Relative geobarometry from high-pressure rocks of quartzofeldspathic composition from the Sesia Zone, Western Alps, Italy

Abstract

In the Sesia Zone of the Western Alps, Italy, early Alpine blueschist to eclogite facies metamorphism of rocks of quartzofeldspathic composition has produced the same high-pressure assemblage of; quartz, Na-pyroxene, Na-amphibole, paragonite, phengite, zoisite, garnet, magnetite, sphene and Fe-sulphide (=the QFS assemblage) over an area (> 150 km2. Relative gradients in pressure and temperature over this region are reflected in the variations in mineral chemistries of the individual phases of the quartzofeldspathic assemblage through continuous reactions. Mineralogical discontinuities do not occur in the QFS assemblage of this region. Increases in the Jadeite content of the pyroxenes (XJd 0.48 to XJd 0.93) and in the glaucophane content of the amphiboles (XGl 0.89 to XGl 0.96) occur from the southwest to the northeast of the region studied. Analysis of coexisting garnets and pyroxenes indicate that the compositional variation of amphiboles and pyroxenes is associated with a decrease in the grossular component of the coexisting garnet. Zoned pyroxenes and garnets, together with the regional trends in mineral chemistries suggest that the evolution of the QFS assemblage with increasing pressure may be modelled by pressure-sensitive continuous reactions in which amphibole, zoisite and the more jadeitic pyroxene constitute the high-pressure assemblage. Chemographic constraints permit the positioning in pressure/temperature space of the compositional isopleths of those model continuous reactions involving these phases which meet the textural and chemical criteria observed in the natural assemblages. The low dP/dT slope (−20 bars/° C) of these isopleths causes the continuous reactions to be useful for geobarometric calculations at pressures above the absolute breakdown of albite to jadeite plus quartz. In addition the pseudobinary loops for the other continuous reactions which are potentially useful geobarometers and involve either the NaAlCa−1Mg−1 exchange or the MgCa−1 exchange are calculated. Comparison of mineral chemistries with the isopleths yields a relative barometric scheme for the localities studied. With these barometric observations, it is possible to show that the P-T path which the Sesia body travelled towards the final recorded state was one of increasing pressure. Other blueschist and eclogite occurrences from Syros and Sifnos which contain rocks of quartzofeldspathic composition are also examined.