Chemical evolution of high-grade metamorphic rocks — Anatexis and remotion of material from granulite terrains

Abstract

The geochemistry of acid, sub-acid metamorphic rocks of sedimentary origin from a sequence of increasing metamorphism of the Western Italian Alps has been studied. Analysis of bulk chemistries and of pairs of geochemically coherent elements (K—Rb, K—Pb, Na—Sr, Fe—Zn, etc.) indicates that metamorphism up to amphibolite facies is essentially an isochemical process even if anatexis is active, the products of initial melting recrystallizing together with unmelted solids. Rocks of granitic composition have been strongly depleted during granulite facies metamorphism in lithophile trace elements and probably in silicate fractions rich in alkalis. Such depletions have been correlated to extensive melting of pelitic rocks under water pressure condition determined by the progressive release of H 2O from hydrated phases involved in the melting. According to fractional melting models, differentiated low-viscosity fractions of silicate melts rich in water and granitophile elements have been removed and diffused outwards under a pressure gradient established by the increase in total volume of the rocks undergoing extensive anatexis. Most of the melt which has not migrated, because of the continuous loss of water and minimum-granite fractions, enhances progressively solidus conditions and recrystallizes within the unmelted solids giving granulitic parageneses. The intimate association between melted and unmelted materials causes granulites to present small-scale elemental and isotope disequilibria. Extensive melting may deeply modify the primary distribution of elements, favouring the attainment in granulites of magmatic-like trends.