bulk rock (ophite) from the vallée du Job. (7): Golberg et al.,
1986; 40Ar–39Ar. (8): Nicolas, 1998, unpublished master thesis; 40Ar–39Ar on muscovite
and biotite from the charnockite and various schists of the Agly Paleozoic massif. (9): Schärer et al.,
1999; U–Pb on xenotime and monazite from talc deposit. (10): Boulvais et al., 2007; 40Ar–39Ar on muscovite
from albitite. (11): Poujol et al., 2010; U–Th–Pb on titanite and monazite from albitite. (12): Fallourd et
al., 2014; U–Pb on titanite in albitite (faded orange field). (13) Henry et al., 1998; 40Ar–39Ar
on amphibole from amphiclasite. (14): Castanares et al., 1997; 2001; chronostratigraphy of submarine volcanic
episodes. (15): Lopez-Horgue et al., 1999; chronostratigraphy of submarine volcanic episodes. (16):
Lopez-Horgue et al., 2009; chronostratigraphy of submarine volcanic episodes. (17) Monié,
unpublished; 40Ar–39Ar on Ms from marble. (A): This study; 40Ar–39Ar on muscovite from marble. (B):
This study; 40Ar–39Ar on amphibole from marble. (C): This study; 40Ar–39Ar on amphibole
from gabbro and teschenite. (D): This study; 40Ar–39Ar on amphibole from meta-ophite. (E):
This study; 40Ar–39Ar on phlogopite from talc deposit. (F): This study; 40Ar–39Ar on
amphibole in gypsum. (G): This study; U–Pb on titanite in albitite dyke. Spots in transparency must be
considered with caution since they were obtained with 40K–40Ar on bulk rock.
The HT-LP metamorphism: ages and coeval magmatismThe age of HT-LP metamorphism: a review
The metasediments of the NPZ are affected by a HT-LP metamorphism with
temperatures commonly higher than 600∘C and pressures lower than 4 kbar (Fig. 2; Bernus-Maury, 1984; Golberg and Leyreloup, 1990; Vauchez et al., 2013).
The NPZ is characterized by strong temperature gradients affecting
mostly the pre- and syn-rift Mesozoic sediments, ranging in age from the
Trias to the base of the Upper Cretaceous (although the extension of the
thermal anomaly to Paleozoic material has rarely been tested). The youngest
metamorphosed terrains in the NPZ are Santonian in age and were only affected
by an epizonal metamorphism (Mattauer, 1964; Choukroune, 1972; Debroas, 1987).
General map of the ductile lineation measured in the NPZ, in the Mesozoic material (blue) and in the Paleozoic basement (red).
The HT-LP metamorphism has alternatively been related to late-Cretaceous
compressive events (Mattauer, 1968; Choukroune, 1976) or to a “pre-Cenomanian
crustal fracturation” (Ravier, 1959; Souquet et al., 1977). Choukroune and
Mattauer (1978) re-interpreted this thermal event as the consequence of an
important thinning of the continental crust for which, due to slow migration
of the isotherms, the maximum of the thermal anomaly would only be reached
during the initiation of the convergent tectonics.
Since the 1980s, the thermal anomaly responsible for the development of the
metamorphic event is attributed to very high geothermal gradients related to
an intense crustal and lithospheric thinning episode during the Cretaceous
(Vielzeuf and Kornprobst, 1984; Golberg, 1987; Golberg and Maluski, 1988;
Dauteuil and Ricou, 1989; Golberg and Leyreloup, 1990). This assumption is
based on four main geological features: (i) HT-LP assemblages are recorded
along the extensive/transtensive domain represented by the NPZ; (ii) HT-LP
metamorphism is contemporaneous with an episode of alkaline magmatism;
(iii) HT-LP metasediments are spatially related to tectonically exhumed deep-seated
material (granulites and peridotites); and (iv) the metamorphic event affected
Albian–Cenomanian flyschs during and just after their deposition.
Previous geochronological studies (Fig. 3) of the Cretaceous metamorphism of
the Pyrenees revealed ages ranging from the Albian to the Santonian, e.g.,
mainly in the range 105–85 Ma (Albarède and Michard-Vitrac, 1978a;
Albarède and Michard-Vitrac, 1978b; Montigny et al., 1986; Golberg and
Maluski, 1988; Golberg et al., 1986; Bandet Y. and Gourinard Y. in Thiébaut
et al., 1988).
In addition to the HT-LP metamorphism well developed in Mesozoic material,
one must also consider the hydrothermal alteration responsible for
albitization and dequartzification of the North Pyrenean massifs and the
formation of massive talc deposits during the late Aptian to early Cenomanian
(Moine et al., 1989; Demange et al., 1999; Schärer et al., 1999; Boulvais et al., 2007; Poujol et al., 2010) at temperatures ranging from 250∘C
(Moine et al., 1989; Boulvais et al., 2006) to more than 500∘C
(sodic-calcic metasomatism; Fallourd et al., 2014). Radiogenic chronometers
in the Paleozoic basement of the North Pyrenean massifs have been reset
during Cretaceous times (e.g., Costa and Maluski 1988; St Blanquat et al.,
1990; Boutin et al., 2015, in the St Barthelemy massif), which is
indicative of general heating of the NPZ. Cretaceous ages have also been
reported in mylonitized granitoids from the Axial Zone in the eastern
Pyrenees (Monié et al., 1994), suggesting that reheating and related
deformation propagated further south of the NPF.
Examples of deformed Mesozoic fossils from the NPZ. Boudinaged (a) and elongated (b) belemnites and pecten in the Bas-Agly.
Ductiley deformed bivalves and belemnites at Port de Saleix (c) and Col Dret (d).
Deformed Urgonian rudists in the Bas-Agly (e and f).
According to previous studies of the eastern NPZ, the intensity of the HT-LP
metamorphism is thought to be directly related to the magnitude of crustal
attenuation, with high-grade metasediments systematically associated with
peridotites or granulites, and lower-grade metasediments associated with mid-
to upper-crustal units (Golberg and Leyreloup, 1990). Our new data set,
extended to the whole NPZ, confirms this trend. To push forward the
characterization and significance of the HT-LP Cretaceous metamorphism, we
will discuss our results at the scale of the whole Pyrenean realm, then
zooming at the scale of the basin and at the scale of the sedimentary pile.
Metamorphism and deformation: the S1 deformation (Choukroune, 1976) is syn-extensional
All along the inner NPZ, Triassic to Albian–Cenomanian rocks bear evidence of
intense ductile deformation generally transposed on the original
stratification of the Mesozoic metasediments. Stretching lineations are
marked by the elongated aspect of some marbles and by preferred orientations
of the HT-LP minerals (scapolite). A map of the HT ductile lineation of the
eastern NPZ is presented in Fig. 4. Despite a relative dispersion probably
due to the later compressional movements, the lineation is generally trending
along a NW–SE direction. Although it may be difficult, the distinction of
Mesozoic marbles from Paleozoic (mainly Devonian) ones can be done by
comparison with less metamorphosed facies (e.g., Debroas in Ternet et al.,
1997); some fossils allow for unambiguous attribution to the Mesozoic series
(Fig. 5). Boudinage and folds are frequent and are observed at different
scales. Sheath folds and isoclinal folds are commonly observed in both
pre-Albian marbles and Albian–Cenomanian metapelites. In the high-grade
marbles of the Internal Metamorphic Zone (IMZ; e.g., Aulus and Arguenos–Moncaup
areas), isoclinal folds are commonly refolded and show complex 3-D geometries.
Such characteristics are evocative of flow folds (viscous folds without
visible axial-plane cleavage or foliation; Wynne-Edwards, 1963).
The analysis of metamorphic parageneses indicates that the metamorphism can
be both static (Ravier, 1959) and syn-kinematic (Bernus-Maury, 1984). According
to Choukroune (1976), the long-lasting Cretaceous metamorphism is
contemporaneous with a first phase of deformation (S1) and continues during a
part of a second phase (S2 of Choukroune, 1976).
Boudinage of dolomitic and silicic layers is commonly observed within the
marbles due to their rheological contrast with the calcitic host. The boudins
and the boudin necks form symmetric lenses displaying a “chocolate tablet”
aspect (Vauchez et al., 2013). The synchronicity between deformation and
metamorphism has also been demonstrated by the HT fabrics of calcite in the
marble of the IMZ (Vauchez et al., 2013;
Lagabrielle et al., 2015).
Considering that the onset of the convergence in the Pyrenean realm is
estimated to occur during the Santonian (Garrido-Megias and Rios, 1972;
McClay et al., 2004), the pre-Santonian radiometric ages obtained for the HT
metamorphism are in agreement with a pre-convergence event (Fig. 3).
Magmatism
The North Pyrenean realm is affected by moderate but well-distributed
magmatic activity during the Mesozoic, responsible for the emplacement of two
main groups of rocks: (i) tholeitic dolerites of Triassic age, generally
referred to as “ophites” (Montigny et al., 1982; Azambre et al., 1987), and
(ii) a wide variety of small intrusive and effusive Cretaceous alkaline
magmatic rocks (Montigny et al., 1986; Azambre et al., 1992; Rossy et al., 1992).
The age of the Cretaceous magmatism is often constrained by
stratigraphic correlations with the sedimentary formations in which volcanic
or intrusive rocks are observed (Fig. 3; San Miguel de la Camara 1952; Rossy
1988; Castanares et al., 1997; López-Horgue et al., 1999; Castanares et al., 2001; López-Horgue et al., 2009). In Buzy (Pyrénées-Atlantiques),
teschenites outcrop as sills intrusive in the Turonian flysch where they
developed a hornblende-hornfels contact metamorphism aureole (Casteras et al., 1970). In the Cantabrian basin, thick formations of lava flows with pillows
are interbedded within the upper Albian to Santonian sedimentary formations
(Castanares et al., 1997, 2001; Carracedo et al., 1999). In
addition, absolute ages are available on Cretaceous magmatic rocks, with a
spread from 113 to 85 Ma (Fig. 3; Golberg et al., 1986; Montigny et al.,
1986; Henry et al.,1998). Some authors distinguish an early phase of
intrusive magmatism followed by a paroxysm from the Cenomanian to the end of
the Turonian (Debroas and Azambre, 2012). Some manifestations of the
Cretaceous alkaline magmatism have also been recognized within the peridotite
bodies of the NPZ, where they appear as dykes of amphibole-bearing
pyroxenites and hornblendes (“ariégites à amphiboles” and
“lherzites” of Lacroix 1917) considered to represent trans-mantellic melt
conduits for the Cretaceous alkaline magmatism (Bodinier et al., 1987;
Conquéré 1971; Vétil et al., 1988). Radiometric dating of these
rocks gave ages around 100 Ma (Vershure et al., 1967; Golberg et al., 1986;
Henry et al., 1998). The peridotite bodies of Tuc de Desse, Arguenos–Moncaup,
Montaut and Turon de la Técouère also contain gabbroic intrusions
that present chilled margins along their contacts, indicating that some of
the mantle rocks were already cooled during the emplacement of the magmatic
rocks (Azambre and Monchoux, 1998).
Raman spectroscopy of carbonaceous materials: analytical method and thermometry
Raman spectroscopy has been used successfully to characterize the
structural evolution of carbonaceous material (CM), reflecting a
transformation from disordered to well-ordered CM during metamorphism
(Wopenka and Pasteris, 1993). The irreversible polymerization and
reorganization of these materials is reflected in their Raman spectrum by the
decreasing width of the graphite G band and gradual disappearance of the
defect bands, first D3 and D4, then D1 and D2. The Raman spectrum of
well-ordered CM (perfect graphite) contains only the G band. This spectral
evolution with increasing graphitization was related to temperature and
quantified, providing a means to determine peak temperatures attained by
metamorphic rocks (Beyssac et al., 2002). This is the basis of the RSCM geothermometer, which was calibrated in the range
330–650∘C by Beyssac et al. (2002) and extended to the range
200–320∘C by Lahfid et al. (2010). In this study, we have applied
these two calibrations to estimate paleotemperatures in marbles and
pelitic–psammitic metasedimentary rocks from the Paleozoic to Upper
Cretaceous series of the NPZ.
RSCM peak temperatures from the NPZ. The parameters
RA1Lahfid (Lahfid et al., 2010) and R2Beyssac
(Beyssac et al., 2002) are used to estimate temperatures <320 and >330∘C, respectively. RA1Lahfid, R2Beyssac
and T are expressed in terms of mean value and SD of all the data obtained
within each of the 106 samples from the NPZ sample. Standard errors
ε are given for temperatures (=1σ SD divided by the
square root of the number of measurements).
Raman analyses were performed using a Renishaw (Wotton-under-Edge, UK) InVIA
Reflex microspectrometer at ENS Paris. Before each session, the spectrometer
was calibrated with silicon standard. The light source was a 514 nm Spectra
Physics argon laser. The output laser power is around 20 mW, but only around
1 mW reached the surface sample through the DMLM Leica (Wetzlar, Germany)
microscope with a 100× objective (NA = 0.90). Edge filters eliminated the
Rayleigh diffusion, and the Raman light was dispersed using a 1800 g mm-1 grating before being analyzed by a Peltier-cooled RENCAM CCD
detector. Measurements were done on polished thin sections cut normal to the
foliation and parallel to the lineation (xz structural plane). To avoid the
effect of polishing on the CM structural state, the CM particles analyzed
were below a transparent adjacent mineral, usually calcite or quartz. The
results are presented in Table 1 and Figs. 2, 7 and 8.
Example of variable mode of extension along a single linear plate boundary. In this example, the eastern domain of the
future NPZ undergoes transtension with a strong left-lateral obliquity, while the western domain opens with a moderate obliquity.
Geological map of the Arguenos–Moncaup area (after Barrère et al., 1984a; Hervouët et al., 1987; Canérot and Debroas, 1988) with Raman temperature
values and location of the samples used for 40Ar–39Ar dating. A clear increase of
peak temperature appears in the vicinity of the mantle peridotites.
Geochronology
In order to corroborate previous results and to extend the data set of the
ages of the North Pyrenean magmatism and metamorphism, we selected 19
samples from the NPZ. We focused on the marbles and gabbros of the
key localities of Lherz (LHZ samples) and Arguenos–Moncaup (MP samples).
Eighteen metamorphic and magmatic samples were dated using
40Ar–39Ar step heating of muscovite, phlogopite and amphibole, and
a sample of albitite vein cross-cutting Paleozoic material has been dated by
U–Pb on titanite.
40Ar–39Ar dating of metamorphic and magmatic samples from the NPZ
Fresh samples were selected in the field for step-heating laser probe
40Ar–39Ar dating. The samples were crushed and sieved, and single
grains of micas (biotite and muscovite) and amphibole were handpicked under
binocular microscope and cleaned in an ultrasonic bath using acetone and
distilled water. Micas and amphiboles were packaged in aluminium foils and
irradiated in the core of the TRIGA Mark II nuclear reactor of Pavia (Italia)
with several aliquots of the Fish Canyon sanidine standard (28.03 ± 0.08 Ma; Jourdan and Renne, 2007) as flux monitor. Argon isotopic
interferences on K and Ca were determined by irradiation of KF and CaF2
pure salts from which the following correction factors were obtained:
(40Ar/39Ar)K=0.00969± 0.00038,
(38Ar/39Ar)K=0.01297± 0.00045,
(39Ar/37Ar)Ca=0.0007474± 0.000021 and
(36Ar/37Ar)Ca=0.000288± 0.000016. Argon
analyses were performed at Géosciences Montpellier (France) with an
analytical system that consists of (a) an IR CO2 laser of 100 kHz used
at 5–15 % during 60 s; (b) a lense system for beam focusing; (c) a steel
chamber, maintained at 10-8–10-9 bar, with a drilled copper
plate;(d) an inlet line for purification of gases including two Zr–Al
getters; and (e) a MAP215-50 mass spectrometer or an Argus VI Thermo-Fisher
multi-collector mass spectrometer. Custom-made software controlled the
laser intensity, the timing of extraction/purification, the data acquisition
and reduction to calculate ages. To measure the argon background within the
system, one blank analysis was performed every three sample analyses. The
1σ errors reported on plateau, isochron and total gas ages include the
error on the irradiation factor J. Atmospheric 40Ar was estimated using
a value of the initial 40Ar–36Ar of 295.5.
Geological map of the central-eastern part of the Aulus basin, with Raman temperature values and location of samples used for 40Ar–39Ar dating. No clear thermal
trend can be deciphered in the basins, which we interpret as an indication of post-peak-metamorphism disruption of the sedimentary pile.
Metasediments samples
Muscovite of samples LHZ120 and FREYCH was extracted from marbles of the
Lherz areas (Fig. 8). Sample LHZ107 is a centimeter-sized muscovite crystal
extracted from a calcite vein cutting through the Liassic metapelites of
Port de Saleix. Muscovites LHZ125 and LHZ152 were sampled in the breccias
reworking clasts of Mesozoic marbles exposed near the peridotite bodies of
Freychinède (LHZ125) and Etang de Lherz (LHZ152). Similar breccias are
described by Clerc et al. (2013). The muscovite grains were extracted from
the matrix of the breccias (Fig. 9). Sample 11MP92 is a centimeter-sized muscovite
crystal found in the rim of a mafic intrusion exposed in the Arguenos marble
quarry, on the western side of Montégut (Fig. 10a and b). Muscovite
StB comes from the Jurassic white marbles of the Rapp quarry, near St
Béat. The amphiboles of samples 11MP86 and 11MP87 were obtained from
metamorphosed Liassic tuffs (B. Azambre, personal communication, 2014) exposed in fresh
cuts along road D39 close to the intersection with roads D618. The tuffs are
highly recrystallized, hosting centimeter-long unoriented amphiboles (Fig. 10c, d
and e).
Microscopic view of breccia sample LHZ152. In
plane-polarized light (a); redrawn (b); in cross-polarized light (c and d).
Magmatic rock samples
Three amphibole samples were extracted from the metadolerites of
Freychinède (LHZ146, LHZ148) and from Cazaunous (11MP88). These rocks,
generally referred to as ophites, were emplaced during the Triassic (Montigny
et al., 1982). The ophites of the NPZ are strongly affected by a post-magmatic
transformation responsible for their recrystallization and the neoformation
of Al-rich amphibole, clinopyroxene, plagioclase and scapolite under
amphibolite facies conditions (Azambre et al., 1987, 1971;
Golberg and Leyreloup, 1990). This recrystallization has been attributed to
the Cretaceous thermal event, and the neoformation of amphiboles was dated at
95 Ma by 87Rb-87Sr on amphibole in Lherz (Montigny et al., 1986).
The centimeter-long green amphibole of ARI was extracted from gypsum at the Arignac
gypsum quarry. TRI is a phlogopite sampled in the hydrothermally altered rim
of a pegmatite dyke in the footwall of the Trimouns talc ore deposit. The
outcrop is now partly destroyed due to mining excavations.
Two gabbros were sampled in the localities of Port de Saleix (LHZ102, Fig. 8)
and Col de Menté (11MP62, out of Fig. 7). At Port de Saleix, the
hectometric gabbro body is found on the southernmost part of the Aulus basin,
between the NPF and a complex unit containing marble breccias and
granulitized metasedimentary rocks including olivine- and pyroxene-bearing
marbles. At col de Menté, the gabbro constitutes a small intrusion within
the Mesozoic marbles. BUZ is a sample of amphibole from a small
teschenite dyke close to the village of Buzy (Mauléon basin).
40Ar–39Ar ages measured in the 14 muscovites and amphibole samples from the Arguenos–Moncaup and Lherz
area.
The results are presented in Table 2, and relevant areas in Fig. 3 are
expanded in Fig. 7 (Moncaup), 8 (Lherz) and 11. In the Lherz area, age
spectra are mainly flat for a large percentage of the argon released, with
only minor evidence of age scattering due to the presence of tiny inclusions
and of weak weathering. Muscovites and amphiboles from six metamorphic rocks
provide ages ranging from 89.5 ± 0.3 to 92.6 ± 1.1 Ma (late
Cenomanian to Turonian), with the exception of one sample (LHZ120) for which
muscovite has a plateau date of 100.1 ± 1.2 Ma. In the Arguenos–Moncaup
areas (Fig. 7), the age spectra of amphiboles are more discordant due the low
amount of potassium and probably the contribution of some inclusions.
Nevertheless, the cooling ages of all the metamorphic samples are clustering
between 101.1 ± 1.3 and 96.7 ± 7.5 Ma (Albian to Cenomanian).
Among the metamorphic ages, breccias from the Lherz area gave the youngest
ones and are all restricted to the Turonian (92.0 ± 0.7, 89.8 ± 1.1 and 89.6 ± 1.6 Ma).
(a) Boudinaged sill in the Montégut marble quarry;
(b) detail of (a) showing centimetric muscovite at the contact (sample 11MP92b);
(c) two beds of Early Jurassic tuffs intruding marbles at the new outcrop of
road D39 (samples 11MP86 and 11MP88).
40Ar–39Ar step-heating data for the samples ARI,
TRI, StB, 11MP62 and BUZ.
In both the Arguenos–Moncaup and the Lherz areas, undeformed gabbros yielded
amphibole 40Ar–39Ar ages in the range 94–100 Ma (samples LHZ102,
11MP62), which confirms that the magmatic and metamorphic activities are
contemporaneous. We note that this age is younger than the 107–109 Ma
40K–40Ar age reported on a gabbro from Les Plagneaux (Fig. 3;
Montigny et al., 1986), which may indicate either that magmatism occurred in
several pulses between 109 and 94 Ma or that amphiboles from Les Plagneaux
have been contaminated by excess argon. However, this age is consistent with
the 109.2 ± 3.5 Ma age obtained on amphiboles from the Arignac
Triassic gypsum (ARI, Fig. 11), which indicates that both metamorphism and
magmatism start to affect the Mesozoic cover as soon as the early Albian.
In Tarascon Valley, the 100.3 ± 1 Ma age obtained on phlogopite
(sample TRI, Fig. 11) from the rim of the Trimouns talc deposit is in
agreement with the 112–97 and 99 Ma ages obtained from U–Pb dating of
xenotime and monazite (Schärer et al., 1999) and constrains the main
period of hydrothermal activity to the Albian–Cenomanian transition.
To the west, in Buzy, the teschenite yielded an amphibole
40Ar–39Ar age of 92.9 ± 1.3 Ma, in good agreement with a
previous datum of 93 ± 4 Ma by Montigny et al. (1986) which confirms
contemporaneity of metamorphism with the emplacement of the previously dated
undeformed gabbro bodies in the Cenomanian–lower Turonian.
Albitite veins intruding the Paleozoic basement at
Arguenos–Moncaup (localized at point 594 near Cazaunous in Fig. 6).
U–Pb dating of albitite veins
In the Arguenos–Moncaup area, small decametric units of Paleozoic basement
resting between the mantle peridotites and the pre-Albian Mesozoic marbles
are cross cut by albitite veins (Fig. 12). Ten analyses were performed in
10 different titanite grains present in one thin section (see Supplement
for detailed results and analytical procedure). In a Tera
Wasserburg diagram (Fig. 13), data plot in a concordant to discordant
position and define a lower intercept age of 98.4 ± 1.1 Ma
(MSWD = 1.4) if anchored to a common 207Pb/206Pb value calculated at
100 Ma using a single-stage Stacey and Kramers (1975) value
(98.3 ± 0.9 Ma, MSWD = 1.7 Ma if the regression is free). Therefore we infer that the
titanite associated with this albitite crystallized 98 My ago. This age is
consistent with the ages of the metamorphic event obtained in this area
(Albian to Cenomanian, cf. Sect. 6.1.3).
207Pb/206Pb–238U/206Pb concordia diagram for
the 10 spots analyzed in titanite from the albitite veins of Agruenos–Moncaup (Fig. 11).
DiscussionSpatial distribution of the thermal anomaly at the scale of the NPZ
A first-order zonation of the HT-LP metamorphism appears at the scale of the
whole Pyrenean domain. In agreement with the previous results (Choukroune
and Séguret 1973; Choukroune 1976; Bernus-Maury 1984; Golberg 1987;
Golberg and Leyreloup 1990), we distinguish three main thermal domains along
the NPZ (Fig. 2c). From west to east, these are the following:
The western domain, corresponding to the Béarn and Pays Basque basins, shows
low-grade HT-LP metamorphism with Raman temperatures generally lower than 350∘C.
In this part of the belt, the thermal anomaly is largely distributed in the prerift
metasediments and in the Albian–Cenomanian flyschs, with the highest temperatures found close to exposures of mantle rocks and Paleozoic basement (Saraillé,
Roquiague).
The central domain – which includes the Ballongue, Barousse and Baronnies
basins – displays a higher grade of HT-LP metamorphism, with RSCM temperatures of 300
to 450∘C, locally exceeding 550∘C close to mantle exposures
(Arguenos–Moncaup peridotites body). The temperatures decrease toward the west.
The eastern domain – including the Boucheville, Agly, Pays de Sault, and
Aulus basins – has the highest grade of HT-LP metamorphism, with Raman
temperatures up to 600∘C and above. In addition to the east–west
thermal zonation, a north–south thermal gradient is well observed in this
domain. The highest temperatures are found in the southernmost regions
(Boucheville and Aulus basins), whereas the northern regions are
characterized by a rapid decrease of the HT-LP metamorphic imprint. The
isograds are here widely disturbed by later faulting locally outlined by
tectonic breccias.
Clerc and Lagabrielle (2014) correlated this thermal zonation across the NPZ
to a variable mode of crustal thinning across the pre-Pyrenean paleomargins,
suggesting the existence of “hot” margins (eastern domain) in opposition to
the cooler western domain. For the authors the three domains can be described
as follows:
The basins of the cooler western domain are characterized by well-developed
coarse clastic formations related to paleoscarps on the border of the troughs, in relation
to high-angle faulting and dominant brittle behavior of the continental crust (e.g., the
Igountze and Mendibelza breccias; Boirie 1981; Boirie and Souquet 1982; Masini et al., 2014).
In contrast, the central part of these basins is a domain of exhumed mantle capped by tectonic
lenses of both ultramafic and sialic compositions tectonically underlying the prerift metasediments.
Here, thinning of the crust occurred in a ductile mode, but the continental crust does not form large
massifs in the center of the basins.
The central domain is characterized by basins of smaller dimensions separated by
blocks of continental material, the North Pyrenean massifs, whose size globally increases
eastward. The basins are frequently triangle- or losange-shaped (Debroas, 1990) with mantle
rocks frequently exposed in their central regions. Progressively, toward the east, the
continental crust of the North Pyrenean massifs is boudinaged and tends to form spindle-shaped lenses.
The eastern domain is characterized by an intense HT deformation of the Triassic to
Albian sedimentary pile and by the boudinage of the continental crust in the distal regions of
the paleomargin. The Mesozoic sequence is everywhere severely thinned under a ductile regime, with
frequent boudinage and tectonic subtractions of comprehensive portions of the original succession.
The Albian–Cenomanian deposits are controlled by a wide wavelength undulation of the top of the
lithosphere in relation to the boudinage of the hyper-thinned continental crust.
Several hypotheses are proposed in the following that may explain such a variation of
the thermal anomaly along the paleomargin:
The whole domain underwent similar HT-LP metamorphic evolution, but later tectonic
inversion led to underthrusting and burial of the highest-grade rocks in the western part of the
NPZ. When considering the general plunge of the Axial Zone toward the west, it is also clear that
the eastern part of the belt underwent more exhumation, hence extensively exposing the deepest
units that underwent more HT-LP metamorphism. In this case, the western and eastern part of the NPZ
could be considered to represent both the shallow (west) and deep (east) processes occurring during
the contractional evolution of a single passive margin. However, in that case the pre-rift material
should have undergone temperatures much higher than the 366 and 298∘C
obtained in the Liassic and Jurassic of the Barlanès and Saraillé areas (Table 1; Mauléon basin in Fig. 2).
By contrast, and in better agreement with the structural variations
observed along the paleomargin, we may also consider the thermal zonation of
the NPZ as a consequence of a lateral variation of the mode of opening of
the basins. Although the kinematic history of the domain is still poorly
constrained, approximately N–S extension is reported from field observations
in the western NPZ (Jammes et al., 2009, 2010; Masini, 2011;
Masini et al., 2014), whereas a transcurrent to oblique motion is generally
reported in the eastern NPZ (Choukroune et Mattauer, 1978; Debroas, 1990;
Debroas, 2003; Clerc et al., 2012, Fig. 5). As exemplified in Fig. 6, a
rotation pole for the Iberian plate located anywhere in the northeast of the
NPZ could lead to differential movement with strike-slip to transtensional
extension in the eastern domain and orthogonal to oblique extension in the
western domain. Since the deformation is prone to be more localized in a
transform system, thermal fluxes are expected to increase with transcurrent
motion (Golberg and Leyreloup, 1990; Muffler and White, 1969; McDowell and
Elders, 1980, 1983). Furthermore, because the
deformation is more localized in the eastern domain than in the western
domain, isotherms may have been more spaced in the wider western NPZ than in
the narrower eastern NPZ. Regional strain partitioning of a transtensional
kinematic into orthogonal extension and transcurrent movement within the
future NPZ, the Parentis, Cameros, Le Danois, Basque–Cantabrian and
Organya–Pedraforca basins may explain the localization of the HT-LP
metamorphism in the basins dominated by transtensive movements (central and
eastern NPZ, Nappe des Marbres, Fig. 4), whereas extension under cooler
conditions is registered in the basins undergoing a dominant orthogonal
extension (Mauléon basin – Jammes et al., 2009, 2010;
Masini et al., 2014). In addition, we can envisage that
various modes of opening induced a variety in hydrothermal circulations. If
some circulations were externally derived, which is actually not identified
in Albian basins using a stable-isotope characterization of veins (Boulvais
et al., 2015), they may have produced differences in regional temperature, the
coolest temperatures being likely found in the basins with the most
efficient cooling circulations (e.g., Souche et al., 2014).
It is also possible to correlate the intensity of the HT metamorphism
to the rising velocity of the mantle peridotites. For the eastern NPZ
peridotites, Fabriès et al. (1991) identified a single and rapid
decompression and cooling step attributed to their ascent from 50–45 km
depth to the surface. In contrast, for the western NPZ peridotites,
Fabriès et al. (1998) identified a slower decompression and cooling from
25–15 km depth up to the surface (see Fig. 11 in Clerc et al., 2013, for a
synthetic exhumation chronology of the NPZ peridotites).
Finally, the lateral variability of the thermal anomaly along the NPZ
might be related to the scissors-shape opening of the extended domain during
the Cretaceous (Masson and Miles, 1984; Srivastava et al., 1990; Sibuet and
Collette, 1991; Roest and Srivastava, 1991; Sibuet 2004; Jammes et al., 2010).
Spatial distribution of the thermal anomaly at the basin scaleReconstructing the initial thermal gradient related to crustal thinning and mantle exhumation: the Arguenos–Moncaup case study
The spatial distribution of the isograds at the scale of the entire belt
shows a first-order relationship between the highest grades of HT
metamorphism and the vicinity of deep material exposed in the NPZ
(peridotites, granulites, migmatites). Such a relationship was already
observed in the eastern part of the NPZ (Golberg and Leyreloup, 1990). Based
on their metamorphic analysis in the eastern part of the NPZ, these authors
also suggested that these primary relationships were often lost and disturbed
by later faulting. Our new data set, extending further west, reveals that the
primary relationships between thermal anomalies and basement rocks are better
preserved in the central part of the NPZ, and especially in the
Arguenos–Moncaup area (Fig. 7), where undisturbed thermal gradients are still
recognizable.
The Arguenos–Moncaup ultramafic body is part of a group of peridotite
exposures lying around the Milhas massif, in the central Pyrenees (Fig. 2).
They are associated with basement rocks, variably brecciated Triassic
sediments, ophites and Albian mafic intrusions. The Arguenos–Moncaup
peridotites are overlain in tectonic contact by highly metamorphosed Mesozoic
marbles (Debeaux and Thiébaut 1958; Hervouët et al., 1987;
Barrère et al., 1984a, b). Although the peridotites
have risen to near-surface levels, there is no evidence for sedimentary
reworking indicating their exhumation on the basin floor. Indeed, on the
basis of their geological setting, it can be deduced that the mantle rocks
have remained capped by the Mesozoic marbles together with small slices of
continental crust, during their ascent along a detachment fault (Lagabrielle
et al., 2010). There, the highest temperatures (500–600∘C) are
recorded directly in marbles on top of the peridotite massif, confirming the
general trend already suggested from the eastern domain. These Raman
temperatures reach values around 400–450∘C in the metasediments
resting over the Paleozoic crustal slice of Job Valley, and they finally
decrease to values around 300–350∘C in the regional background. As
suggested by Golberg and Leyreloup (1990), this correlation is the result of
the very strong thermal gradient in the locus of the extreme crustal
thinning.
At the Pyrenean scale, the original distribution of the thinning-related
thermal gradient has not been preserved as well as in the Arguenos–Moncaup
area since it has been disrupted by tectonic and sedimentary processes in
many places. However, there exist additional remnants of initial
relationships, with temperature increasing when approaching the exhumed mantle
rocks in some locations (e.g., Saraillé, Roquiague, Montaut, Salvezines).
Pre- and post-metamorphic disruption of the sedimentary pileTriassic to Albian metasediments
An important, well-observed feature of the eastern domain is that the
intensity of the peak metamorphism is not correlated with the stratigraphic
age. High temperatures are found in Triassic or Albian sediments as well.
Also, in the Aulus basin (Fig. 8) peak temperatures of 420 to 600∘C have been measured in Neocomian material as well as in Jurassic or Triassic
material; e.g., one of the coolest temperatures of the Aulus basins has been
obtained in the Liassic fossiliferous marbles of the Col Dret. This apparent
chaotic disposition of the isotherms within the Triassic to Albian
sedimentary pile can be explained by a combination of several mechanisms:
It can be the consequence of the propagation of the thermal anomaly by
magmas or fluid circulations, as already proposed by Dauteuil and Ricou (1989).
Fluids circulating through the sedimentary pile could be responsible for their
hydraulic brecciation. We did not observe any correlation between the intensity of
peak metamorphism and the position of the breccias. For example, the Neocomian material
exposed in the western part of the Aulus basin is nearly devoid of breccias, but it
displays peak temperatures of 500–600∘C. However, other evidence for fluid
circulations may be inconspicuous (cf. Sect. 5).
It can result from the tectonic dismembering affecting the sedimentary
pile before and/or during the peak of metamorphism. This phenomenon is observed
and described further east, e.g., on the southern flank of the Mouthoumet massif
and on the northern flank of the Agly massif, where sedimentary formations of
various ages, from the Triassic to the Aptian, are intensely truncated and/or
scalped (Durand-Delga, 1964; Bessière et al., 1989; Clerc and Lagabrielle, 2014).
Tectonic contacts sealed by mid-Cenomanian deposits are commonly observed between
Liassic and Paleozoic, Jurassic and Paleozoic, Middle or Upper Jurassic and Triassic,
Aptian and Liassic or Triassic, and Cretaceous and Paleozoic. No stratigraphic
repetition of any member of the sedimentary series has been observed, and such a
disposition can be interpreted as the result of a pre-mid-Cenomanian extensional
tectonics described in the literature as the pre-Cenomanian phase (Casteras, 1933;
Mattauer and Proust, 1965; Durand-Delga, 1965).
Finally, later Alpine compressive inversion of the NPZ may also be
responsible for the juxtaposition of metamorphic domains of various grades.
Cenomanian and younger metasediments
By contrast, the mid-Cenomanian, Turonian and younger metasediments always
display metamorphic record of lower grade with respect to the metasediments
on which they lie (up to 350∘C in the Lherz and Vicdessos area;
Fig. 8). This observation indicates that their deposition was contemporaneous
to or followed the extensional tectonics described above. The Cenomanian and
later flyschs probably acted as a blanket on the basins, facilitating the
temperature increase.
Estimating the importance of fluid circulations
The HT-LP Cretaceous metamorphism was first considered as isochemical by
Ravier (1959) and Ravier and Thiebaut (1982). Evidence of fluid circulations
during the metamorphism was later presented by Bernus-Maury (1984). For
Bernus-Maury (1984), Golberg (1987) and Dauteuil et al. (1987), CO2- and
H2O-rich fluids released by the decarbonation reaction of siliceous
dolomitic limestones are responsible for local brecciation of the Mesozoic
metasediments. For Minnigh et al. (1980) massive quenching of the peridotites
and decarbonation reaction would have generated most of the carbonate- and
peridotite-bearing breccias. However, this hypothesis does not explain (i) the
sedimentary fabrics observed in some of these breccias and (ii) the
syn-metamorphic foliation observed in the Mesozoic clasts of these breccias
(Choukroune, 1980; Lagabrielle and Bodinier, 2008; Clerc et al., 2012, 2013).
Top: Field indications of fluid circulations in the Urgonian marbles near
Estagel (Bas-Agly syncline). Centimeter-long scapolite ghosts are found in the immediate
vicinity of fractures (a) or interbeds (b). Note that the former scapolite crystals show
no preferential orientation, whereas the marble presents a clear ductile foliation responsible
for the stretching and flattening of fossils. In this case, the fluid circulation hence postdates the main hot deformation event.
Bottom: Percolation (c) and segregation (d) of fluids in the Triassic meta-evaporites of the Col
d'Agnes (Aulus basin). Such meta-evaporitic material is a potential source of Na- and Cl-rich fluids responsible for the precipitation of scapolites.
Most of the scapolite occurrences are limited to the Triassic and Liassic
units, which are known to be meta-evaporitic sequences (Ravier and Thiebaut
1982). Furthermore these minerals often display a chemical zonation resulting
from the interaction with Na- and Cl-rich fluids, suggesting small-scale
fluid circulation within the sedimentary pile (Golberg and Leyreloup, 1990).
Evidence of small-scale intra-basinal fluid circulation is proposed in Fig. 14. In the NPZ, scapolite is commonly observed in various marbles where it is
localized along discontinuities such as fractures (Fig. 14) or stratigraphic
joints. At the Col d'Agnes (Lherz Area), lagoonal deposits of Hettangian age
(Ravier and Thiebaut 1982) show a dotty aspect with numerous millimeter-size
rounded pockets filled by whitish carbonates (Fig. 14c). These pockets
locally interconnect and gather into veinlets and veins (Fig. 14d). We
interpret this feature as evidence of percolation, segregation and migration
of fluids (Fig. 14c and d). Such fluids originating from Triassic and
Liassic lagoonal deposits are good candidates to explain the allochemical
scapolites observed higher up in the sedimentary pile (Fig. 14a and b).
According to Dauteuil and Ricou (1989), the high thermal gradient
responsible for the HT-LP metamorphism (> 600∘C – 2 to
4 kbar) cannot be reached solely by thermal conduction. Instead, enhancement
of the heat propagation by the fluid circulations would be efficient enough
to reach gradients of more than 100∘C km-1. Similar gradients are
reported in the Salton Sea (Muffler and White, 1969; McDowell and Elders,
1980; McDowell and Elders, 1983), where Younker et al. (1982) identified
small-scale convection cells beneath an impermeable cap rock.
Thermal conductivity of calcite, halite and pure water at 0, 50, 100, 200, 300 and 400 ∘C, after Clark (1966) and Clauser and Huenges (1995).
Due to their high thermal conductivity, evaporitic rocks such as anhydrite,
halite or sylvite are very efficient at transferring heat to the surrounding
layers. This phenomenon, referred to as the “chimney effect” in salt diapirs
(Noack et al., 2010; Kaiser et al., 2011), can be responsible for positive
anomalies of a few degrees in the covering and adjacent layers, and negative
anomalies of a few tens of degrees in the subjacent layers (e.g., Yu et al.,
1992). The temperature effect of the salt is therefore well below the
temperature differences recorded in the IMZ. In the North Pyrenean realm, the
abundant Triassic evaporites may hence have played a minor role in the
propagation of the Cretaceous thermal anomaly. Moreover, the contrast of
conductivity between limestone and evaporitic salts strongly diminishes with
increasing temperatures (Table 3); e.g., the thermal conductivities of halite
and calcite become equivalent around 400∘C. The pumping-up effect
of the evaporites is hence probably negligible in the hottest parts of the
eastern NPZ. But in the cooler central and western domain, it may explain the
focalization of the thermal anomalies around the Saraillé and Roquiague
areas that are considered as former diapiric structures (Canérot 1989;
Canérot and James, 1999). Moreover, facilitation of heat transfer by the
evaporitic layers may also be expressed in a horizontal direction, which
could explain the strong contrast of temperature observed between the
Triassic evaporites and the overlying series in the Arnave, Arignac,
Bonrepaux, Betchat, Salies du Salat, Gotein and Caresse-Salies du Béarn
areas (Thiébaut et al., 1988, 1992). A similar process
may explain the higher temperatures obtained on the Mesozoic cover (up to
494∘C in the Triassic of the Agly Paleozoic massif) than in the
massif itself (351∘C in the Silurian).
Schematic representation of the evolution of the Pyrenenean
Cretaceous metamorphism and magmatism in response to crustal extension. Top:
early hydrothermal-dominated phase affecting the crystalline basement. Down:
Phase of HT-LP metamorphism, occurring mainly in the pre- and syn-rift
sedimentary cover, in response to the attenuation of the continental crust.
By contrast, claystone, shales and organic-matter-rich sediments are
characterized by remarkably low thermal conductivities in the range of
0.2–1.0 W m-1/∘C, lower by a factor of 2 or more than other common
sedimentary rocks (Blackwell and Steele, 1989). These rocks are known to act
as a thermal blanket retaining heat within the underlying rocks (Blackwell
and Steele, 1989; Pollack and Cercone, 1994; Nunn and Lin, 2002). We hence
propose that the organic-matter-rich black shales of the Albian–Cenomanian
black flysch (Souquet et al., 1985; Debroas, 1990) may have participated in
the strong thermal anomaly registered within the Albian–Cenomanian black
flysch itself and in the underlying Mesozoic series. In addition, its
relative impermeability due to high clay content must have consistently
limited the penetration of basinal and meteoric water in the system, hence
annihilating convective cooling (Boulvais et al., 2015). In contrast with the
starved paleotethys margin (Manatschal and Bernoulli, 1999; Manatschal, 2004;
Masini et al., 2012) and Iberian margin (Shipboard Scientific Party, 1987;
Manatschal and Bernoulli, 1999; Soares et al., 2012), the Pyrenean
paleomargin seems to have developed in a sediment-rich environment,
favorable to a marked blanketing effect.
Timing and relationship between metamorphism and magmatism
The ages obtained on our samples confirm the contemporaneity of the
Cretaceous magmatism and metamorphism in the NPZ. This is well demonstrated
in the Lherz area where, with the exception of one sample, metamorphic
muscovite and amphiboles provide ages similar to the amphibole age of an
undeformed gabbro at 94.7 ± 1.3 Ma. The age of about 100 Ma reported
for a single muscovite grain from this area suggests that the thermal anomaly
started to develop earlier, which is in accordance with the
40Ar–39Ar data reported further east in the Arignac and Trimouns
places (100–109 Ma) as well as with the 40K–40Ar age of Les
Plagneaux gabbros (107–109 Ma; Montigny et al., 1986) and the U–Pb age of
several metasomatic albitite bodies in the eastern Pyrenees (117–98 Ma;
Fallourd et al., 2014, and references therein). In the Moncaup area, the
magmatic, metamorphic and hydrothermal activity is restricted to a shorter
period as indicated by 40Ar–39Ar and U–Pb ages close to 100 Ma.
Alteration and recrystallization of some of the NPZ Cretaceous alkaline
magmatic rocks by the HT-LP metamorphism indicate that magmatism cannot
account for the regional thermal anomaly (Azambre, 1967; Azambre et al., 1971, 1992; Azambre and Rossy, 1976; Ternet et al., 1997; Azambre
and Monchoux, 1998). Consistently, the present-day distribution of the
Cretaceous magmatic rocks is clearly not correlated to the distribution of
the isograds. For example, magmatism is abundant in the cold Mauléon
basin whereas it is scarce in the Lherz or Boucheville areas.
Hot versus cold margins?
Direct access to the present-day passive margin is limited by thick
sedimentary deposits, and information about the thermal history of the margin
is scarcely gathered. The use of fossil margins exposed in mountain belts
offers a unique opportunity to study the metamorphic imprint of the
extension. However, when not overprinted by the subduction metamorphism, the
Alpine analog indicates only low-grade metamorphism. At present, very few
examples of hot passive margin presenting evidence of exhumed
subcontinental mantle or deep crust have been reported. In the Zagros
Mountains, mapping reveals that pre-rift cover and mantle were superposed early
in the Kermanshah ophiolite (Wrobel-Daveau et al., 2010), where
high temperatures are recorded in the Mesozoic sediments along their contact
with the peridotites (Hall, 1980). In the Zagros of Iraq, Jassim et al. (1982)
described a similar metamorphism affecting sediments close to exhumed
ultramafic rocks with temperatures up to 750∘C over 2.5 km
thickness. In the light of our results, we propose a distinction between
“cold” Iberian or Alpine-type passive margins and “hot” Pyrenean –type
margins. The cause of this thermal variability along passive margins is
still unclear. I could be explained by several factors such as the kinematic
context (transtension versus extension), the mantle dynamics (hot versus
cold mantle), the sedimentary input or the extension rate.
Conclusions
In this work, we measured more than 100 RSCM peak temperatures
relevant to the HT-LP metamorphism in the North Pyrenean Zone, and we report
19 new 40Ar–39Ar and ages from metamorphic and magmatic samples.
Our results are in full agreement with previous data and confirm the
first-order link between the metamorphism and the Cretaceous crustal
thinning. The primary link between mantle exhumation and thermal anomaly is
particularly well illustrated by our new data set in the Arguenos–Moncaup
area.
At the scale of the whole NPZ, we observe a clear increase of the
temperatures from west to east that could be explained by a combination of
several scenarios: (i) the whole domain underwent the same HT-LP metamorphism,
but later tectonic inversion led to underthrusting and burial of the
highest-grade rocks in the western part of the NPZ. (ii) The thermal zonation
may be a consequence of the lateral variation of the kinematics along the
NPZ, implying a dominant N–S extension in the west and a NW–SE transtension
in the east, leading to enhanced thermal gradients with variable hydrothermal
circulation patterns. (iii) The intensity of the HT metamorphism seems
correlated to the rising velocity of the exhumed peridotites. (iv) The lateral
variability of the thermal anomaly along the NPZ may be related to the
scissors-shape opening of the extended domain during the Cretaceous.
Isotherms may have been more spaced in the wider western NPZ than in the
narrower eastern NPZ.
Different parameters may have played a role in the propagation of the
metamorphism. We suggest that the circulation of fluids originating from the
basin rocks themselves may have relayed the thermal anomaly across the
sedimentary column (Fig. 14). In addition, it is noteworthy that the pre-
and syn-rift material of the NPZ is generally limited at its base by the
highly conductive evaporites of the Triassic and at its top by the thick and
insulating organic-rich Albian–Cenomanian black flysch. This salt effect and
thermal blanket effect of the organic-rich black flysch probably facilitated
the elevation of the temperatures.
Fig. 15 proposes a conceptual model in which the continental crust is
first weakened, then thinned and altered by hydrothermal circulations during the
upper Aptian–Albian. This first phase led to the extension and thinning
of the continental crust where the North Pyrenean massifs constitute crustal
boudins. During the Cenomanian to Coniacian, the hyper-thinned domains
opened on each side of these boudins concentrated most of the thermal
anomaly in relation to the development of thick sedimentary basins.
The presence of a long-lasting HT-LP metamorphic event along the Pyrenean
paleomargins has to be taken into consideration when analyzing the
thermo-structural history of passive margins. Metamorphic events of this
kind has never yet been described along the fossil margins exposed within the Alpine
or Appalachian–Caledonian orogens. This bears important consequences
regarding (i) the variability of the thermal regime that we may expect at
the foot of worldwide passive margins and (ii) the variability of structural
style resulting from this changing thermicity.
The Supplement related to this article is available online at doi:10.5194/se-6-643-2015-supplement.
C. Clerc conducted and interpreted the structural
and a part of the thermometric data and wrote most of the paper; A. Lahfid brought
his expertise in RSCM and conducted part of the analysis; P. Monié conducted the
40Ar–39Ar dating; Y. Lagabrielle, C. Clerc and J. C. Ringenbach directed C. Clerc's PhD and as
such proposed the extensive use of the RSCM on the NPZ. M. Poujol conducted U–Th
dating on titanite from Arguenos–Moncaup albitites; P. Boulvais contributed ideas on
the crustal distribution of the HT-LP metamorphism. E. Masini participated in a
sampling campaign in the western NPZ. M. de St Blanquat furnished some of the ductile
lineation measurements from the eastern NPZ. All authors contributed
intellectually to the paper.
Acknowledgements
This work was made possible thanks to the CNRS and TOTAL
S.A. through a joint PhD grant to C. Clerc. We are grateful to
B. Goffé, who encouraged the application of the Raman thermometry to the NPZ.
Edited by: D. J. J. van Hinsbergen
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