More widespread than supposed: preserved lawsonite in the blueschist-facies ophiolitic bodies from the Albergian unit (Liguria-Piemonte zone, Western Alps)
Keywords:lawsonite, thermodynamic modeling, tectono-metamorphic evolution, Albergian unit, Liguria-Piemonte zone, Western Alps
The Albergian unit (AU), belonging to the Liguria-Piemonte oceanic domain of the Western Alps, comprises a thick sequence of calcschists and minor meta-ophiolitic bodies, from metric to kilometric in size. The occurrence of fresh lawsonite in its meta-sedimentary cover is known since the end of the XIX century. This paper reports the occurrence of fresh lawsonite also in the meta-mafic rocks and presents petrological data to constrain the tectono-metamorphic evolution of one of these meta-mafic bodies exposed in the upper Chisone valley. In the Monte Albergian – Gran Mioul area, the AU consists of a kilometric body of basalt and/or diabase meta-breccias (with minor meta-gabbros and meta-plagiogranite clasts and blocks), covered by discontinuous layers of quartzitic meta-sandstone, black micaschists, and eventually a thick sequence of calcschists. This paper focuses on the petrographic and mineral chemistry features of these lithologies, with emphasis on the preserved magmatic textures in the meta-mafic clasts, and on fresh lawsonite occurrence and its retrogression/replacement. The second part of the paper deals with phase equilibrium modeling of two representative samples from the area: a Mg-Al meta-gabbro and a meta-breccia with plagiogranite clasts. Thermodynamic modeling of isochemical phase diagrams (i.e. P-T pseudosections), performed for the first time in this unit, allowed constraining the metamorphic peak in the lawsonite blueschist-facies conditions, at about 18-21 kbar and 380-430 °C, followed by a partial re-equilibration in the epidote-stability field.