Only cooling or re-heating and cooling?

The estimates of the Grosina unit indicate that this unit was initially at T > 400°C before 260 Ma, as indicated by the youngest ⁴⁰Ar/³⁹Ar ages on muscovite. The youngest

40Ar/³⁹Ar ages on biotite from the Grosina unit indicates that in Triassic times, at the end of the

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Permian cooling stage, the unit was at temperatures lower than 300°C at an unknown depth, and was never subsequently re-open. At the onset of the Jurassic cooling event, around 190–185 Ma, the temperature of the Campo basement was between 400 and 500°C, as indicated by the oldest ⁴⁰Ar/³⁹Ar age of muscovite and eventually by the ⁴⁰Ar/³⁹Ar age of amphibole. The unit was subsequently cooled rapidly to reach temperatures lower than 300°C after ca. 170 Ma, as pointed by the youngest ⁴⁰Ar/³⁹Ar on biotite age. Such a temperature–time path is at odds with the presence of Rb–Sr ages on biotite older than 185 Ma, as their closure temperature should be close to 300°C (Dodson, 1973; Jäger et al., 1967). Moreover with such fast cooling rates,

40Ar/³⁹Ar and K–Ar ages on muscovite and biotite should be encompassed in a narrow time window, not older than 190 Ma for muscovite and older than 180 Ma for biotite.

Therefore, two theoretical scenarios can be envisaged. The first scenario (Fig. V-15ac) involves one single cooling from 500 to 470°C from Triassic (250 Ma) to Jurassic times (185 Ma), not intersected by re-heating periods, and subsequently exhumed at near-surface conditions (50°C, 0.5 kbar) from 185 to 165 Ma, in agreement with the expected fast cooling rates during Jurassic rifting. In such a context, old ages may be due to different closure temperature of minerals due to different grains size, different chemistry, etc. The second scenario (Fig. V-15bd) involves a first Triassic cooling stage continuous to the Permian thermal high, to reach 270°C in the Triassic, before the onset of rifting. Such a temperature is in line with expected temperatures at the onset of rifting by taking a geothermal gradient of 25°C/km suggested by lower crustal environmental conditions at that time (Müntener et al., 2000). Subsequently, the argon system is re-open in Jurassic times from 190 to 185 Ma, followed by a rapid cooling associated to

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(a) (b)

Fig. V-14: (a) Temperature (T) versus time (t) diagram for the different thermo-chronometers for the Grosina and the Campo units. Arrows represent different cooling rate. Error bars are plotted at 2σ level of confidence for ages, and are arbitrarily plotted at ± 50°C to denote the uncertainty in closure temperature determination. (b) Calculated cooling rates for samples having different thermo-chronometers represented in function of the oldest age of the different thermo-thermo-chronometers. Dotted line indicates the poorly constrained cooling-rate range for sample BPA 031-11a (relying on the amphibole age). The complete dataset is provided in Table V-4. See text for details.

182 Formation et exhumation des granulites permiennes

the exhumation at near surface conditions (50°C, 0.5 kbar) around 165 Ma. In such context, old ages should be regarded as mixed ages between Permo-Triassic cooling and Jurassic re-opening/cooling. In order to test both hypotheses, we modelled the diffusion of argon in micas with DiffArgP (Warren et al., 2011), a version of DiffArg (Wheeler, 1996) implemented with the P-dependent diffusion of muscovite (Harrison et al., 2009).

The first model (Fig. V-15e) preserves old ages (> 200 Ma) in the core of big muscovite, whereas small muscovite grains will preserve only ages around 190 Ma, due to the initially slow cooling. This scenario explains the presence of old muscovite ages in the Campo unit as e.g. the 217 ± 11 Ma of Hanson et al. (1966). Conversely, biotite present systematically the same age of 180 Ma with any modelled grain size, and that is not consistent with the 216 ± 2 Ma of the biotite from sample BPA 034-11c. Such pattern is due to the fast cooling between 180 and 165 Ma, allowing closure of the crystal in a narrow time range with respect to the argon system. In this context, the Rb–Sr ages older than ⁴⁰Ar/³⁹Ar ages on biotite implies that the closure temperature of Rb–Sr is higher than the proposed value of 300°C. This model implies also to maintain a high geothermal gradient in Triassic time, inconsistent with the lack of potential thermal support by magmatism in the Austroalpine realm, in contrast to the Southern Alps domain. Moreover, it does not fit with the aggradation of shallow-marine sediments at the surface during Triassic times, suggesting subsidence, i.e. either cooling or thinning of the crust).

The second model (Fig. V-15f) with 400°C as peak temperature allows big biotites to preserve ages older than 200 Ma, whereas muscovite is only mildly affected by diffusion, keeping its initial cooling age close to 250 Ma. Higher peak temperature allows partial resetting of muscovite, already visible with 450°C, but with such conditions, biotites are totally re-open and would not preserve old ages, not even for big grains. This model is supported by the supposed geothermal gradient at the onset of rifting in line with the shallow-marine sedimentation in upper crustal levels. Moreover, the re-heating of crustal units during rifting was already described (e.g. Clerc & Lagabrielle, 2014). This model explains well the existence of ⁴⁰Ar/³⁹Ar on biotite ages older than 200 Ma (e.g. sample BPA 034-11c), but is unable to reconcile the measured muscovite ages.

Neither one of the two end-member models fits perfectly with the measured ages.

Whereas the first model explains the spread in muscovite ages it does not explain the old ages of biotite and the second model is supported by more geological observations but is unable to explain old and young ages for both muscovite and biotite. The actual temperature-time evolution should lie between the two modelled end-members. We suggest that potentially, one problem may arise from strong differences in mineral chemistry, e.g. for the magmatic biotite of sample BPA 034-11c, that shift the closure of the system to higher temperatures. Another

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15 Scenario 1: Only cooling Scenario 2: Cooling and re-heating

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Fig. V-15: Two possible scenario of temperature–time–depth evolution for the Campo unit corresponding to (a) one cooling from Triassic to Jurassic times or (b) a Triassic cooling followed by a late-Triassic heating and Jurassic cooling and exhumation. For each thermal evolution (c,d), diffusion of argon in muscovite and biotite is presented for different grain size (e,f). Diffusion was calculated using DiffArgP (Warren et al., 2011; Wheeler, 1996) with initial conditions of 4 kbar/500°C and final exhumation after 185 Ma to reach 0.5 kbar/50°C with one continuous cooling (case 1) or with a re-heating event (case 2).

P–T evolution of the Malenco unit is from Müntener et al. (2000).

184 Formation et exhumation des granulites permiennes

explanation could also be strong lateral variations in temperature in a rifted margin context (e.g.

Scheck-Wenderoth & Maystrenko, 2008).

5. e

Our results suggest that the basements of the Adriatic margin, nowadays preserved in the Austroalpine units, records a polyphase tectonic history characterized notably by a Permian post-orogenic cooling, and the subsequent Jurassic thinning. Based on information from the Grosina unit, the Campo unit, and additional data from the Austroalpine and Upper Penninic nappes (Fig. V-16), we propose a polyphase evolutionary model for the formation of the Adriatic necking zone from Permian to Jurassic times.