Missing lawsonite and aragonite found: P–T and fluid composition in meta-marls from the Combin Zone (Western Alps)

Abstract

We report the first findings of several occurrences of lawsonite and metamorphic aragonite in the meta-sediments from the Combin Zone (Piemonte–Liguria ocean, Western Alps), where the early blueschist-facies episode is poorly documented. New field and metamorphic data (thermodynamic modelling and Raman spectroscopy on carbonaceous material) are used to elucidate the P–T evolution and fluid composition of the Combin Zone and investigate the lawsonite growth and breakdown reactions. Two tectonometamorphic units have been identified within the Combin Zone with distinct geometry, lithological content and P–T conditions. In the higher grade unit, metamorphic aragonite occurs as inclusions in titanite. Lawsonite and garnet were stable at peak P–T conditions (~ 16–17 kbar and 460–480 °C) at very low X(CO2) values. Lawsonite is systematically pseudomorphed, but preserves hourglass zoning or 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 continental Dent Blanche nappe and records P–T conditions similar to those recorded by the Dent Blanche nappe. A metamorphic discontinuity is, therefore, documented between the largest part of the Combin Zone on the one hand, and the Dent Blanche nappe on the other hand. The discovery of lawsonite and metamorphic aragonite allows a better understanding of the large-scale metamorphic structure of the Western Alps.