Missing lawsonite found ! Resolving paradoxes of the metamorphic structure of the Western Alps

Lawsonite is a strongly hydrated (12 weight % H₂O) Ca- and Al-rich silicate, exclusively stable along low P/T gradients, typical of subduction zones. The distribution and preservation of lawsonite at the scale of a subduction/collision belt reflect the occurrence of rocks with favourable chemical composition (mainly hydrothermally altered metabasalts and marly limestones (i.e. calcschists), two lithologies especially common in the oceanic units) and their pressure-temperature-fluid history (with preservation favoured by decreasing T during decompression).

The distribution of lawsonite in the Western Alps has been investigated since several decades. In the blueschist-facies units from the South-Western Alps (Queyras, Ubaye), lawsonite is well preserved in the external domain, at the contact with the Briançonnais domain, but is largely pseudomorphed in the more internal domain, at the contact with the Viso Unit. Further North, neither lawsonite nor lawsonite pseudomorphs have been reported in the supposedly blueschist-facies Combin Zone, taken by most studies as an equivalent of the Queyras-Ubaye units. This constitutes a paradox with respect to the overall metamorphic structure of the Alpine belt.

This study documents for the first time several occurrences of lawsonite and garnet in the calcschists from the Combin Zone. Field and metamorphic data (thermodynamic modelling and Raman spectroscopy on carbonaceous material) point to the occurrence of two tectonometamorphic units within the Combin Zone, characterised by distinct geometry, lithological content and Alpine P-T conditions.

In the higher grade unit, lawsonite and garnet were stable at peak P-T conditions (~14-16 kbar and ~460-490 °C) at very low X(CO₂) values. Although lawsonite is systematically pseudomorphed, we have been able to recognize hourglass zoning in lawsonite or preservation of an internal fabric associated with the prograde ductile deformation.

The lower grade unit (~8 ± 1 kbar ~370-400 °C) is discontinuously exposed along the western base of the Dent Blanche nappe and records Alpine P-T conditions similar to those reached by the Dent Blanche nappe (Manzotti et al. 2020).

Our data show that lawsonite is not missing in the Combin Zone, and resolve a paradox about the large-scale metamorphic structure of the Alps.

 

Manzotti, P., Ballèvre, M., Pitra, P., Müntener, O., Putlitz, B., Robyr, M. (2020). Journal of Petrology, egaa044, https://doi.org/10.1093/petrology/egaa044.