Experimental and field-related investigations on the metamorphic history of eclogites in the Sesia-Lanzo Zone, Western Alps (Italy)

The lack of applicable geobarometers in many eclogite facies rocks led to the evaluation of the assemblage K-feldspar-jadeite-quartz (KJQ) as a possible geobarometer in Chapter II. Use of appropriate mixing models to K-feldspar and jadeite solid solutions allowed application of the reaction albite = jadeite + quartz to the KJQ assemblage, which was used to constrain pressures and also a(H{dollar}sb2{dollar}O) in eclogite facies metagranitic and metapelitic rocks from the Sesia-Lanzo Zone.; Based on textural evidence and thermobarometric calculations the P-T conditions of five different stages of mineral growth in eclogites from Ianca Valley in the Sesia-Lanzo Zone were performed in Chapter III. It was possible to reconstruct a P-T-a(H{dollar}sb2{dollar}O) path for these eclogites. The data show that the eclogites underwent a nearly isothermal decompressional path from 570-650{dollar}spcirc{dollar}C at 21 kbar to 500-600{dollar}spcirc{dollar}C at 11-13 kbar and finally to 300-500{dollar}spcirc{dollar}C and 3-7 kbar. During these stages, a(H{dollar}sb2{dollar}O) remained relatively high (0.7-1.0).; Piston-cylinder experiments on sodic amphiboles were undertaken in Chapters IV and V. Synthetic sodic amphiboles are displaced from stoichiometric glaucophane, and the glaucophane component increases with decreasing temperature and increasing pressure. The compositions of these amphiboles are comparable to those from ultra-high pressure rocks. Additional piston-cylinder experiments on the reaction 2 glaucophane = 4 jadeite + 3 enstatite + 2 coesite + 2 H{dollar}sb2{dollar}O were undertaken in Chapter V. They yielded a bracket between 37 and 38 kbar at 800{dollar}spcirc{dollar}C which allowed the calculation a {dollar}Delta{dollar}G{dollar}spcircsb{lcub}rm f,298{rcub}{dollar} for glaucophane. The resulting {dollar}Delta{dollar}G{dollar}spcircsb{lcub}rm f,298{rcub}{dollar} is -11253 {dollar}pm{dollar} 2 kJ/mol, 5-20 kJ/mol more negative than previous estimates.; An experimental investigation on the synthesis of F-Al-sphene (CaAlSiO{dollar}sb4{dollar}F), which yielded Al contents of 50 to 98 mol percent, was performed in Chapter VI. No significant pressure or temperature dependency of the (Al + F) substitution could be found. Activity measurements by dilution of the reaction anorthite + rutile = sphene + kyanite with fluorite revealed only a slight non-ideality in the system CaTiSiO{dollar}sb5{dollar} CaAlSiO{dollar}sb4{dollar}F at 1000{dollar}spcirc{dollar}C and 1100{dollar}spcirc{dollar}C. These preliminary data strongly support the use of a molecular mixing model for sphene.