Amphibole evolution in Variscan eclogite-amphibolites from the Savona crystalline massif (Western Ligurian Alps, Italy): Controls on the decompressional P- T- t path

Abstract

The Variscan metamorphic evolution of eclogite-amphibolites from the Savona Crystalline Massif is marked by the occurence of calcic amphiboles. Microtextural relations together with coupled electron microprobe and X-ray investigations on amphiboles pointed out a complex decompressional evolution. A prograde stage of the evolution in the eclogite facies (temperature conditions estimated as ca. 620°C) is recorded by compositional zoning and inclusions in garnet. The eclogitic event (pressure conditions 12kbar) produced assemblages of garnet, omphacite, Ca-amphibole, zoisite, quartz and rutile. Composition and zoning of matrix eclogitic amphiboles reveal that the subsequent evolution in the eclogite facies contemplates a late re-equilibration under lower conditions of temperature (ca. 540°C) and pressure. The eclogitic amphiboles are pargasitic to edenitic hornblendes, and are marked by rather high Na contents at the M4 site (up to 0.5 apfu). The forming of coarse-grained diopside-plagioclase symplectites and of amphibole-plagioclase coronas represent the early results of the destabilization of omphacites and garnets in the amphibolite facies. Pressure conditions of this stage have been estimated of ca. 10 kbar, while temperature conditions are poorly constrained. However, the increase in edenite (NaAlSi −1) at the outermost rims of the matrix eclogitic amphiboles and the very high values of edenite and tschermack (Al 2Mg −1Si −1) components in the coronitic amphiboles (subsilicic pargasites) suggest that temperature increase accompanied the early decompression to the amphibolite facies. Subsequent retrogression in the low-grade amphibolite facies resulted in the complete breakdown of omphacite, producing finer-grained symplectites. This stage is also recorded by the amphibole growth (magnesio-hornblendes to edenitic hornblendes with negligible Na amounts at the M4 site) at the expense of the clinopyroxene symplectites and as outward rims around the coronitic amphiboles. Frequently, amphibolites are completely recrystallized and do not retain any relic of high pressure assemblages. Temperature conditions for this late amphibolitic event have been estimated of ca. 510°C, while pressure conditions are inferred in the range 4–7 kbar. Such a complicated decompressional evolution is indicative of complex uplift tectonics after a subduction event.