Implications for the Permian post-Variscan extension

In the Permian post-orogenic magmatic system, numerous authors have proposed a genetic link between lower crustal mafic magmas (e.g. Ivrea-Verbano mafic complex) and upper crustal granitoids (e.g. Serie dei Laghi, Schaltegger & Brack, 2007; Sinigoi et al., 2011). The main arguments are the comparable ages (Schaltegger & Brack, 2007), geochemical affinities (see e.g. Boriani & Giobbi, 2004) and close field associations (e.g. Hansmann et al., 2001;

Müntener et al., 2000). In this study, we suggest that the Sondalo gabbro may represent a feeding system, transferring magma through the middle crust and therefore providing a link between magmatic systems in the lower and upper crust.

Evidence of how magmas are extracted from lower crustal bodies to reach shallower levels are documented in the Ivrea-Verbano zone (e.g. Handy & Streit, 1999; Schaltegger &

Brack, 2007). The mafic complex is unfortunately truncated at its top by the Permian Cossato-Mergozzo-Brissago line/shear zone (CMB; Snoke et al., 1999). Nevertheless, the CMB is often

“decorated” by syn-kinematic intrusions of mafic melts (the so-called appinites; Boriani et al., 1990) supposed to be extracted from the underlying mafic body (Handy & Streit, 1999; Mulch et al., 2002). Therefore, mafic melts may be extracted from lower crustal magmatic systems along shear zones (Fig. IV-20). Such structures may trigger the movement of mafic magma from high hydrostatic pressure areas to lower pressure ones, i.e. to mid-crustal levels. This suggests a strong link between tectonics and mafic magma ascent, and suggest that if no shear zone is developed, mafic magma will remain trapped in the lower crust, as modelled by Gerya & Burg (2007). The Sondalo gabbro may represent the continuation of the ascent of mafic magma in the

0

10

20

30

depth (km)

Diorite Granitoid

Gabbro Foliation Magma flow Sed. basin Upper crust Middle crust Lower crust Subcont. mantle

Fig. IV-20: Synthetic model of ascent of magma through the crust. The magma is extracted from lower-crustal mafic bodies thanks to lower-crustal-scale shear zones. Modified after Handy & Streit (1999) and Rutter et al. (1993). See text for details.

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Chapitre IV : Mise en place d’un gabbro en croûte moyenne

continental crust linked to major shear zones although no evidence for such a structure has been documented in our study area. Eventually, mafic magma may evolve and differentiate before reaching upper crustal levels, where it eventually forms felsic intrusions.

8. c

The intrusion of the Permian Sondalo Gabbro occurred in two stages, between 289 and 285 Ma. During the first stage, magma flowed upward along the sub-vertical foliation of the metasedimentary Campo unit, without apparent deformation. Synchronously, a migmatitic aureole was developed, while granulitic xenoliths were left behind by the magma progression.

The second stage is characterized the en-masse rise of the core of the pluton, during which xenoliths equilibrated at 18 km were juxtaposed with the host-rock that remained at 12 km.

The exhumation of the core occurred by shearing in the migmatitic contact aureole and in the border zone of the pluton, transposing the pre-existing magmatic and metamorphic foliations.

Apparently, the intrusion is not directly associated to a shear zone or any other large-scale deformation structure. Therefore, the mechanical reasons for the magma ascent remain unclear.

a

R. van Elsas and A. Aubert are thanked for the help with mineral separation, and P.

Vonlanthen and G. Morvan for the BSE and CL imaging, A. Ulianov for the help with the LA-ICPMS, and A. Longeau for the 3axes-IRM measurements.

142 Formation et exhumation des granulites permiennes

a

a: g

:

Zircons were separated from samples after crushing, sieving (50–250 µm) and cleaned ultrasonically. Zircons were carefully selected by handpicking under a binocular microscope and mounted in epoxy after concentration of zircon using a gold pan, hand magnet and Frantz magnetic separator. U–Pb isotopic measurements were performed at the Institute of Mineralogy and Geochemistry of the University of Lausanne using a ThermoFischer Element XR ICP–MS coupled to a NewWave UP-193 ArF excimer laser ablation system. Analytical conditions are identical to those described by Ulianov et al. (2012). Zircon grains were ablated with a 25–35 µm spot size at 2.2–2.5 J.cm^-2 with a 5 Hz repetition rate. Isotopic ratios were normalized after each ten spot analyses by the GEMOC GJ-1 primary standard (CA-ID-TIMS ²⁰⁶Pb-²³⁸U age of 600.5 ± 0.4 Ma; Schaltegger et al., unpubl. In: Boekhout et al., 2012) and with the 91500 secondary standard (1065.4±0.3 Ma, Wiedenbeck et al., 1995). Ages were calculated using LAMTRACE (Jackson, 2008). Additional data reduction methods can be found in Ulianov et al. (2012).

Zircon trace element analyses were also carried out at the University of Lausanne on an Element XR ICP–MS with a NewWave laser ablation system and normalized to the SRM 612 glass reference material (Pearce et al., 1997) after each ten analyses. Apart from a spot size of 35 µm, analytical conditions are identical to those used for U–Pb dating. Data were regressed using SILLS (Guillong et al., 2008).

a

b: a

:

At least 2 cores per site were collected, with a minimum of 10 standard specimens (23 mm in diameter and 22 mm in height) and oriented with a magnetic compass. AMS was characterized for all samples whereas magnetic mineralogy was determined for one specimen per site following the methods described below.

AMS was measured with a MFK1–A Kappabridge (AGICO, Inc operating in low field at the University of Strasbourg. Data were regressed using Anisoft 4.2 (AGICO, Inc.). To infer the magnetic susceptibility tensor (K), the specimen is slowly rotated in a coil with an applied field of 80 A.m^-1 and magnetic susceptibility is measured in 48 directions. This operation is repeated for three mutually perpendicular directions. This procedure allows a very precise determination of magnetic susceptibility tensor K by least square inversion (Jelinek, 1977). The susceptibility tensor can be decomposed into three eigenvalues (principal susceptibilities, K₁ ≥ K₂ ≥ K₃) and three associated orthogonal eigenvectors. While the eigenvectors are represented on a stereogram (equal area lower hemisphere projection), eigenvalues are usually combined to

143

Chapitre IV : Mise en place d’un gabbro en croûte moyenne

infer the shape of the ellipsoid. We use the shape parameter T defined as:

and the anisotropy parameter Pj defined as:

The shape parameter is interpreted as reflecting the shape of the ellipsoid: linear (-1<T<-0.5), linear to linear (-0.5<T<-0.2), linear (-0.2<T<0.2), planar to plano-linear (0.2<T<0.5) or planar (0.5<T<1). The Pj parameter increases from 1 (isotropic) up to values depending on the magnetic constituents of the rocks. A third useful parameter is the mean susceptibility K_m=(K₁+K₂+K₃)/3, which generally relates to the magnetic phases of the rock (Rochette et al., 1992).

Two ways of combining these parameters are commonly used in the AMS studies. The first one links Pj vs K_m and gives information on the influence of the magnetic mineralogy on the measured anisotropy (Fig. IV-13b). The second, which links T vs Pj (Fig. IV-13f), shows the relationship between the measured anisotropy and the shape of the ellipsoids (see e.g.

Borradaille, 1988).

The investigation of coercivity-unblocking temperature spectra of ferromagnetic minerals was obtained by applying decreasing isothermal remanant magnetization saturation in three perpendicular directions (so-called 3axes–IRM) on a standard cylindrical sample, which was then stepwise thermally demagnetized in a paleomagnetic furnace, free of external magnetic fields. The isothermal remanant magnetization (IRM) in each direction is provided by an ASC impulse magnetizer (ASC Model IM-10) enabling a high field up to 1.1 T. Stepwise demagnetization was obtained in a home-made oven with a residual field of 100 nT. At each step, the magnetization was measured with a JR6-A spinner magnetometer (Agico, Inc.). The three saturation magnetizations are determined by both technical considerations and rock magnetic properties. The first applied magnetic field (associated to M(z)) corresponds to the maximum 1.1 T provided by the device and is supposed to magnetize the high coercivity minerals. This is the high coercivity axis. The second field of 0.5 T (associated to M(y)), applied perpendicular to the first one, magnetizes the intermediate coercivity fraction softer than 0.5 T and is called the medium coercivity axis. The last field of 0.1 T (associated to M(x)) applied normal to the two first directions saturates the low coercivity fraction and is called the low coercivity axis. A sharp

𝑇𝑇 =ln(𝐾𝐾₂⁄𝐾𝐾₃)−ln(𝐾𝐾₁⁄𝐾𝐾₂) ln(𝐾𝐾2⁄𝐾𝐾3) + ln(𝐾𝐾1⁄𝐾𝐾2)

𝑃𝑃𝑃𝑃= exp�2��ln�𝐾𝐾1

𝐾𝐾_𝑚𝑚��²+�ln�𝐾𝐾2

𝐾𝐾_𝑚𝑚��²+�ln�𝐾𝐾3

𝐾𝐾_𝑚𝑚��²�

(Eq. IV-1)

(Eq. IV-2)

144 Formation et exhumation des granulites permiennes

decrease in the magnetization of a rock indicates the unblocking temperature of a phase, which is slightly lower than its Curie temperature (Lowrie, 1990).