High-temperature metamorphism in marbles as a consequence of volatile release from crystallizing anatectic melts, Naxos, Greece

Two generations of high-temperature silicate mineral assemblage occur in vesuvianite- and amphibolite-bearing calcite- marbles in the leucogneiss core of the Miocene M2 metamorphic complex of Naxos, Greece. The first (1p) generation formed during prograde metamorphism, whereas the second retrograde (2r) generation developed along contacts between prograde amphibolites and calcite marbles, in veins that crosscut pure marbles, and at the contacts between pegmatite-aplite bodies and calcite marble, and clearly grew during the infiltration of an external fluid into the marbles. Phase equilibrium calculations indicate a water-rich fluid. The oxygen isotope composition of 2r silicate minerals shows a significant decrease relative to the 1p composition in prograde marbles. The calculated δ¹⁸O values of water in isotopic equilibrium with the 2r silicate minerals (δ¹⁸O garnet = 9.0 to 9.6‰; δ¹⁸O clinopyroxene = 10.3 to 11.0‰; δ¹⁸O hornblende = 10.8 to 11.2‰) match those of water in equilibrium with the minerals of the pegmatite-aplite bodies, which formed during late-stage crystallization of peak-metamorphic anatectic melts. The volatiles exsolved during the pegmatite-aplite crystallization are thus indicated to be the source of the infiltrating fluid. Recrystallized calcite marble adjacent to 2r silicate minerals shows isotopic evidence for limited water-rock interaction and indicates that fluid infiltration was strongly localized in the metasomatic zones. The study confirms that high-temperature metamorphism occurs as the result of volatile release during the crystallization of anatectic partial melts and that peak metamorphism in such terrains is a continuum spanning both prograde heating and retrograde cooling.