Possible reasons for excess 40 Ar

40Ar/³⁹Ar dating relies on the assumption that the system was initially open above the

“closure temperature”, where argon is expected to leave the host mineral in an infinite reservoir.

This should occur for long term stagnation in the middle and lower continental crust at normal temperature and pressure (e.g. Warren et al., 2012). We suggest that some of our samples present excess-Ar based on: (1) amphibole sampled in and around the Permian Sondalo gabbro have older ⁴⁰Ar/³⁹Ar ages than the age of intrusion of the pluton determined by U–Pb on zircon and Sm–Nd ages (Bachmann & Grauert, 1981; Tribuzio et al., 1999), and (2) few mica ages are inconsistent with respect to the other dated samples coming from the same unit.

We suggest that excess-Ar was mobilized due to the heat introduced into the unit by

176 Formation et exhumation des granulites permiennes

the mantle-derived mafic intrusion that disturbed the ⁴⁰Ar/³⁹Ar ratio for amphibole and micas.

As the mafic magma was generated in the mantle, it has a low K-content and therefore a low radiogenic Ar-production. Therefore, it has a ⁴⁰Ar/³⁶Ar ratio of the mantle (Sarda et al., 1985;

Staudacher et al., 1989; Trieloff et al., 1997). The high ⁴⁰Ar/³⁶Ar of the magma is the first possible source of excess-Ar in the pluton. The second possible source may be related to the intrusion of the Sondalo gabbro in a K-rich environment. Although Ar is expected to leave the mineral lattice and diffuse out of the mineral, the higher environmental Ar inhibited this process and eventually triggered back diffusion from the external reservoir into the mineral.

This process can even affect the country rock few meters around the intrusion (Hyodo & York, 1993). As the analyzed disturbed minerals are up to 5 km from the intrusion, it questions the extent of the gabbro below the nowadays outcropping surface, or the ability of a postulated excess ⁴⁰Ar “wave” to affect the host.

The amphiboles were significantly more affected by excess-Ar than the micas, probably due to the more sensitive character of amphibole due to a low ⁴⁰K content and therefore a lower internal radiogenic ⁴⁰Ar concentration (Kelley, 2002). Otherwise micas are likely affected by

310 290 270 250 230 210 190 310 290 270 250 230 210 190

310 290 270 250 230 210 190

0 0.5 1.0

Cumulative probability

0 0.5 1.0

Cumulative probability

U-Pb on Zrn (7) Sm-Nd (2)

Rb-Sr on Bt (8) Rb-Sr on Ms (7)

Ar-Ar on Bt (2) Ar-Ar on Ms (7) Ar-Ar on Ms (3)

Grosi.Campo Ar-Ar on Bt (7)

Age (Ma) Age (Ma)

Age (Ma)

(a) (b)

(c)

Fig. V-11: Cumulative probability histogram of (a) crystallization (Sm-Nd and U-Pb) ages of the Sondalo gabbro, (b) Rb–Sr ages on muscovite and biotite from the Campo unit and (c) ⁴⁰Ar/³⁹Ar and K–Ar ages on muscovite and biotite from the Grosina and the Campo units from both bibliography and present work. The complete dataset is provided in Table V-4.

177

Chapitre V : Exhumation et refroidissement pendant le rifting

extraneous argon incorporation due to the higher partition coefficient between extraneous fluids and micas (Kelley, 2002).

Biotite age from the sample BPA 034-11c is ca. 40 Ma older than other dated biotites from the Campo basement. Similarly, the muscovite age from the sample BPA 003-11b is ca. 30 Ma older than other dated muscovites. The older “sub-plateau” age presented in Figures V-7b and V-8c may be due to extraneous ⁴⁰Ar incorporation homogeneously distributed in the biotite lattice (Foland, 1983; McDougall & Harrison, 1999) eventually brought by fluids (Kelley,

270 250 230

190 210

170

150-1.0 -0.5 0 0.5 1.0 1.5 Distance

to ESZ (km)

Age (Ma) Eita Shear Zone

Grosina

unit Campo

unit

Ar-Ar on Ms Ar-Ar on Hbl Ar-Ar on Bt K-Ar on Ms K-Ar on Bt

Fig. V-12: Time versus distance diagram representing the spatial distribution of the ages with respect to the sub-horizontal Eita shear zone. Errors are represented at a 2σ level of confidence.

2002). Nevertheless, a possible alternative explanation may be brought by the temperature-time evolution of the unit, as discussed in a further section with diffusion modelling.

4.2 Exhumation and cooling history of the Campo and Grosina basements These new ages, together with a compilation of published U–Pb, Sm–Nd, Rb–Sr, K–

Ar and ⁴⁰Ar/³⁹Ar ages performed on both the Grosina and Campo basements (see Fig. V-11 and Table V-4), constitute a consistent dataset to estimate the respective cooling history using the notion of rapid or slow cooling. Commonly, the cooling history is inferred from the difference in age of two chronometers with different temperature-dependent closure of the isotopic system.

Overlapping or comparable ages record a rapid cooling whereas disparate ages may attest of a slow cooling. In addition, this history may be estimated by calculating the cooling rate, but has to be used with caution, as closure temperatures may be sensitive to other parameters such as the cooling rate itself, the mineral chemistry, the mineral size… In this study, geochronological data are linked to the closure temperature of the probed chronometer/mineral system (Fig. V-13) using for ⁴⁰Ar/³⁹Ar: 500°C for hornblende (Harrison, 1982), 400°C for muscovite (Harrison et

178 Formation et exhumation des granulites permiennes

Lat Lon Are Err.

(°N) (°E) Local. prec. Reference Sample Lithology Unit Area Method Mineral (Ma) (2σ)

46.347 10.349 Mid Tribuzio et al., 1999 SO5/1 Troctolite Campo Sondalo Gabbro Sm-Nd WR-Min 300 12 46.348 10.349 Mid Tribuzio et al., 1999 SO5/11 Norite Campo Sondalo Gabbro Sm-Nd WR-Amp-Pl 280 10 46.322 10.353 Mid Bachmann & Grauert, 1981 ? Diorite Campo Sondalo gabbro U-Pb Zrn 270

46.396 10.342 High This study M28B73 Diorite Campo Vendrello U-Pb Zrn 289 4

46.360 10.311 High This study GM 601 Diorite Campo Above Alto U-Pb Zrn 285 2

46.345 10.325 High This study M1B14 Diorite Campo Above Sondalo U-Pb Zrn 285 6

46.360 10.331 High This study BPA 003-12d Migmatite Campo Alto U-Pb Zrn 288 5

46.360 10.331 High This study BPA 003-12c Migmatite Campo Alto U-Pb Zrn 289 4

46.394 10.343 High This study BPA 109-12c Migmatite Campo Vendrello U-Pb Zrn 277 3

46.347 10.350 Mid Tribuzio et al., 1999 SO5/1 Troctolite Campo Sondalo Gabbro Rb-Sr Pl-Amp 266 10 46.348 10.350 Mid Tribuzio et al., 1999 SO5/11 Norite Campo Sondalo Gabbro Rb-Sr Pl-Amp 269 16 46.430 10.170 Low Del Moro & Notarpietro, 1987 PB2 Granitoids Campo Pizzo Bianco Rb-Sr Ms 277 10 46.311 10.288 Low Del Moro & Notarpietro, 1987 VR1 Granitoids Campo Vernuga Rb-Sr Ms 269 8 46.328 10.336 Mid Bachmann & Grauert, 1981 ? Ms-pegmatite Campo Sant'Agnese Rb-Sr Ms 257 46.322 10.352 Mid Bachmann & Grauert, 1981 ? Ms-pegmatite Campo Mondadizza Rb-Sr Ms 251

46.343 10.271 Low Hanson et al., 1966 10-M Ms-pegmatite Campo Val Grosina Rb-Sr Ms 252 15

46.362 10.249 Mid Thöni, 1981 T 946 Pegmatite Campo Val Grosina Rb-Sr Ms 329 15

46.430 10.170 Low Del Moro & Notarpietro, 1987 PB2 Granitoids Campo Pizzo Bianco Rb-Sr Bt 263 8 46.275 10.164 Low Del Moro & Notarpietro, 1987 VF6 Granitoids Campo Val Ferrata Rb-Sr Bt 220 6

46.264 10.157 Mid Del Moro et al., 1981 VA 2 Granitoids Campo Val Ferrata Rb-Sr Bt 224 8

46.414 10.149 Low Del Moro & Notarpietro,1987 V6 Granitoids Campo Val Viola Rb-Sr Bt 205 6 46.414 10.149 Low Del Moro & Notarpietro, 1987 V12 Granitoids Campo Val Viola Rb-Sr Bt 233 8 46.311 10.288 Low Del Moro & Notarpietro, 1987 VR1 Granitoids Campo Vernuga Rb-Sr Bt 127 4

46.314 10.288 Mid Del Moro et al., 1981 VA 79-4 Granitoids Campo Vernuga Rb-Sr Bt 107 4

46.362 10.249 Mid Thöni, 1981 T 946 Pegmatite Campo Val Grosina Rb-Sr Bt 97 4

46.350 10.173 High This study GM264 Micaschist Grosina Above Val de Sach Ar-Ar Ms 261 2

46.388 10.254 High Mohn et al., 2012 Sample 208 Chl-Orthogneiss Grosina ESZ (100m from) Ar-Ar Ms 260 2 46.387 10.253 High Mohn et al., 2012 Sample 257 Chl-Orthogneiss Grosina ESZ(1m from) Ar-Ar Ms 273 2

46.396 10.230 High This study GM119 Bt-orthogneiss Grosina Bt-zone Ar-Ar Bt 246 13

46.350 10.173 High This study GM264 Micaschist Grosina Above Val de Sach Ar-Ar Bt 245 2

210 4 177 13

46.332 10.311 High This study BPA28-11c Micaschist Campo Sommacologna Ar-Ar Ms 178 5

46.359 10.250 High This study GM260 Micaschist Campo Eita valley Ar-Ar Ms 182 2

46.363 10.325 High Mohn et al., 2012 Sample 17 Granitoid dyke Campo ESZ (1m from) Ar-Ar Ms 203 1 46.383 10.271 High Mohn et al., 2012 Sample 233 Deformed pegmatite Campo ESZ Ar-Ar Ms 209 2

46.343 10.271 Low Hanson et al.,1966 10-M Ms-pegmatite Campo Val Grosina K-Ar Ms 217 11

46.362 10.249 Mid Thöni, 1981 T 946 Pegmatite Campo Val Grosina K-Ar Ms 182 8

46.353 10.251 Mid Thöni, 1981 T 945 Micaschist Campo Val Grosina K-Ar Ms 187 8

46.332 10.311 High This study BPA28-11c Micaschist Campo Sommacologna Ar-Ar Bt 174 2

46.335 10.318 High This study BPA31-11a Diorite Campo Sondalo Ar-Ar Bt 173 2

46.359 10.250 High This study GM260 Micaschist Campo Eita valley Ar-Ar Bt 171 1

46.363 10.325 High Mohn et al., 2012 Sample 32 Banded migmatite Campo Migmatitic aureole Ar-Ar Bt 185 1 46.358 10.331 High Mohn et al., 2012 Sample 6 Leucosome Campo Migmatitic aureole Ar-Ar Bt 189 1

46.362 10.249 Mid Thöni, 1981 T 946 Pegmatite Campo Val Grosina K-Ar Bt 164 7

46.353 10.251 Mid Thöni, 1981 T 945 Micaschist Campo Val Grosina K-Ar Bt 122 6

46.335 10.318 High This study BPA31-11a Diorite Campo Sondalo Ar-Ar Hbl

Grosina unit, Ar-Ar ages Campo unit, U-Pb and Sm-Nd ages

Campo unit, Ar-Ar and K-Ar ages Campo unit, Rb-Sr ages

Table V-4: Compilation of pre-Alpine ages from the Campo/Grosina area (N-limb of the Mortirolo anticline, completed after Meier, 2003).

179

Chapitre V : Exhumation et refroidissement pendant le rifting

al., 2009), 300°C for biotite (Harrison et al., 1985); for Rb–Sr: 500°C for muscovite (Jäger et al., 1967), 450°C for amphibole and 300°C for biotite (Dodson, 1973; Jäger et al., 1967). U–Pb and Sm–Nd ages are expected to record the crystallization of the rock. We consider here that these ages record the cooling of the rock below 800°C being roughly when the crystallization of felsic magmas occurs. Cooling rates were calculated for each chronometer pair per sample in order to rule out potential spatial variations of the geothermal gradient (e.g. Mancktelow &

0 100 200 300 400 500 600 700

U-Pb Zrn

Ar-Ar Hbl U-Pb Ttn U-Pb Mnz

U-Pb Aln

Rb-Sr Ms

Rb-Sr Bt FT Ttn

Ar-Ar Bt Ar-Ar Ms

FT Zrn Ar-Ar Kfs

FT Ap (U-Th)/He

T(°C)

Age

Fig. V-13: Synthetic T–t path presenting the closure temperature of different thermo-chronometers (modified after Spear, 1993).

Grasemann, 1997). Results are presented in Figure V-14b. Two distinct cooling “events” can be identified from the data presented in Figures V-11 and V-14: a Permio-Triassic cooling, and a subsequent Jurassic cooling.