Exhumation of the Schistes Lustrés complex: in situ laser probe 40Ar/39Ar constraints and implications for the Western Alps
Article first published online: 29 JUL 2002
Journal of Metamorphic Geology
Volume 20, Issue 6, pages 599–618, August 2002
How to Cite
Agard, P., Monié, P., Jolivet, L. and Goffé, B. (2002), Exhumation of the Schistes Lustrés complex: in situ laser probe 40Ar/39Ar constraints and implications for the Western Alps. Journal of Metamorphic Geology, 20: 599–618. doi: 10.1046/j.1525-1314.2002.00391.x
- Issue published online: 29 JUL 2002
- Article first published online: 29 JUL 2002
- Received 11 June 2001; revision accepted 1 March 2002.
- exhumation velocities;
- HP–LT metamorphism;
- laser-probe 40Ar/39Ar dating;
- Western Alps
The Schistes Lustrés (SL) suture zone occupies a key position in the Alpine chain between the high-pressure (HP) Brianconnais domain and the ultrahigh-pressure (UHP) Dora Maira massif, and reached subduction depths ranging from c. 40–65 km (Cottian Alps). In order to constrain the timing of HP metamorphism and subsequent exhumation, several phengite generations were differentiated, on the basis of habit, texture, paragenesis and chemistry, as belonging to the first or second exhumation episode, respectively, D2 or D3, or to earlier stages of the tectono-metamorphic evolution.
Ten carefully selected samples showing D2, D3 (D2 + D3), or earlier (mostly peak temperature) phengite population(s) were subjected to laser probe 40Ar/39Ar analysis. The data support the results of the petrostructural study with two distinct age groups (crystallization ages) for D2 and D3 phengite, at 51–45 and 38–35 Ma, respectively. The data also reveal a coherent age cluster, at 62–55 Ma, for peak temperature phengite associated with chloritoid which were preserved in low strain domains.
The age of the D3 event in the SL complex appears very similar to ages recently obtained for greenschist facies deformation on the border of most internal crystalline massifs. Exhumation rates of the order of 1–2 mm yr−1 are obtained for the SL complex, which are compatible with velocities documented for accretionary wedge settings. Similarly, cooling velocities are only moderate (c.5 °C Myr−1), which is at variance with recent estimates in the nearby UHP massifs.